## July 2004 • NREL/SR-510-36244



## J. Van Gerpen, B. Shanks, and R. Pruszko Iowa State University



## D. Clements Renewable Products Development Laboratory



## G. Knothe

USDA/NCAUR 

# Biodiesel Production Technology

August 2002–January 2004

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

Operated for the U.S. De partment of Energy Office of Energy Effici ency and Renewable Energy by Midwest Research Institute • Battelle

Contract No. DE-AC36-99-GO10337

## July 2004 • NREL/SR-510-36244



# Biodiesel Production Technology

August 2002–January 2004

J. Van Gerpen, B.Shanks, and R. Pruszko Iowa State University



## D. Clements Renewable Products Development Laboratory



## G. Knothe

USDA/NCAUR

NREL Technical Monitor: K. Shaine Tyson

Prepared under Subcontract No. ACO-2-35016-01



### National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

Operated for the U.S. De partment of Energy Office of Energy Effici ency and Renewable Energy by Midwest Research Institute • Battelle

Contract No. DE-AC36-99-GO10337NOTICE

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## Biodiesel Production Technology

Background

1. Basics of Biodiesel Production .............................................................................................. ...................1

2. Basic Organic Chemistry ..................................................................................................... .....................7

3. Biodiesel Specifications and Properties..................................................................................... .............22

Biodiesel Production Processes

4. Types of Biodiesel Production Processes ..................................................................................... ..........30

Laboratory - Exercise 1........................................................................................................ ..........41

5. Basic Plant Equipment and Operation ........................................................................................ ...........43

6. Chemical Plant Controls ..................................................................................................... ....................48

7. Pretreatment of High Free Fatty Acid Feedstocks ............................................................................. .....52

8. Patent Discussion........................................................................................................... .........................56

9. Patent List for Biodiesel................................................................................................... .......................62

10. Post Reaction Processing ................................................................................................... ...................66

11. Treatment and Recovery of Side Streams..................................................................................... ........75

Laboratory – Exercise 2........................................................................................................ .........78

Biodiesel Plant Logistics

12. Feedstock Preparation...................................................................................................... .....................79

13. Feedstock Quality Issues................................................................................................... ....................90

14. Plant Safety ............................................................................................................... ............................92

15. Biodiesel Transportation and Storage....................................................................................... ............98

16. Product Quality ............................................................................................................ .......................101

Laboratory – Exercise 3........................................................................................................ .......1051 

## 1. Basics of Biodiesel Production

Biodiesel is an alternative fuel for diesel engine s that is gaining attention in the United States after reaching a considerable level of success in Europe. Its primary advantages are that it is one of the most renewable fuels currently available an d it is also non-toxic and biodegradable. It can also be used directly in most diesel engines without requirin g extensive engine modifications. The purpose of this book is to describe and e xplain the processes and issues involved in producing this new fuel.

The most cursory look at the l iterature relating to biodiesel will soon reveal the following relationship for prediction of bi odiesel from fats and oils.

100 lbs of oil + 10 lbs of methanol → 100 lbs of biodiesel + 10 lbs of glycerol

This equation is a simplified form of the following transesterfication reaction.

O O || ||

CH 2 - O - C - R 1 CH 3 - O - C - R 1 | | O O CH 2 - OH | || || |

CH - O - C - R 2 + 3 CH 3 OH → CH 3 - O - C - R 2 + CH - OH | (Catalyst) | | O O CH 2 - OH | || ||

CH 2 - O - C - R 3 CH 3 - O - C - R 3

triglyceride methanol mixture of fatty esters glycerol

Figure 1. Transesterification Reaction

where R 1 , R 2 , and R 3 are long chains of carbons and hydroge n atoms, sometimes called fatty acid chains. There are five types of chains that are common in soybean oil and animal fats (others are present in small amounts):

Palmitic: R = - (CH 2 ) 14 – CH 3 16 carbons, (including the one that R is attached to.) (16:0)

Stearic: R = - (CH 2 ) 16 – CH 3 18 carbons, 0 double bonds (18:0)

Oleic: R = - (CH 2 ) 7 CH=CH(CH 2 ) 7 CH 3 18 carbons, 1 double bond (18:1)2 Table 1. Composition of Various Oils and Fats.

Oil or fat 14:0 16:0 18:0 18:1 18:2 18:3 20:0 22:1
Soybean 6-10 2-5 20-30 50-60 5-11

Corn 1-2 8-12 2-5 19-49 34-62 trace

Peanut 8-9 2-3 50-65 20-30

Olive 9-10 2-3 73-84 10-12 trace

Cottonseed 0-2 20-25 1-2 23-35 40-50 trace

Hi linoleic Safflower 5.9 1.5 8.8 83.8

Hi Oleic Safflower 4.8 1.4 74.1 19.7 Hi Oleic Rapeseed 4.3 1.3 59.9 21.1 13.2 Hi Erucic Rapeseed 3.0 0.8 13.1 14.1 9.7 7.4 50.7

Butter 7-10 24-26 10-13 28-31 1-2.5 .2-.5

Lard 1-2 28-30 12-18 40-50 7-13 0-1

Tallow 3-6 24-32 20-25 37-43 2-3

Linseed Oil 4-7 2-4 25-40 35-40 25-60 Yellow grease (Typical) 2.43 23.24

16:1=3.79 12.96 44.32 6.97 0.67

Data derived from Organic Chemistry , W.W. Linstromberg, D.C. Heat h and Co., Lexington, MA, 1970.

Linoleic: R = - (CH 2 ) 7 CH=CH-CH 2 -CH=CH(CH 2 ) 4 CH 3 18 carbons, 2 double bonds (18:2)

Linolenic: R = - (CH 2 ) 7 CH=CH-CH 2 -CH=CH-CH 2 -CH=CH-CH 2 -CH 3 18 carbons, 3 double bonds (18:3)

These chains are designated by two numbers se parated by a colon. The first number designates

the number of carbon atoms in the chain and th e second number designates the number of double

bonds. Note that the number of carbon atoms incl udes the carbon that is double bonded to the oxygen atom at one end of the fatt y acid (called the carboxylic car bon). This is the end that the methanol attaches to when methyl esters are produced. Table 1 shows the percentages of each fatty acid chain present in common oils and fats.

For simplicity, consider an oil such as soybean oil to consist of pure triolein. Triolein is a triglyceride in which all three fatty acid chains ar e oleic acid. This is near the actual number of carbons and hydrogens and gives a molecular weight that is near the value for soybean oil. If triolein is reacted with methanol, the reaction w ill be that shown in Figure 2. Note that weights for each of the compounds in the reaction are given. These are based on the fact that one

molecule of triolein reacts with 3 molecules of methanol to produce 3 molecules of methyl oleate, the biodiesel product, and one mole of glycerol. Chemists typically multiply all the terms of this equation by a large number that corresponds to the number of molecules in a quantity equal to the molecular weight of the substance. This quantity is called a mole of the substance. To calculate the molecular weight of triolein, we count the number of carbons in the molecule (57) and multiply3 O ||

CH 2 - O - C - (CH 2 ) 7 CH=CH(CH 2 ) 7 CH 3 | | O | ||

CH - O - C - (CH 2 ) 7 CH=CH(CH 2 ) 7 CH 3 + 

## 3 CH 3 OH → | (KOH) | O | ||

CH 2 - O - C - (CH 2 ) 7 CH=CH(CH 2 ) 7 CH 3

Triolein Methanol (885.46 g) (3 x 32.04 = 96.12 g)

O CH 2 - OH || |
3 CH 3 - O - C - (CH 2 ) 7 CH=CH(CH 2 ) 7 CH 3 + CH - OH |

CH 2 - OH

Methyl oleate (biodiesel) Glycerol (3 x 296.50 = 889.50 g) (92.10 g)

Figure 2. Transesterification of Triolein

this by 12.0111, the molecular weight of car bon. Doing the same thing for hydrogen and oxygen gives:

57 x 12.0111 = 684.63

104 x 1.00797 = 104.83

6 x 16.000 = 96.00 Total = 885.46 grams per mole

Therefore, the molecular weight of triolein is 885.46 and one mole of triolein weighs 885.46 grams. Three moles of methanol weigh 96.12 g, 3 moles of methyl oleate weigh 889.50 g, and 1 mole of glycerol weighs 92.10 g.

We do not actually conduct the reaction this way. We usually add 60% to 100% excess methanol to ensure that the reaction goes to completion. In general, reactions can be encouraged to progress by adding an excess of one of the reac tants or by removing one of the products. The reaction of triolein with 100% excess (X S) methanol is shown in Figure 3.4 Triolein + 2X Methanol (885.46 g) (6 x 32.04 = 192.24 g)



# → Methyl oleate + Glycerol + XS Methanol
(Catalyst) (3 x 296.50 = 889.50 g) (92.10 g) (96.12 g)

Figure 3. Transesterification of Tr iolein with 100% Excess Methanol

On the basis of 100 lb of oil, the reaction ma ss balance with 100% XS methanol becomes:

100 lb oil + 21.71 lb methanol → 100.45 lb biodiesel + 10.40 lb glycerol + 10.86 lb XS methanol

The reaction also requires about 1% (based on the weight of oil) of sodium hydroxide or a similar catalyst that mostly ends up in the glycer ol. These quantities can be converted to volumes by including the densities of the reac tants and products given in Table 2.

Table 2. Densities of Biodiesel Reactants (kg/liter)
(from the Handbook of Chemistry and Physics, 51 st Edition, CRC, 1970-1971.)
Triolein: 0.8988

Methanol 0.7914 Methyl Oleate 0.8739 Glycerol 1.2613

On a volume basis, the reaction becomes:

100 liters of oil + 24. 65 liters methanol

→ 103.3 liters methyl oleate + 7.42 liter s glycerol + 12.33 liters XS methanol

Product Quality

The standard for biodiesel allows 0.24% total glycerol in the final product. What does this actually mean? It is cl ear that a molecule of a triglycerid e can be considered to contain a molecule of glycerol, sometimes called the glycerol backbone. In the case of triolein, the mole of glycerol would weigh 92.10 g and the mole of tr iolein weighs 885.46 g. Therefore, triolein can be considered to consist of 92.10/885.46 = 0.104, or 10.4% glycerol. This glycerol is called bound glycerol because it is chemically bound to the triolein molecule. Bound glycerol can also be associated with monoglycerides and digl ycerides, the partial r eaction products of the conversion of triglycerides to al kyl esters. The structures of these molecules are shown in Figure

4.5 CH 2 - OH CH 3 - OH | | | O | | || |

CH - O - C - R 2 CH 3 - OH | | | O | O | || | ||

CH 2 - O - C - R 3 CH 3 - O - C - R 3

Diglyceride Monoglyceride

Figure 4. Chemical Structure of Diglyceride and Monoglyceride

Bound glycerol is added to a ny fully reacted glycerol, or free glycerol , that may still be in the biodiesel, to get the total glycerol . If the original oil contai ns 10.4% glycerol, and the final biodiesel can only contain a total glycerol level of 0.24%, then the transesterification

reaction must be

% 7

. 97 100 4

. 10 24

. 0 4

. 10

= − x or 97.7% complete.

Competing Reactions

It is common for oils and fats to contain sma ll amounts of water and free fatty acids. Free fatty acids consist of the long carbon chains describe d that are disconnected from the glycerol backbone. They are sometimes called carboxylic acids.

O ||

HO - C - R

Figure 5. Carboxylic Acid (R is a carbon chain)

The oleic group we have used earlier gives oleic acid, one of the free fatty acids that can be found in unrefined vegetable oils and animal fats.

O || HO - C - (CH 2 ) 7 CH=CH(CH 2 ) 7 CH 3

Figure 6. Oleic Acid6 If an oil or fat containing a free fatty acid such as oleic acid is used to produce biodiesel, the alkali catalyst typically used to encourage the reaction will react with th is acid to form soap. Figure 7 shows this reaction when the catalyst is potassium hydroxide (KOH).

O || + KOH HO - C - (CH 2 ) 7 CH=CH(CH 2 ) 7 CH 3

Oleic Acid Potassium Hydroxide

O ||

→ K +

- O - C - (CH 2 ) 7 CH=CH(CH 2 ) 7 CH 3 + H 2 O

Potassium oleate (soap) Water

Figure 7. Formation of Soap

This reaction is undesirable because it binds the cat alyst into a form that does not contribute to accelerating the reaction. Excessive soap in the products can inhibit later processing of the biodiesel, including glycerol se paration and water washing.

Water in the oil or fat can also be problem. When water is present, particularly at high temperatures, it can hydrolyze the triglycerides to diglycerides and form a free fatty acid. Figure 8 shows a typical hydrolysis reaction.

When an alkali catalyst is present, the free fatty acid will react to form soap following the reaction given earlier (Figure 7). When water is present in the reaction it generally manifests itself through excessive soap production. The soap s of saturated fatty acids tend to solidify at ambient temperatures so a reaction mixture with excessive soap may gel and form a semi-solid mass that is very difficult to recover.

O ||

CH 2 - O - C - R 1 CH 3 - OH | | | O | O O | || | || ||

CH - O - C - R 2 + H 2 O → CH 3 - O - C - R 2 + HO – C - R 1 | | | O | O | || | ||

CH 2 - O - C - R 3 CH 3 - O - C - R 3 Triglyceride Water Diglyceride Fatty acid

Figure 8. Hydrolysis of a Triglyceride to Form Free Fatty Acids7 

## 2. Basic Organic Chemistry

As demonstrated in the preceding chapter, biodiesel production involves many chemical processes. In order to facilitate comprehensi on of the organic chemistr y underlying biodiesel, its feedstocks, production and analysis, we will intro duce some basic chemistry definitions. Some or parts of some definitions given here are taken from Webster’s Third New International Dictionary , published by Merriam-Webster, Springfield, MA, 1993.

Basic Chemistry

Chemistry is the science that deals with the compositi on, structure, and prope rties of substances

and of the transformations that these substances undergo. In most cases , those transformations

are called reactions

.

More than 100 elements are now known and most of them o ccur naturally. An element is a substance that cannot be separa ted into simpler substances by ordinary chemical means. Elements are arranged in a systematic fashi on based on their atomic numbers (see definition below) and properties in the periodic table

.

An atom is the smallest particle of an element that can exist either alone or in combination with similar particles (other atoms) of the same or of a different element. It is the smallest particle of an element that enters into the composition of mo lecules (which leads us to the definition of a molecule below). An element is comprised exclusively of atoms having the same atomic

number . The atoms of each element have a specific atomic weight , which is the average relative weight of an element as it occurs in nature referred to some element taken as standard. Atoms are composed of protons

, electrons and neutrons , which are responsible for the atomic weight. The nucleus of an atom consists of the protons and neutrons. The nucle us is the bulk of the weight of the atom. Electrons are smaller and lighter and they orbit the nucleus. It is very important that a proton has a positive charge and an electron has a negative charge. The number of protons, neutrons and electrons that compri se an atom give the atom its properties and ultimately the properties of the element. Thus, the element with one proton (and one electron) is hydrogen. “Adding” another proton (and electron as well as neutrons) gives the next element (helium), etc. Carbon has six protons and six electrons while oxygen has eight protons and

electrons.

A molecule is the smallest particle of an aggregate of at least two connected atoms of either the

same or different elements and has a combinati on of properties (chemical and physical) that is specific to it. Each molecule has a molecular weight , which is the weight of the molecule calculated as the sum of the atomic weights of its constituent atoms.

A compound is a chemically distinct substance formed by the union of two or more ingredients (as elements) in definite proportion by weight and with definite stru ctural arrangement.

A mole is the quantity of a chemical substance th at has a weight in mass units (in grams) numerically equal to the molecular weight or that in the case of a gas has a volume occupied by such a weight under specified conditions (as 22.4 liters at 0oC and a pressure of 760 millimeters8 of mercury). Thus, the number of molecules (6.022 x 10 23 ) in one mole of any given substance is equal to the number of molecules in one mole on any other substance. The concept of mole is very important in chemistry for calculating the amounts of compounds that undergo a reaction

with each other. Often reactions are influenced by the amounts of compounds reacted with each other, the molar ratio (a related term to molar ratio is stoichiometry of a reaction).

Elements are in most cases comprised of the same kind of elementary particles known as atoms

. Atoms can form aggregates known as molecules. Molecules can consis t of atoms of many

different elements or of atoms of one element. If a molecule is comprised of atoms of different elements, it is also known as a compound.

Each element has a symbol, for example H for hydrogen (hydrogen is the lightest element, each atom consisting only of one proton and one elec tron, see above; a hydrogen atom with the

electron removed is referred to as a proton that can be written as H + ) and O for oxygen, so that a

chemical formula for each compound can be written. If mo re than one atom of an element is in the compound, a subscript number indicates that. Thus, water, as a compound comprised of two hydrogen atoms and one oxygen atom, has the molecular formula H 2 O. In other words, each water molecule is composed of two hydrogen atoms and one oxygen atom.

Organic chemistry

Although there are many compounds that bridge the gap, most compounds are still classified

either as inorganic or organic . The reason goes back far into the history of chemistry when it was assumed that all compounds came either fr om inanimate matter (inorganic compounds) or living matter (organic compounds). With some exceptions (an example is carbon dioxide, CO 2 ) that will not concern us here, organic chemistry is the chem istry of the element carbon (symbol C). If organic chemistry deals only with th e chemistry of one element, then why is it distinguished from inorganic chemistry? The reason is that most known compounds (approximately 90%) are organic compounds. If you think of the as tounding variety of plant and animal life and that, on a basic le vel, organic compounds are responsib le for that life, it becomes clear why there are so many organic compounds. More scientifically, carbon atoms are very versatile in forming compounds with atoms of othe r elements. So why are we going to deal with some organic chemistry here? The answer is th at both biodiesel and conventional diesel fuel (derived from petroleum) are mixtures of organic compounds.

Besides carbon, several other elements are very im portant to organic chemistry. Three especially important elements are hydrogen, oxygen, a nd nitrogen. Other important elements are phosphorus, sulfur and the so-called halogens (f luorine, chlorine, brom ine and iodine). The following table contains some basic informa tion on common elements (except halogens) in organic chemistry.9 Table 3. Some elements of importance to organic chemistry.

Name Symbol Atomic Number Atomic Weight

Carbon C 6 12.011

Hydrogen H 1 1.008

Nitrogen N 7 14.007

Oxygen O 8 15.9994

Phosphorus P 15 30.974

Sulfur S 16 32.06

For biodiesel purposes, carbon, hydrogen and oxyge n are especially impor tant and it is a good idea to memorize their atomic weights. For (rough) estimates, it suffices to just use the values H = 1, C = 12, O = 16, etc.

When atoms form molecules, they connect via bonds . For organic chemistry, it is extremely
important that each carbon atom in a molecule forms four bonds to other atoms. These other atoms can be other carbon, hydrogen, or oxygen or atoms of other elements. The bonds can be formed to a mixture of other atoms, for example, one carbon atom can form a bond to one hydrogen, one oxygen and two carbon atoms to meet the requirement of four bonds. When a

carbon atom bonds to four other atoms, regardless of the nature of these atoms, these bonds are single bonds. Hydrogen can form only one bond to other atoms and oxygen can form two bonds

to other atoms. It is also extremely important that carbon atoms can form double bonds or triple

bonds . Compounds that have double or triple bonds are often called unsaturated compounds (can you explain why?). Thus, one carbon atom can be connected to an oxygen atom by a single bond, another carbon by a single bond, and another car bon by a double bond to meet the requirement of four bonds. Two carbon atoms can also be connected by triple bonds. A carbon atom and an oxygen atom can also be connected by a double bond (obviously, then the oxygen atom cannot form a bond with another atom). This versatility in bonding is responsible for the many different, often complex, structures of organic compounds.

As in the case of water disc ussed above, organic molecules ca n be written as formulas, for example, CH 4 for methane. For more complex molecules, it is a good idea and indeed customary

to write structural formulas because of the ma ny possible structures for a given combination of

carbon atoms with other at oms (a feature known as isomerism ). That way, features like double or triple bonds can also be indicat ed by the corresponding number of lines between carbon atoms or carbon and other atoms. Examples of some si mple organic molecules containing only carbon, hydrogen, and oxygen that also show the diversity of organic compounds (check to see that there are always four bonds per carbon) are shown in Figure 9.

Organic compounds can be categorized into various classes of compounds . In the above examples, note that methanol, ethanol (the alcoho l contained in alcoholic beverages), and 1- and 2-propanol (note the difference in 1- and 2-propanol; what does the difference in nomenclature indicate?) belong to the class of compounds known as alcohols . Their common feature is the10 O ||

CH 4 CH 3 -CH 3 CH 3 -CH 2 -CH 3 CH 2 =CH 2 CH ≡ CH CH 3 -C-CH 3 Methane Ethane Propane Ethylene Acetylene Acetone (Ethene) (Ethyne) (Propanone)

OH OH OH OH | | | |

CH 3 OH CH 3 -CH 2 -OH CH 3 -CH 2 -CH 2 -OH CH 3 -CH-CH 3 CH 2 —CH---CH 2 Methanol Ethanol 1-Propanol 2-Propanol Glycerol ( iso -Propanol) (Glycerine; 1,2,3-Propanetriol)

O O O || || ||

CH 3 -C-OH CH 3 -C-OCH 3 CH 3 -C-O-CH 2 CH 3 Acetic acid Methyl acetate Ethyl acetate (Acetic acid methyl ester) (Acetic acid ethyl ester)

Figure 9. Simple Organic Molecules

presence of an OH group (also known as hydroxy gr oup). A feature such as an OH group is also

known as a functional group . Besides OH, there are many more functional groups. A functional group usually imparts specific properties and the propensity to undergo certain reactions to a molecule. There are innumerable compounds that belong to the class of compounds known as alcohols. Many organic molecules contain the charac teristics of several or even many classes of compounds usually because they contain seve ral or many different functional groups.

Note how the names of alcohols are derived from the compounds in the top row known as hydrocarbons (hydrocarbons are compounds containi ng carbon and hydrogen) by dropping the -e and adding the suffix -ol (Why is the name “glycerol ” preferable to “glycerine ” for the same compound?). Similarly, note that the CH 3 moiety as it is found in methyl esters is called a methyl group and CH 2 CH 3 is called ethyl , again a systematic change in nomenclature. Such changes in nomenclature, especially rational nomenclature, ar e common in organic chemistry. For some of the compounds, more than one name is given. This results of many co mpounds having so-called trivial names besides their rational names (histo rically, the trivial names are older than the rational names). Trivial names are important because for many organic compounds of more complex structure the rational names become unwiel dy, so that the trivial names are much easier

to learn and use.11 O O O O || || || ||

R-OH R-C-OH R 1 -C-O-R 2 R 1 -C-R 2 R-C-H

Alcohol Carboxylic acid Este r Ketone Aldehyde

Figure 10. Classes of Organic Molecules

When reading (or writing) the structural formulas of organic compounds, you will often

encounter R or R 1 , R 2 or R 3 , etc. “R” means “ organic rest ” and is an abbreviation for a part of the molecule which then has to be defined. General formulas for some classes of compounds can be written as shown in Figure 10.

The specific structural features of organic co mpounds (such as the hydroxy groups in alcohols) are responsible for the fact that many compound s can react with other compounds to form new compounds. The new may then belong to a diffe rent class of compounds. These structural features are also responsible for the differen ces in physical properties (melting point, boiling point, etc.) of the organic compounds. For example, acetic acid can react with methanol to form methyl acetate (which belongs to the ester class of compounds; esters are formed by the reaction of an acid with an alcohol), in which case water is also formed as a byproduct:

O O
|| ||
CH 3 -C- OH + CH 3 O H → CH 3 -C- O-CH 3 + H 2 O

Figure 11. Chemical Reaction to Form Methyl Acetate

You can also perform the back reaction in which methyl acetate reacts with water to form acetic acid and methanol. When methanol is the alcohol pa rticipating in the reacti on, the ester formed is

a methyl ester , when ethanol is the alcohol pa rticipating in the reaction, an ethyl ester is formed.

Vegetable oils and biodiesel

Now we can start to deal with biodiesel. As you know, biodiesel is derive d from vegetable oils. The major components of vegetable oils are triglycerides. The term triacylglycerols is being used more and more, but we will use the cl assical term in this discussion. Triglycerides are esters of glycerol (see above; an alcohol with a hydr oxy group on each of its three carbon atoms) with long-chain acids, commonly called fatty acids . Tables 4 and 5 list the most common fatty acids and their corresponding methyl esters.

Note from the comparison of the rational names of the fatty acids with thei r structural formulas how the position of the double bonds is define d by numbers. The number of carbon atoms is counted by beginning with the first carbon havi ng the functional group defining the fatty compound as acid or ester. As you can see from the former example (for example, 1-propanol and 2-propanol), this way of c ounting holds for other functional groups as well. The trivial names of fatty acids and their esters are far mo re commonly used than their rational names.12 Table 4. Chemical Structure of Common Fatty Acids and Their Methyl Esters Fatty acid (trivial name / rational name); Structure a) Common

acronym Methyl ester (trivial name / rational name)

Palmitic acid / Hexadecanoic acid; R-(CH 2 ) 14 -CH 3 C16:0 Methyl palmitate / Methyl hexadecanoate

Stearic acid / Octadecanoic

acid R-(CH 2 ) 16 -CH 3 C18:0 Methyl stearate / Methyl octadecanoate

Oleic acid / 9( Z )-

octadecenoic acid R-(CH 2 ) 7 -CH=CH-(CH 2 ) 7 - CH 3 C18:1 Methyl oleate / Methyl 9(Z)-octadecenoate

Linoleic acid / 9( Z ),12( Z )- octadecadienoic acid; R-(CH 2 ) 7 -CH=CH-CH 2 - CH=CH-(CH 2 ) 4 -CH 3 C18:2 Methyl linoleate / Methyl 9(Z),12(Z)-

octadecadienoate

Linolenic acid / 9( Z ),12( Z ),15( Z )- octadecatrienoic acid; R-(CH 2 ) 7 -(CH=CH-CH 2 ) 3 - CH 3 C18:3 Methyl linolenate / Methyl 9(Z),12(Z),15(Z)-

octadecadienoate

a) R = COOH (CO 2 H) or COOCH 3 (CO 2 CH 3 ); (CH 2 ) 7 = CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 , etc.

Table 5. Characteristics of Common Fa tty Acids and Their Methyl Esters

Fatty acid Methyl ester Formula Molecular weight Melting point ( o C)

Palmitic acid Methyl palmitate C 16 H 32 O 2 C 17 H 34 O 2 256.428

270.457 63-64

30.5

Stearic acid Methyl stearate C 18 H 36 O 2 C 19 H 38 O 2 284.481

298.511 70

39

Oleic acid Methyl oleate C 18 H 34 O 2 C 19 H 36 O 2 282.465

296.495 16

-20

Linoleic acid Methyl linoleate C 18 H 32 O 2 C 19 H 34 O 2 280.450

294.479 -5

-35

Linolenic acid Methyl linolenate C 18 H 30 O 2 C 19 H 32 O 2 278.434

292.463 -11

-52 / -57

A glance at the differences in the nomenclature will indicate why. Often fatty acids and their esters are referred to, especially in writing, by the abbreviations given in the last column of Table 4. (Question: How do those abbreviations arise?)

All double bonds in the rational nomen clature of fatty acids and esters are also defined by a letter ( Z ) , which is the result of the possibility of two kinds of double bonds. These two kinds of

double bonds are a result of how the other parts of the molecu le are attached in space to the carbon atoms forming the double bond. Besides this technical aspect, we will not discuss any13 further details of such arrangements of mol ecules in space except nomenclature. In rational nomenclature, the counterpart to Z is defined by the letter E . However, in the literature the terms

cis and trans are also often used. In organic chemistry, a cis double usually, but not always

, corresponds to a Z double bond and a trans double bond usually, but not always , corresponds to

an E double bond. In the case of the fatty acids we are interested in here, cis and Z correspond to

each other as do trans and E . However, each of these fatty acid s has a counterpart in which the

double bonds are trans (or E ), but their occurrence (in nature) is far less common. (Question: Have you heard health reports on trans fatty acid s?) Under certain conditions, the double bonds can change from Z to E . These conditions incl ude heating and types of reactions called hydrogenations (a hydrogenation reaction is th e production of more saturated fats from unsaturated ones by adding hydrogen to double bonds).

The two kinds of double bonds cause the compounds to have different physical properties such as melting point. For example, while the melting point of oleic acid is 15oC (compare to that of methyl oleate above; note that fatty acids ha ve higher melting points than their corresponding methyl esters), that of elai dic acid (the acid in which the Z double bond in oleic acid is replaced by E ; the methyl ester would be called methyl el aidate) is 45oC. The existence of different compounds for the same formula (both oleic acid and elaidic acid are C 18 H 34 O 2 ; their methyl

esters are C 19 H 36 O 2 ) is called isomerism . The corresponding compounds are then isomers . There are many different kinds of isomerism. A more detailed discussion of isomerism is beyond the scope of this course.

It is extremely important to realize that vegetable oils are mixtures of triglycerides from various fatty acids. The composition of vegetable oils varies with the plant source. Often the terms fatty acid profile or fatty acid composition are used to describe the sp ecific nature of fatty acids occurring in fats and oils. Several examples were described in Chapter 1 and profiles for common vegetable oils and animal fats were given in Table 1. The chemical and physical properties of fats and oils and the esters derive d from them vary with the fatty acid profile. For example, imagine how mixtures containing larg er amounts of methyl palmitate or methyl stearate will solidify at higher temperatures than those containing lesser amounts (see melting points in Table 5).

The corresponding esters of those fatty acids in Tables 4 and 5 with glycerol, are known as tripalmitin, tristearin, triolein, trilinolein and trilinolenin. (Exercise: Calculate the molecular weights of these triglycerides)

In a variation of the formation of esters from aci ds and alcohols as discussed above, an ester can react with another alcohol. In that case, the ne w alcohol is derived from the original ester is formed and the new ester is derived from the origin al alcohol. Thus, an ethyl ester can react with methanol to form a methyl ester and ethanol. This process is called transesterification

. Transesterification is extremely important for biodiesel. Biodiesel as it is defined today is obtained by transesterifying the tr iglycerides with methanol. Meth anol is the preferred alcohol for obtaining biodiesel because it is the cheapest (and most available) alcohol. However, for the reaction to occur in a reasonable time, a substance called a catalyst (catalysts are substances that, often present in small amounts, accelerate the speed of a reaction; in many cases virtually no reaction would occur without a catalyst), must be added to the mixture of the vegetable oil and14 methanol. The transesterification reaction for bi odiesel production was provided in Figure 1 and is repeated in Figure 12 in more generic form.

O ||

CH 2 -O-C-R CH 2 -OH | | | O O | | || Catalyst || |

CH-O-C-R + 3 R-OH → 3 R--O-C-R + CH-OH | | | O | | || |

CH 2 -O-C-R CH 2 -OH

Triacylglycerol Alcohol Alkyl ester Glycerol (Vegetable oil) (Biodiesel)

Figure 12. The transesterification reaction.

R is a mixture of various fatty acid chains. The alcohol used for producing biodiesel is usually methanol (where the R- corresponds to CH 3 ).

As noted in the figure caption, R can be any of the fatty acid chains (for example, -(CH 2 ) 14 -CH 3; see Table 4 for more examples; note how R or anything containing it is flexible and must be defined each time it is used). The number 3 indicates the number of moles needed to satisfy the formation of the methyl esters. When one mole is necessary, the number 1 is usually not written. It is also important to keep it mind that this only formally states the molar ratios of starting materials and products. In practice, the molar ratios may need to vary to obtain a nearly complete

reaction.

For the transesterification to occur, usually 6 moles of alcohol are used for every mole of triglyceride, which is more than the equation indi cates. The reason is that the reaction is desired to proceed in the direction of the arrow, i.e., to the right. In other terms, the equilibrium of the reaction needs to be shifted toward the right side of the equation. As the term “equilibrium” indicates, not all reactions eas ily proceed to completion and after some time the starting materials and reaction products ar e present in constant amounts (the equilibrium has been attained). In many cases, the fact that a reaction can proceed in th e reverse fashion (from right to left in the equation) also plays a role in formation of the equilibrium. To force the equilibrium in the direction of the products (as is almost alwa ys desired), one or more parameter(s) of the reaction may need to be changed. Such parameters include the molar ratio as well as others such as temperature, pressure and use of a catalyst.

In accordance with the names of fatty acids and th eir esters (see Tables 4 and 5), the methyl ester of soybean o il is often called methyl soyate . The term soybean oil methyl ester (SME) is15 also very common. The same holds for the es ters of other vegetabl e oils. Another common abbreviation that is commonly used is FAME (fatty acid methyl ester).

Besides triglycerides, mono- and diglycerides can also exist. They are formed as intermediates during the transesterification reaction. This is one of the problems when conducting chemical reactions in general, not only the transesterification reaction. It is almost always the goal of chemical reactions to obtain products that are as pure as possible. However, hardly any chemical reaction proceeds to full completion (see the discussion of equilibrium above). Therefore, often intermediates (in the case of transesterification the intermediates are partially reacted triglycerides, i.e., the mono- and diglycerides) can contaminate the final product. Other materials

that can contaminate biodiesel are residual metha nol (or other alcohol), gl ycerol, and catalyst.

When the transesterification react ion is conducted, you can visually observe that not all materials can be readily mixed with each other. A term of ten used for this phenomenon is that there are

two phases . At the start of the reacti on, you can notice that methanol and vegetable oil do not readily mix. At the end of the reaction, you can notice that there are two layers (phases), one consisting mainly of glycerol, the other of the methyl esters. Obviously, glycerol and methyl esters do not mix readily. How readily one compound will dissolve in another depends on the structural features of the compounds, for exam ple the existence of OH groups. Thus, compounds containing OH groups and those not containi ng OH groups often will not readily mix. (Question: Why is water effective in “washing” biodiesel after the transe sterification reaction?)

The catalyst used for carrying out the transesterif ication is usually sodium hydroxide (NaOH) or potassium hydroxide (KOH). These compounds be long to a class of materials known as bases and also are inorganic compounds (inorganic compounds are often us ed in organic chemistry for carrying out or catalyzin g reactions). Other bases are also suitable for the transesterification reaction. The counterparts of bases are known as acids . Many acids can also be used as catalysts in the transesterification reaction. However, the base-catalyzed reaction has advantages such as a higher reaction rate. More deta ils are covered in the produc tion part of this book.

In the soap formation reaction described in Figure 7, a fatty acid and base reacted to form a new compound, which was called soap, and water. Compounds such as soap, in which the hydrogen ( proton ) of an acid has been replaced with a metal ion, are often called salts . The reason that such compounds exist is that materials such as NaOH (or KOH) can split apart ( dissociate ) in a fashion that gives Na + and OH - (or K + and OH - ) in which the protons and electrons are not evenly distributed, leading to charged part icles. Thus, having the same charge, Na + or K + can replace H + here.

Another important aspect of the chemistry of fa tty acids and their esters is that the more unsaturated fatty acids and their esters (linoleic, linolenic) can relatively easily react with air (more specifically, the oxygen in air) and form de gradation products with time (the time depends also on other factors such as temperatur e). This is due to the existence of CH 2 between two carbons double-bonded to other carbons (see th e structural formulas in Table 4).16 Why are methyl esters of fatty acids suitable as diesel fuel?

Earlier, we briefly discussed compounds su ch as methane, ethane, and propane. These compounds belong to a class of compounds called alkanes . Alkanes are compounds consisting only of carbon and hydrogen and they only contain single bonds. Therefore, the general formula for straight-chain alkanes is CH 3 -(CH 2 )

n -CH 3 . Other alkanes, for example with branching in the chain, or with the carbons arranged in a ring, can also exist. However, in each case, the carbons are connected only with single bonds.

When dealing with diesel fuel s, you may have heard the term cetane number , which derives its name from hexadecane (trivial name: cetane) , a straight-chain al kane with 16 carbons (C 16 H 34 ).

The cetane number is a dimensionless indicator of the ignition quality of a diesel fuel and is conceptually similar to the better-known octane number used for gasoline. Hexadecane is the high-quality standard on the cetane scale and ha s been assigned a cetane number of 100. A highly branched alkane, 2,2,4,4,6,8,8-heptamet hylnonane (HMN; nine carbons in a chain with

seven CH 3 groups attached to it besides the two terminal CH 3 groups, thus it also has 16 carbon atoms; try drawing its structure) is the low- quality compound on the cetane scale and has a cetane number of 15. Therefore, long, unbranched alkanes comprise an “i deal” petrodiesel fuel. The analogy to hexadecane as “ideal” petrodiese l component shows why biodiesel is suitable as an “alternative” diesel fuel. The fatty acids whose methyl esters are now used as biodiesel also are long-chain compounds similar to long-chai n alkanes such as hexadecane which make good petrodiesel.

Petrodiesel consists of many components. Besi des hydrocarbons such as those mentioned above, petrodiesel often contains signifi cant amounts of compounds known as aromatics . Aromatics are cyclic compounds such as benzene or toluene (see structures in Figure 13). The carbon atoms are not shown here explicitly in the structures. They are assumed to be at the intersections of the

lines that show the bonds between the carbons.

Figure 13. Aromatic molecules

Thus, benzene has the formula C 6 H 6 and toluene has the formula C 7 H 8 . The common feature of all aromatic compounds is such a ri ng structure. There are many aromatic compounds in which the ring structures are fused, for example, naphthalene (formula C 10 H 8 ). They have low

cetane numbers and therefore are undesirable components of petrod iesel. However, they have high densities and thus help elevate the energy cont ained in a gallon of the fuel. Biodiesel’s lack of aromatic compounds is ofte n cited as an advantage.17 Compounds in which several benzene rings are fuse d together (even more than in naphthalene)

are termed polyaromatic hydrocarbons (PAHs). They are found in exhaust emissions of petrodiesel and, in reduced amounts, of biodiesel fuel.

Why are vegetable oils transes terified to produce biodiesel? This question will mainly be dealt with in other parts of the course(s), but briefly, vegetable oil methyl esters have lower viscosities (resistance to flow of a liquid) than the parent vegetable oils (think of honey or syrup, which have high viscosities and flow with difficulty, vs . water or milk, which have low viscosities and flow easily). Compared to the viscosities of the pa rent vegetable oils, the viscosities of vegetable oil methyl esters are much closer to that of petrodiesel. High viscosity causes operational problems in a diesel engine such as poor quality fuel injection and the formation of deposits.

The Concept of Acids and Bases

Many compounds, both organic and inor ganic, can be classified as acids and bases. The most useful concept of acids and bases as these materials relate to biodiesel is that of proton donors (acids) or proton acceptors (bases), also often called Brønsted (or Lowry ) acids and bases. Transferring a proton from an acid to a base is an acid-base reaction. A proton is a hydrogen atom stripped of its electron. Thus, a proton ca rries a positive charge and is often written H +

.

Whether a compound can behave as an acid or base depends on some of the atoms contained in it. Thus, organic compounds containing nitroge n often have base character, while those containing oxygen in form of OH (hydroxy) gro ups often have acid character (there are compounds that have the characteristics of both but that will not concern us here). Also, there are acids and bases in which a proton has been replaced by another particle carrying a positive charge called a cation , derived from another element. No te that a charged particle, be it negatively or positively charged, is called an ion . Without going into further detail, that acids and bases split ( dissociate ) into ions instead of atoms results fr om the position of the atoms in that compound in the periodic table of the elements and the corresponding electron configuration of

those atoms.

Thus, in water (H 2 O), one proton (H + ) can be replaced by the positively charged cation of sodium (Na + ) or potassium (K + ) to give NaOH (sodium hydroxide) or KOH (potassium hydroxide). Obviously, the remaining hydroxy part ( OH) is then negatively charged to give OH - (hydroxide). As a result, NaOH a nd KOH are strong bases as they ha ve a tendency to take up a proton. Similarly, in an alcohol such as methanol (CH 3 OH), the proton attached to the oxygen atom can be replaced by, for example, Na + . The resulting CH 3 ONa (sodium methylate; sodium methoxide; sodium methanolate) is a strong base, CH 3 O - being the reason.

Similarly, but in the reverse fashion, there are acids, the best-known being sulfuric acid (H 2 SO 4 ), hydrochloric acid (HCl) and nitric acid (HNO 3 ). Acids have a tendency to give off protons, leaving a negatively charged particle called an anion . For example, the anion of sulfuric acid is

HSO 4 -

.18 After the above discussion, it is not too surprising that water can act as an acid and/or a base as

water can be seen as H + and OH - giving H 2 O. Actually, however, water reacts with itself as acid and base in the following fashion:

H 2 O + H 2 O → H 3 O + + OH The double arrow indicates that a reaction can go in both directi ons. The concentrations of both

H 3 O + and OH - in neutral water are very low. In chem ical terminology it is then said that the equilibrium of the reaction is on the left side of the equation. (Also H 2 O + H 2 O would usually

be written as 2 H 2 O.)

The above reaction forms the basis of the important concept of the pH value . The pH value is

defined as the concentration of H 3 O + (hydronium ion) in an aqueous solution.

It is very important to know that the pH of neutral water is 7 (this results from the concentration

of H 3 O + in neutral water. (Question: How does the concentration of OH - in neutral water compare to that of H 3 O + ?). An acidic solution has a pH below 7 while a basic solution has a pH

above 7.

The pH value of an aqueous solution can be c onveniently measured directly in a matter of seconds using a pH meter . There are also strips of paper ( indicator paper ; a simpler version is litmus paper ) that when held into a solution change co lor due to an acid-base reaction with the

indicator on the paper. Comparison of the obtained color with a chart on the box in which the indicator paper is stored then gives the pH.

Thus, an acid can be seen as a compound that produces H 3 O + when dissolved in water beyond

the H 3 O + present in neutral water anyway . Conversely, a base produces OH - when dissolved in water beyond the OH - present in neutral water. (Exercis e: Try writing the above equation for dissolving an acid or base in water. For an ac id use the generic formula HA and for a base A - .)

Another important part of the acid-base chemistry is that of neutralization . Many chemical reactions, including the transester ification reaction that yields biodiesel, require the use of catalysts. Catalysts are compounds that are often presen t in small amounts that cause a reaction to proceed at a faster rate. Often such catalysts can be acids or bases. That means that even when the reaction is over, the reaction solution will still be acidic or basic, depending on the kind of catalyst used (and the other materials present). Then it is usually necessary to neutralize the solution, i.e., the solution needs to attain the ne utral pH. Not surprisingly, a basic solution can be

neutralized with an acid and, vice versa, an acidic solution can be neutralized with a base. In many cases, a reaction of an acid and a base forms a salt , i.e., a product of part of the acid and base (the anion and cation). Actually, common tabl e salt (NaCl) is an exam ple as it can be seen as arising from sodium hydroxide a nd hydrochloric acid: NaOH + HCl → NaCl + H 2 O.

Many other salts are possible. When producing biodiesel under basic (more common) or acidic (less common) conditions, neutralizat ion is required. That neutraliza tion may lead to a salt, that, although a side product of the process, may even be sold. It may be worthy of consideration to choose the neutralizing acid (or base) based not only on its cost but also on the market potential19 of the salt. However, attention is required to possible contamina tion of the glycerol by the salt, which may influence the purification, applicabil ity, and market potentia l of the glycerol. The salts may possess different properties, incl uding retention, rela tive to glycerol.

For application of the acid-base co ncept to the organic chemistry of fats and oils, biodiesel, and the transesterification reaction, it is important to recall that fatty acids ar e acids (organic acids

are often termed carboxylic acids ), although rather weak ones. The important functionality in the fatty acid molecule is -COOH. The proton can dissociate to give -COO - and H + . Thus fatty acids can also form salts, in which the H + is replaced by some cation such as Na + or K + . Such salts of fatty acids form the basis of soaps , as described earlier.

Fatty acids can also form esters, i.e. react with an alcohol, for example, methanol:

O O || ||

R--C- OH + CH 3 O H → R--C- OCH 3 + H 2 O

.

The italicized and bolded parts of the equation i ndicate which parts are exchanged. Actually in the esterification of the acid with methanol, it is OH - that is exchanged for CH 3 O - . Thus, under the right conditions, a carboxylic acid can give off either H + or OH - . It is also possible to

transform one ester into another:

O O || ||

R—C- OCH 3 + C 2 H 5 O H → R--C OC 2 H 5 + CH 3 OH

.

However as most carboxylic acids, including fatt y acids, and alcohols are very weak acids and bases, stronger acids or bases need to be added to their mixture to achieve a reaction. These acids or bases function as the catalysts.

Catalysts

Two of the most commonly used catalysts for transesterification are NaOH and KOH. These catalysts operate by reacting with the alcohol according to the reaction given below (written using methanol and NaOH but other alcohols and catalysts could be substituted).

CH 3 OH + NaOH → CH 3 O-Na + H 2 O (Eq. 1)

Similar to H 2 O “consisting” of H + and OH - , CH 3 O-Na can be seen as consisting of

CH 3 O - (alkoxide; alkylate) and Na + . CH 3 O - is the species that attacks the ester moieties in the glycerol molecule in the following fashion:20 O O || ||

CH 2 -O-C-R CH 2 -O-C-R | | | O O | | || || |

CH 3 -O - + CH-O-C-R → CH 3 -O-C-R + CH-O - (Eq. 2) | | | O | O | || | ||

CH 2 -O-C-R CH 2 -O-C-R

Alkoxide Triglyceride Methyl ester Triglyceride anion

While the ester molecule is complete after this reaction, the anion of the triglyceride needs to pick up a proton to give a stable product (a diglyc eride in this case). If this proton is taken from methanol, then the alkoxide catalyst will be recovered as shown in Eq. 3.

Several other reactions could be written that would allow the anion of the triglyceride to pick up a proton, such as reaction with free fatty acids and water. The presence of water affects the transesterification negatively because the triglyceride anion in Eq. 3 will react

O ||

CH 2 -O-C-R CH 2 -O-C-R | | | | | |

CH-O - + CH 3 OH → CH-OH + CH 3 -O - (Eq. 3) | | | O | | || |

CH 2 -O-C-R CH 2 -O-C-R

with the water to form OH - , which can behave in a fashion similar to CH 3 -O - but a free fatty acid will result instead of a methyl ester.21 O O || ||

CH 2 -O-C-R CH 2 -O-C-R | | | O O | | || || |

OH - + CH-O-C-R → HO-C-R + CH-O - (Eq. 4) | | | O | O | || | ||

CH 2 -O-C-R CH 2 -O-C-R

Hydroxide Triglyceride Free fatty acid Triglyceride anion

The free fatty acid can react with the Na + to form soap. Some water in the system can be tolerated, because R-O - is a stronger base than OH - so the transesterification reaction occurs at a higher rate than the “saponification” of glycerol leading to free fatty acids. The equilibrium of the following reaction (formation of free fatty acids from the ester) is far on the left side of the reaction equation:

O O ||

 ||

OH - + R-O-C-R → R-O - + HO-C-R (Eq. 5)

alkyl ester free fatty acid

As a result of the slow reaction rate, only ve ry minor amounts of free fatty acids are formed during transesterification if the reaction is free of water at the beginning. Another aspect of the weaker basicity of OH - vs. R-O - is that the ester moieties in th e triglyceride molecules will react

less with OH - than R-O - . However, when too much catalyst (or water) is present, Eq. 5 becomes more prevalent and causes enhanced formation of mono- and diglyceride molecules instead of reactions with all positions in the glycerol backb one. Thus, the effect of too much catalyst or too much water leads to the same result, namely , enhanced formation of undesirable mono- and diglycerides as well as free fatty acids. Too mu ch catalyst can lead to soap formation when conditions encourage free fatty acid production.

Because of the possibility of the reactions desc ribed above leading to free fatty acids and monoand diglycerides, direct use of sodium or potassium alkylate (R-ONa or R-OK; the alkylate moiety must correspond to the R- moiety in the alc ohol) as catalysts is becoming of greater

interest. The reaction of alc ohol and XOH given above (Eq. 1) cannot occur in this case (assuming, of course, that the reaction system is free of water). Instead, the transesterification according to Eq. 2 can occur directly.22 

## 3. Biodiesel Specifications and Properties

Introduction

This module will acquaint you with the fuel speci fication that defines and sets the quality

standards for biodiesel. The standard is framed as a set of property specifications measured by specific ASTM test methods. The sta ndard for biodiesel is ASTM 6751-02.

ASTM D 6751 – 02 sets forth the specifications th at must be met for a fatty acid ester product to carry the designation “biodiesel fu el” or “B100”or for use in ble nds with any petroleum-derived diesel fuel defined by ASTM D 975, Grades 1-D, 2-D, and low sulfur 1-D and 2-D.

The instructional goals for this module are: 1. Learn the Specifications for B 100 fuel. 2. Introduce the Methods used to measure the performance parameters for B 100 fuel.

3. Describe the Methods and measurements need ed for a basic quality control labo ratory for a production facility.

Definition of “Biodiesel” Biodiesel is defined as: a fuel comprised of mono -alkyl esters of long chain fatty acids derived from vegetable oils or animal fats, designated B100.

A “mono-alkyl ester” is the product of the reaction of a straight ch ain alcohol, such as methanol or ethanol, with a fat or oil (tri glyceride) to form glycerol (gly cerin) and the esters of long chain fatty acids.

Biodiesel can be used as B 100 (neat) or in a blend with petroleum diesel. A blend of 20 % biodiesel with 80 % petrodiesel, by volume, is te rmed “B 20”. A blend of 2 % biodiesel with 98 % petrodiesel is “ B 2”, and so on.

Property Requirements and Specified Methods for B100

The values of the various biodiesel properties specified by ASTM D 6751 are listed in Table 6. Each of these properties and the test method used to measure it are described below.

Method: ASTM D93- Flash point, closed cup Requirement: 130 oC min

The flash point is defined as the “lowest temperat ure corrected to a barome tric pressure of 101.3 kPa (760 mm Hg), at which application of an ig nition source causes the vapors of a specimen to ignite under specified conditions of test.” This te st, in part, is a measur e of residual alcohol in

the B100.23 Table 6. ASTM D 6751 – 02 Requirements
Property Method Limits Units

Flash point, closed cup D 93 130 min °C

Water and sediment D 2709 0.050 max % volume

Kinematic viscosity, 40 ° C D 445 1.9 – 6.0
mm 2 /s
Sulfated ash D 874 0.020 max wt. %

Total Sulfur D 5453 0.05 max wt. %

Copper strip corrosion D 130 No. 3 max

Cetane number D 613 47 min

Cloud point D 2500 Report to customer °C

Carbon residue D 4530 0.050 max wt. %

Acid number D 664 0.80 max mg KOH/g

Free glycerin D 6584 0.020 wt. %

Total glycerin D 6584 0.240 wt. %

Phosphorus D 4951 0.0010 wt. %

Vacuum distillation end poi nt D 1160 360 °C max, at

90% distilled oC

Storage stability To be

determined To be determined To be

determined

The flash point is a determinant for flammability classification of materials. The typical flash point of pure methyl esters is > 200 ° C, classi fying them as “non-flammable”. However, during production and purification of biodi esel, not all the methanol may be removed, making the fuel flammable and more dangerous to handle and store if the flash point falls below 130oC. Excess methanol in the fuel may also affect engine se als and elastomers and corrode metal components. Generally, a production Quality Control (QC) labor atory should include a flash point apparatus for quality control and as a means of detecting excess alcohol levels.

Method: ASTM D 2709 – Water and sediment Requirement: 0.050 % volume max

Water and sediment is a test that “determines the volume of free water and sediment in middle distillate fuels having viscosities at 40 °C in the range 1.0 to 4.1 mm 2 /s and densities in the range of 700 to 900 kg/m 3 .”

This test is a measure of cleanliness of the fu el. For B100 it is particularly important because water can react with the esters, making free fa tty acids, and can support microbial growth in storage tanks. Water is usually kept out of the production process by removing it from the feedstocks. However, some water may be form ed during the process by the reaction of the sodium or potassium hydroxide catalyst with alcohol . If free fatty acids are present, water will be formed when they react to either biodiesel or soap. Finally, water is deliberately added during the24 washing process to remove contaminants from the biodiesel. This washing process should be followed by a drying process to ensure the final product will meet ASTM D 2709.

Sediments may plug fuel filters and may contribute to the formation of deposits on fuel injectors and other engine damage. Sediment levels in biodiesel may increase over time as the fuel degrades during extended storage. The production QC lab should be equipped to perform this

test on a routine basis.

Method: ASTM D 445 – Kinematic viscosity, 40 o C Requirement: 1.9 – 6.0 mm 2 /s

Kinematic viscosity: “the resistan ce to flow of a fluid under gravit y”. [The kinematic viscosity is equal to the dynamic viscosity/density] The kine matic viscosity is a basic design specification for the fuel injectors used in diesel engine s. Too high a viscosity, and the injectors do not perform properly.

Dynamic viscosity – ‘ratio betw een applied shear stress and rate of shear of a liquid.”

Density – “the mass per unit volume of a substance at a given temperature.”

The viscosity of biodiesel can be predicted ± 15 % using the esters composition determined using ASTM D 6584. The viscosity apparatus to run D 445 is not critical to the QC lab, but it is valuable as a quick assay method for estimating the degree of completion for a reaction batch.

Method: ASTM D 874 – Sulfated Ash Requirement: 0.020 wt. %, max

Sulfated ash is the “... residue remaining afte r a [fuel] sample has been carbonized, and the residue subsequently treated with sulfuric acid an d heated to a constant weight.” This test

monitors the mineral ash residual when a fuel is burned.

For biodiesel, this test is an important indicator of the quantity of residual metals in the fuel that came from the catalyst used in the esterification process. Producers that use a base catalyzed process may wish to run this test regularly. Ma ny of these spent sodium or potassium salts have low melting temperatures and may cause e ngine damage in combustion chambers.

Method: ASTM D5453 – Total sulfur Requirement: 0.05 wt. %, max

“This method covers the determination of total su lfur in liquid hydrocarbons, boiling in the range from approximately 25 to 400 ° C, with viscosities between approximately 0.2 and 20 cSt (mm 2 /s) at room temperature.”

Biodiesel feedstocks typically have very little sulfur, but this test is an indicator of contamination of protein material and/or car ryover catalyst material or ne utralization material from the production process. Producers using rendered or waste feedstocks should have access to this

measurement for their feedstocks. Some biodiesel s from rendered fats and greases have been25 found to have sulfur levels of 40-50 ppm. The su lfur limits for on-highway diesel fuel will be reduced to 0.0015% (15 ppm) in 2006, so producers that do not meet this future specification on a regular basis may need to consider sulf ur removal technology in their facility.

Method: ASTM D 130 – Copper strip corrosion Requirement: No. 3, max

The copper strip corrosion is used “ [for the] detection of the corrosiveness to copper of fuels and solvents.” This test monitors the presence of acids in the fuel.

For B 100, the most likely source of a test failure would be excessive free fatty acids, which are determined in accordance with an additional specification. The producer may choose to run this test periodically, but the acid num ber (D 664) determination is the more important acid content QC measurement.

Method: ASTM D 613 – Cetane number Requirement: 47, min

The cetane number is “a measure of the igniti on performance of a diesel fuel obtained by comparing it to reference fuels in a standardized engine test.” Cetane for diesel engines is analogous to the octane rating in a spark ignition e ngine – it is a measure of how easily the fuel will ignite in the engine.

For B100, the cetane number is seldom an issue because all of the common fatty acid esters have

cetane numbers near or above 47. The cetane numb er can be predicted ± 10 % using the esters composition. It is unlikely that an individual prod ucer will ever run cetane tests on-site because the equipment is extremely expensive.

Method: ASTM D 2500 – Cloud point Requirement: Report in oC to customer

The cloud point is “The temperature at which a cloud of wax crystals fi rst appears in a liquid

when it is cooled down under condi tions prescribed in this test method.” The cloud point is a critical factor in cold weather performance for all diesel fuels.

The chemical composition of some biodiesel feed stocks leads to a B100 that may have higher cloud points than customers desire. The cloud point , however, is another parameter that can be predicted ± 5 % with knowledge of the esters composition, but producers are advised to be equipped to perform this test. Since the saturated methyl esters are the first to precipitate, the amounts of these esters, methyl palmitate and met hyl stearate, are the dete rmining factors for the cloud point.

The producer can modify the cloud point two ways. One is through the use of additives that

retard the formation of solid crystals in the B100 by various mechanisms. The cloud point can also be modified by blending feedstocks that ar e relatively high in saturated fatty acids with

feedstocks that have a lower satu rated fatty acid content. The result is a net lower cloud point for26 the mixture. See the section on pr ediction of physical properties for a more complete discussion of the effect of composition on cloud point.

Method: ASTM D 4530 – Carbon residue Requirement: 0.050 wt%, max

“In petroleum products, the part remaining afte r a sample has been subjected to thermal decomposition...” is the carbon residue. The carbon residue is a measure of how much residual

carbon remains after combustion. The test basi cally involves heating the fuel to a high temperature in the absence of oxygen. Most of th e fuel will vaporize and be driven off, but a portion may decompose and pyrolyze to hard carbonaceous deposits. This is particularly important in diesel engines because of the possibility of carbon residues clogging the fuel injectors.

The most common cause of excess carbon residue s in B 100 is an excessive level of total glycerin. Total glycerin is also measured dir ectly using ASTM D 6584, so this measurement is generally not critical to the producer.

Method: ASTM D 664 – Acid number Requirement: 0.80 mg KOH/g, max

The acid number is “The quantity of base, expr essed as milligrams of potassium hydroxide per gram of sample, required to titrate a sample to a specified end point.” The acid number is a direct measure of free fatty acids in B100. The fr ee fatty acids can lead to corrosion and may be a symptom of water in the fuel. Usually, for a base catalyzed process, the acid value after production will be low since the base catalyst will strip the available free fatty acids. However, the acid value may increase with time as the fuel degrades due to contact with air or water. This test should be performed regularly as a part of the producer QC program.

Method: ASTM D 6584 – Free glycerine Requirement: 0.020 wt. %

Free glycerol is the glycerol present as molecular glycerol in the fuel. Free glycerol results from incomplete separation of the ester and glycerol products after the transesterification reaction. This can be a result of imperfect water washi ng or other approaches that do not effectively separate the glycerol from the biodiesel. The free glycerol can be a source of carbon deposits in the engine because of incomplete combustion. The terms “glycerine,” “glycerin,” and “glycerol” are used interchangeably.

Method: ASTM D 6584 – Total glycerine Requirement: 0.240 wt. %

Total glycerol “is the sum of free and bonded glycerol.” Bonded glycerol “is the glycerol portion of the mono-, di-, and triglyceride mo lecules.” Elevated total glycerol values are indicators of incomplete esterification reactions and predictors of excessive carbon deposits in the engine.27 The ASTM D 6584 test for free and total glyc erol must be done routinely as the key measurement in the producer’s QC program.

Method: ASTM D 4951 – Phosphorus Requirement: 0.0010 wt. %

“This test covers the quantit ative determination of barium, calcium, copper, magnesium, phosphorus, sulfur, and zinc in unused lubricating o ils and additive packages.” In the case of B100, phosphorus can come from incomplete refi ning of the phospholipids (or gums) from the vegetable oil and from bone and proteins enc ountered in the rendering process. The producer should have access to this method for periodic measurements.

Method: ASTM D 1160 – Vacuum distillation end point Requirement: 360 oC max, at 90 % liquid distilled

The vacuum distillation end-point test “covers the determination, at reduced pressures, of the range of boiling points for petroleu m products that can be partially or completely vaporized at a maximum liquid temperature of 400° C.”

Petroleum fractions have tens to hundreds of individual compounds mixed together. The

distillation curves are used to characterize the broad chemistry of a given crude oil source in terms of the boiling temperatures of its constituent compounds.

In B100 there are, at most, ten different esters present, and they can be identified using gas or liquid chromatography. The same chromatograph that determines free and total glycerin can determine the esters composition in the B100. Th is composition allows calculation of the T-90 point for the fuel, without having to perform the te sting for every batch of product. It is unlikely that the producer would have a reason to run this test except to certify compliance with the

ASTM standard.

Storage stability

– Method to be determined

All fuels are subject to degradation over time wh en they are stored. This degradation may be due to microbial action, water intrus ion, air oxidation, etc. The sta ndard and the test methods for determining storage stability for B100 are stil l in the development stage within the ASTM process. In general, the following parameters will change and can be used to determine if the fuel should not be used: acid number, water and sedime nt, and viscosity. Should an aged fuel fail any of these three standards, it should not be used.

Sources for ASTM Standards

Complete copies for the ASTM Standards and Methods referenced above are available from ASTM by mail, fax order, or on-line purchase fo r $35.00 per standard. Mo st universities that have an engineering college, and many larger li braries, include ASTM Standards in hard copy

form in their reference collection.

The Standards and Methods are very detailed and some are quite complex. A number of the Methods include reference to additional Methods, so the overall library needed for a QC lab is28 rather large. The methods are revised occasionally, so it is prudent to ensure that your lab has the most up-to-date revisions annually.

QC Laboratory Recommendations

The QC Laboratory is an essential element of any biodiesel production facility. An individual trained in chemistry through the level of orga nic chemistry, analytical chemistry, and instrumental analysis is recommended for the QC chemist function. Most community colleges offer training in an Associates Degree program th at will meet these criteria. Some of the methods used are quite specialized, so training in the co nduct of the methods is also recommended. Table 7 shows a listing of the specific test methods that are recommended for use in the plant laboratory.

Summary

ASTM Specification D 6751 – 02, specification fo r B 100 (biodiesel) fuel is the basis for ensuring that quality products ar e provided to the fuel distributi on system. The key properties of

B 100 are discussed in terms of thei r Test Methods and specifications.

Essential testing capabilities for the producer include:

• ASTM D 2709, Water and Sediment;

• ASTM D 874, Sulfated Ash;

• ASTM D 2500, Cloud Point;

• ASTM D 664, Acid Number, and;

• ASTM D 6584, Free Glycerine and Total Glycerine29 Table 7. QC Laboratory Recommended Equipment: Needs and Costs

ASTM Method Apparatus Approx.

Cost

D93 – Flash Point Hertzog MP 329 Automatic Pensky-Martens Tester $ 10,300 Koehler Pensky-Martens Flash Cup Tester $ 2,200 D130 - Corrosion Koehler K25330 Copper Strip Test Bath $ 3,700 Standards: ASTM - $ 195; Koehler - $ 466 D445 - Viscosity Koehler AKV9500 Automated Kinematic Viscosity System $ 22,500 Polystat Constant Temperature Bath (Cole – Parmer) $ 2,900 Viscometer Cleaning and Drying Apparatus (Koehler) $ 3,400 Viscometers, various ranges $ 120 ea

D664 – Acid Value KEM AT-150 Automatic Potentiometric Titrator $ 7,000

D874 – Sulfated

Ash Koehler Isotemp Basic Muffle Furnace, 0.58 cu.ft. $ 2,500

D2500 – Cloud

Point Koehler Cloud and Pour Point Bath $ 5,500

Koehler Automated Cloud and Pour Point System $ 21,000

D2709 – Water and

Sediment PAC 67310 Benchtop Centrifuge $ 6,600

D6584 – Total and Free Glycerol Shimadzu GC-17A Gas Chromatograph $ 18,000

Total Cost Recommended Apparatus $ 76,10030 

## 4. Types of Biodiesel Production Processes

Types of Biodiesel Production Processes

Introduction

This module provides an overview of the steps in the production of biodiesel from preparation of the feedstock to the recovery a nd purification of the fatty acid esters (biodiesel) and the coproduct glycerol (also called glycer in). We will review several chem istries used for esterification and different approaches to produc t preparation and purification.

The emphasis throughout the module will be the c hoices to be made in matching feedstock selection, capacity, and operating mode with the selection of the basic process chemistry and layout for a specific location. While no specific pro cess technology is favored in this description, an effort has been made to describe the majo r approaches currently in use in the industry.

Feedstocks Used in Biodiesel Production

The primary raw materials used in the production of biodiesel are vegetable oils, animal fats, and recycled greases. These materials contain triglyce rides, free fatty acids, and other contaminants depending on the degree of pretreatment they have received prior to delivery. Since biodiesel is a mono-alkyl fatty acid ester, the primary alcohol used to form the ester is the other major

feedstock.

Most processes for making biodiesel use a catalyst to initiate the esterification reaction. The catalyst is required because the alcohol is sparingly soluble in the oil phase. The catalyst promotes an increase in solubility to allow the reaction to proceed at a reasonable rate. The most common catalysts used are strong mineral base s such as sodium hydroxide and potassium hydroxide. After the reaction, the base catalyst must be neutrali zed with a strong mineral acid.

Typical proportions for the chemical s used to make biodiesel are:

Reactants •Fat or oil (e.g. 100 kg soybean oil) •Primary alcohol (e.g. 10 kg methanol) Catalyst •Mineral base (e.g. 0.3 kg sodium hydroxide) Neutralizer •Mineral acid (e.g. 0.25 kg sulfuric acid)

Fats and Oils: Choice of the fats or oils to be used in producing biodiesel is both a process chemistry decision and an economic decision. With respect to process chemistry, the greatest difference among the choices of fats and oils is th e amount of free fatty acids that are associated with the triglycerides. Other contaminants, such as color and odor bodies can reduce the value of the glycerin produced, and reduce th e public acceptance of the fuel if the color and odor persist

in the fuel.

Most vegetable oils have a low percentage of associated free fatty acids. Crude vegetable oils contain some free fatty acids and phospho lipids. The phospholipids are removed in a31 “degumming” step and the free fatty acids are rem oved in a “refining” step. Oil can be purchased as crude, degummed, or refined. The selecti on of the type of oil affects the production technology that is required.

Animal tallows and recycled (yellow) grease have much higher levels of free fatty acids. Yellow grease is limited to 15% free fatty acids and is a traded commodity that is typically processed into animal and pet food. Specifications fo r yellow grease are described in the chapter on Feedstock Preparation. Trap greases come from traps under kitchen drains and these greases can contain between 50 and 100% free fatty acids. There is no market for these greases at this time and most are landfilled. Trap grea se is not yet used for biodies el production and may have some technical challenges that have not be fully resolved such as difficult to break emulsifications (gels), fine silt that will cause equipment wear , high water contents, and very strong color and

odor bodies that affect biodiesel and glycerin products. There are also unresolved questions about small quantities of other contaminants such as pesticides that might be present in the fuel.

The options for the triglyceride choice are many. Among the vegetable oils sources are soybean, canola, palm, and rape. Animal fats are product s of rendering operations. They include beef tallow, lard, poultry fat, and fish oils. Yellow gr eases can be mixtures of vegetable and animal sources. There are other less desirable, but also le ss expensive triglyceride sources such as brown grease and soapstock. The free fatty acid content a ffects the type of biodi esel process used, and the yield of fuel from that process. The other contaminants present can affect the extent of feedstock preparation necessary to use a given reaction chemistry.

Alcohol: The most commonly used primary alcohol us ed in biodiesel production is methanol, although other alcohols, such as ethanol, isoprop anol, and butyl, can be used. A key quality factor for the primary alcohol is the water content. Water interf eres with transesterification reactions and can result in poor yi elds and high levels of soap, fr ee fatty acids, and triglycerides in the final fuel. Unfortunately, all the lowe r alcohols are hygroscopic and are capable of absorbing water from the air.

Many alcohols have been used to make biodies el. As long as the product esters meet ASTM 6751, it does not make any chemical difference which alcohol is used in the process. Other issues such as cost of the alcohol, the amount of alc ohol needed for the reacti on, the ease of recovering and recycling the alcohol, fuel tax credits, a nd global warming issues influence the choice of alcohol. Some alcohols also requir e slight technical modifications to the production process such as higher operating temperatures, longer or slower mixing times, or lower mixing speeds.

Since the reaction to form the esters is on a molar basis and we purchase alcohol on a volume basis, their properties make a significant difference in raw material price. It takes three moles of alcohol to react completely with one mole of triglyceride. Toda y, one gallon of methanol costs $0.61. That gallon contains 93.56 gram-moles ofmethanol; at a cost of $ 0.00652 per grammole. By contrast, a gallon of ethanol, at the cu rrent price of $ 1.45 per gallon for fuel-grade ethanol, costs $ 0.02237 per gram -mole, or 3.4 times more.

In addition, a base catalyzed pr ocess typically uses an operatin g mole ratio of 6:1 mole of alcohol rather than the 3:1 ratio required by th e reaction. The reason for using extra alcohol is32 that it “drives” the reacti on closer to the 99.7% yield we need to meet the total glycerol standard for fuel grade biodiesel. The unused alcohol must be recovered and recycled back into the process to minimize operating costs and environmen tal impacts. Methanol is considerably easier

to recover than the ethanol. Ethanol forms an azeotrope with water so it is expensive to purify the ethanol during recovery. If the water is not removed it will interfere with the reactions. Methanol recycles easier because it doesn’t form an azeotrope.

These two factors are the reason th at even though methanol is more toxic, it is the preferred alcohol for producing biodiesel. Methanol has a flash point of 10 °C, while the flash point of ethanol is 8°C, so both are considered highly flammable. You should never let methanol come into contact with your skin or eyes as it can be readily absorbed. Excessive exposure to methanol

can cause blindness and other health effects. For student demonstrations, ethanol may be safer to

use.

Methanol does have a somewhat variable prici ng structure. When the production of MTBE was

mandated for the reduction of emissions from gasoline engines in the winter, there was a significant expansion in world capacity for the material. The excess capacity and crash in demand led to methanol prices of $ 0.31 per gallon in early 2002. However, in late July, 2002, the production/consumption levels regained equilib rium and the methanol price doubled back to the more typical value of $ 0.60± per gallon.

The alcohol quality requirements are that it be un-denatured and anhydrous. Since chemical grade ethanol is typically denatured with poison ous material to prevent its abuse, finding undenatured ethanol is difficult. Purchase ethano l that has been denatu red with methanol if possible.

Catalysts and Neutralizers: Catalysts may either be base, acid, or enzyme materials. The most commonly used catalyst materials for converting triglycerides to biodi esel are sodium hydroxide, potassium hydroxide,

and sodium methoxide. Most base catalyst systems use vegetable oils as a feedstock. If the vegetable oil is crude, it contains small amounts (<2%) of free fatty acids that will form soaps that will end up in the crude glycerin. Refined f eedstocks, such as refined soy oil can also be used with base catalysts.

The base catalysts are highly hygr oscopic and they form chemical water when dissolved in the alcohol reactant. They also absorb water from th e air during storage. If too much water has been adsorbed the catalyst will perform poorly and the biodiesel may not meet the total glycerin

standard.

Although acid catalysts can be used for transesterification they are generally considered to be too slow for industrial processing. Ac id catalysts are more commonly used for the esterification of free fatty acids. Acid catalysts include sulfuric acid and phosphoric acid. Solid calcium carbonate is used as an acid catalyst in one experimental homogeneous catalyst process. The acid catalyst is

mixed with methanol and then this mixture is a dded to the free fatty acids or a feedstock that contains high levels of free fatty acids. The fr ee fatty acids convert into biodiesel. The acids will need neutralization when this process is complete, but this can be done as base catalyst is added to convert any remaining triglycerides.33 There is continuing interest in using lipases as enzymatic catal ysts for the production of alkyl fatty acid esters. Some enzymes work on the triglyceride, converting them to methyl esters; and some work on the fatty acids. The commercial us e of enzymes is currently limited to countries like Japan, where energy costs are high, or fo r the production of speci alty chemicals from specific types of fatty acids. The commercial us e of enzymes is limited because costs are high, the rate of reaction is slow, and yields to methyl esters are typically less than the 99.7% required for fuel-grade biodiesel. Enzymes are being consider ed for fatty acid conversion to biodiesel as a pretreatment step, but this system is not commercial at this time.

Neutralizers are used to remove the base or acid catalyst from the product biodiesel and glycerol. If you are using a base catalyst, the neutralizer is t ypically an acid, and visa versa. If the biodiesel is being washed, the neutralizer can be added to the wash water. While hydrochloric acid is a common choice to neutralize base catalyst s, as mentioned earlier, if phosphoric acid is used, the resulting salt has value as a chemical fertilizer.

Catalyst Selection: Base catalysts are used for essentially all vegetable oil processing plants. The initial free fatty acid content and the water cont ent are generally low. Tallows and greases with free fatty acid contents greater than about 1% mu st be pretreated to either remove the FFA or

convert the FFA to esters before beginning the base catalyzed r eaction. Otherwise, the base catalyst will react with the free fatty acids to form soap and water. The soap formation reaction is very fast and goes to completion be fore any esterification begins.

Essentially all of the cu rrent commercial biodiesel producers use base catalyzed reactions. Base catalyzed reactions are relatively fast, with residence times from about 5 minutes to about 1 hour, depending on temperature, concentration, mixing and alcohol:triglyceride ratio. Most use NaOH or KOH as catalysts, although glycerol refiners prefer NaOH. KOH has a higher cost but the potassium can be precipitated as K 3 PO 4 , a fertilizer, when the pr oducts are neutralized using phosphoric acid. This can make meeting water effl uent standards a bit more difficult because of limits on phosphate effluents.

Sodium methoxide, usually as a 25 % solution in methanol, is a more powerful catalyst on a weight basis than the mixture of NaOH and methanol. This appears to be, in part, the result of the negative effect of the chemical water produced in situ when NaOH and methanol react to

form sodium methoxide.

Acid catalyst systems are characterized by slow reaction rates and high alcohol:TG requirements (20:1 and more). Generally, acid ca talyzed reactions are used to convert FFAs to esters, or soaps to esters as a pretreatment step for high FFA feedstocks. Residence times from 10 minutes to about 2 hours are reported.

Counter current acid esterification systems have b een used for decades to convert pure streams of fatty acids into methyl esters at yields above 99%. These systems tend to force yields to 100% and wash water out of the system at the same time because the feedstock and the sulfuric acid/methanol mix are moving in opposite dire ctions. Acid esterification systems produce a byproduct of water. In batch systems, the water te nds to accumulate in the vessel to the point where it can shut the reaction down prematurely. The sulfuric acid tends to migrate into the34 water, out of the methanol, rendering it unavaila ble for the reaction. All acid esterification systems need to have a water management strategy. Good water management can minimize the amount of methanol required for the reaction. Excess methanol (such as the 20:1 ratio) is generally necessary in batch react ors where water accumulates. Another approach is to approach the reaction in two stages: fresh methanol and su lfuric acid is reacted, removed, and replaced

with more fresh reactant. Much of the water is removed in the first round and the fresh reactant

in the second round drives the r eaction closer to completion. Ac id-catalyzed esterification is discussed in more detail in the chapte r on Pretreatment of High FFA Feedstocks.

Lipase catalyzed reactions have the advantag e reacting a room temper ature without producing spent catalysts. The enzymes can be recycled for use again or immobilized onto a substrate. If immobilized, the substrate will require replacement wh en yields begin to decline. The enzyme reactions are highly specific. Because the alcohol can be inhibitory to some enzymes, a typical strategy is to feed the alcohol in to the reactor in three steps of 1:1 mole ratio each. The reactions are very slow, with a three step sequence requ iring from 4 to 40 hours, or more. The reaction conditions are modest, from 35 to 45 °C. Transest erification yields generally do not meet ASTM standards, but esterification yields can occur re latively quickly and yi elds are good. Excess free fatty acids can be removed as soaps in a later transesterification or caustic stripping step.

Biodiesel Production Process Options

Batch Processing The simplest method for producing alcohol esters is to use a batch, stir red tank reactor. Alcohol to triglyceride ratios from 4:1 to 20:1 (mole:mo le) have been reported, with a 6:1 ratio most common. The reactor may be sealed or equi pped with a reflux condenser. The operating temperature is usually about 65°C, although temperat ures from 25°C to 85°C have been reported.

The most commonly used catalyst is sodium hydroxide, with potassium hydroxide also used. Typical catalyst loadings range from 0.3 % to about 1.5%.

Thorough mixing is necessary at the beginning of the reaction to bring th e oil, catalyst and

alcohol into intimate contact. Towards the end of the reaction, less mixing can help increase the extent of reaction by allowing the in hibitory product, glycerol, to phase separate from the ester – oil phase. Completions of 85% to 94 % are reported.

Some groups use a two-step reaction, with glycer ol removal between steps, to increase the final reaction extent to 95+ percent. Higher temper atures and higher alcohol:oil ratios also can enhance the percent completion. Typical reaction times range from 20 minutes to more than one

hour.

Figure 14 shows a process flow diag ram for a typical batch system. The oil is first charged to the system, followed by the catalyst and methanol. Th e system is agitated during the reaction time. Then agitation is stopped. In some processes, th e reaction mixture is allo wed to settle in the reactor to give an initial separation of the este rs and glycerol. In othe r processes the reaction mixture is pumped into a settling vesse l, or is separated using a centrifuge.35 The alcohol is removed from both the glycerol and ester stream using an evaporator or a flash unit. The esters are neutralized, washed gently using warm, slightly acid water to remove residual methanol and salts, and then dried. The finished biodiesel is then transferred to storage. The glycerol stream is neutralized and washed with soft water. The glycerol is than sent to the glycerol refining section.

Figure 14. Batch Reaction Process

For yellow grease and animal fats, the system is slightly modified with the addition of an acid esterification vessel and storage for the acid cataly st. The feedstock is sometimes dried (down to 0.4% water) and filtered before loading the acid esterification tank. The sulfuric acid and methanol mixture is added and the system is agita ted. Similar temperatures to transesterification are used and sometimes the system is pressurized or a cosolvent is added. Glycerol is not produced. If a two-step acid treatment is used, the stirring is suspended until the methanol phase separates and is removed. Fresh methanol and su lfuric acid is added and the stirring resumes. Once the conversion of the fatty acids to me thyl esters has reached equilibrium, the methanol/water/acid mixture is removed by settli ng or with a centrifuge. The remaining mixture is neutralized or sent straight into transester ification where it will be neutralized using excess base catalysts. Any remaining free fatty ac ids will be converted into soaps in the transesterification stage. The tr ansesterification batch stage processes as described above.

Continuous Process Systems A popular variation of the batch process is the use of continuous stirred tank reactors (CSTRs) in series. The CSTRs can be varied in volume to allow for a longer residence time in CSTR 1 to achieve a greater extent of reaction. After the initial product glycerol is decanted, the reacti on in CSTR 2 is rather rapid, with 98+ completion not uncommon.

An essential element in the design of a CSTR is sufficient mixing input to ensure that the composition throughout the reactor is essentially c onstant. This has the effect of increasing the dispersion of the glycerol product in the ester phase. The result is that the time required for phase separation is extended. Ester

TG

Alcohol Catalyst Ester Biodiesel

Batch Reactor Alcohol Water Water

Dryer

Wash Water

Water

Crude Glycerol Alcohol36 There are several processes that use intense mixi ng, either from pumps or motionless mixers, to

initiate the esterification reaction. Instead of a llowing time for the reaction in an agitated tank,

the reactor is tubular. The reaction mixture move s through this type of reactor in a continuous plug, with little mixing in the axial direction. Th is type of reactor, called a plug-flow reactor (PFR), behaves as if it were a seri es of small CSTRs chained together.

The result is a continuous system that requires rather short residence times, as low as 6 to 10 minutes, for near completion of the reaction. The PFRs can be staged, as shown, to allow decanting of glycerol. Often this type of reactor is operated at an elevated temperature and pressure to increase reaction rate. A PFR system is shown in Figure 15.

Figure 15. Plug Flow Reaction System

High Free Fatty Acid Systems

High free fatty acid feedstocks will react with th e catalyst and form soaps if they are fed to a base catalyzed system. The maximum amount of fr ee fatty acids acceptable in a base catalyzed system is less than 2 percent, and preferably le ss than 1 percent. Some approaches to using high free fatty acid feedstocks use this concept to “refine” the free fatty acids out of the feed for disposal or separate treatment in an acid esterifi cation unit. The caustic is added to the feedstock and the resulting soaps are stripped out using a centrifuge. This is ca lled caustic stripping.

Some triglycerides are lost with the soaps durin g caustic stripping. The soap mixture can be acidulated to recover the fatty acid s and lost oils in a separate reaction tank. The refined oils are dried and sent to the transesterification unit for further processing. Rather than waste the free fatty acids removed in this manner, they can be transformed into methyl esters using an acid esterification process. As descri bed earlier, acid catalyzed processe s can be used for the direct esterification of free fatty acids in a high FFA fe edstock. Less expensive feedstocks, such as tallow or yellow grease, are char acteristically high in free fatt y acids (FFA). The standard for tallow and yellow grease is ≤ 15 percent FFA. Some lots may exceed this standard.

Direct acid esterification of a hi gh free fatty acid feed requires water removal during the reaction, or the reaction will be quenched prematurely. Al so, a high alcohol to FFA ratio required, usually TG

Ester TG

Alcohol

Catalyst Heater Reactor 1 Alcohol

Separator Reactor 2 Alcohol

Glycerol Ester

Alcohol

Glycerol37 between 20:1 and 40:1. Direct es terification may also require ra ther large amounts of the acid catalyst depending on the process used.

The esterification reaction of FFAs with methanol produces byproduct water that must be removed, but the resulting mixture of esters and triglyceride, can be used directly in a conventional base catalyzed system. The water can be removed by vaporization, settling, or centrifugation as a methanol-water mixture. C ounter-current continuous-flow systems will wash out the water with the exiting stream of acidic methanol.

One approach to the acid catalyst system has been to use phosphoric acid as the initial catalyst,

neutralize with an excess of KOH for the base step, and then neutraliz e with phosphoric acid upon completion. The insoluble potassium phosphate is recovered, washed, and dried for use as a fertilizer. Figure 16 shows a typical acid ca talyzed, direct esterification process.

An alternative approach to uti lization of high FFA feedstocks is to use a base catalyst to deliberately form soap from the FFAs. The soap is recovered, the oil dried, and then used in a conventional base catalyzed system. This strategy can lead to a false sense of econom y. If the soapstock is discarded, the effective price of the feedstock is increase d in inverse proportion to the pe rcentage of remaining oil. The soapstock can, however, be converted into este rs by using an acid ca talyzed reaction. The problem with this strategy is that the soapstoc k system contains a large amount of water that must be removed before the product esters can meet the biodiesel standard. The soapstock process is shown in Figure 17.

Base

Hi-FFA TG

Esters

Acid TG+esters

Alcohol Acid Reactor Neutralize Separate Dryer Water

Figure 16. Acid Catalyzed Direct Esterification Process

An alternative procedure for processing high FFA feeds is to hydrolyze the feedstock into pure FFA and glycerine. Typically this is done in a counter current reactor using sulfuric/sulfonic acids and steam. The output is pure free fatty acids and glycerin. Any contaminates in the feedstock partition mostly into the glycerin and some may leave with the steam/water effluent.

Some contaminants continue with the FFA a nd can be removed or left in, depending on the processes and product specifications . The pure FFA are then acid es terified in another counter current reactor to transform them into methyl es ters. The methyl esters are then neutralized and38 dried. Yields can exceed 99%. Equipment needs to be acid resistant but ge nerally feedstock costs are extremely low.

Figure 17. Preparation of Soap stock from a High FFA Feed A variation of the base-catalyzed system that avoids the problem of high FFAs is the use of a fixed bed, insoluble base. An example of this system, using calcium carbonate as the catalyst,

has been demonstrated at the bench-scale. This process is depicted in Figure 18.

TG

Alcohol Heater

Ester

Glycerol

Figure 18. Fixed-Bed, Base Catalyzed Reactor System

Non-Catalyzed Systems – Biox Process

Cosolvent options are designed to overcome sl ow reaction time caused by the extremely low solubility of the alcohol in the TG phase. One ap proach that is nearing commercialization is the Biox Process. This process uses a co-solvent, te trahydrofuran, to solubilize the methanol. The result is a fast reaction, on th e order of 5 to 10 minutes, and no catalyst residues in either the ester or the glycerol phase. The THF co-solvent is chosen, in part, because it has a boiling point very close to that of methanol. After the r eaction is complete, the excess methanol and the tetrahydrofuran co-solvent are r ecovered in a single step. This system requires a rather low operating temperature, 30 °C. Other cosolvents , such as MTBE, have been investigated. Lo-FFA

TG Hi-FFA TG

Na OH Soap Reactor Separator Dryer

Soap Water

TG Feed39 The ester-glycerol phase separation is clean and th e final products are ca talyst- and water-free. The equipment volume has to be larger for the same quantity of final product because of the

additional volume of the co-solvent. The Biox Pro cess is depicted in Figure 19. Cosolvents that are subject to the hazardous and/or air toxic EPA list for air pollutants require special “leak proof” equipment the entire sytem including meth anol/cosolvent recovery and recycling. Fugitive emissions are tightly controled. The co solvent must be completely removed from the glycerin as well as the biodiesel.

Alcohol, Co-solvent

TG

Alcohol

Co-solvent Reactor Ester

Glycerol

Figure 19. Biox Co-Solvent Process

Non-Catalyzed Systems – Supercritical Process

When a fluid or gas is subjected to temperatures and pressures in excess of its critical point, there are a number of unusual properties exhibited. Ther e no longer is a distinct liquid and vapor phase, but a single, fluid phase present. Solvents containing a hydroxyl (OH) group, such as water or primary alcohols, take on the properties of super-acids.

A non-catalytic approach is the use of a high ( 42:1) alcohol to oil rati o. Under supercritical conditions (350 to 400 °C and > 80 atm or 1200 psi) the reaction is complete in about 4 minutes. Capital and operating costs can be more e xpensive, and energy consumption higher.

An intriguing example of this process has been demonstrated in Japan, where oils in a very large excess of methanol have been subjected to ve ry high temperatures and pressures for a short period of time. The result is a very fast (3 to 5 minute) reaction to form esters and glycerol. The reaction must be quenched very rapidly so that the products do not decompose. The reactor used in the work to date is a 5 ml cylinder that is dropped into a bath of molten metal, and then quenched in water.

Clearly, while the results are very interesting, the scale-up to a useful process may be quite difficult. Figure 20 depicts one conception of a configuration for a supercritical esterification

process.40 TG

Alcohol Supercritical Reactor Methanol Esters

Glycerol

Figure 20. Supercritical esterification process

The refining of glycerol is di scussed in a separate chapter, although it is a topic of great importance to the choice of biodiesel production pr ocess. Generally the quality of the glycerol produced and its value as a coproduct is an impor tant economic variable. The typical glycerol produced by a biodiesel plant is 50% glycerol or less and contains water, salts, methanol, methyl esters, and unreacted glycerides, color and odor bodies, dimers, timers, and other minor compounds. This is commonly referred to as “biodi esel crude” and is generally worth less than 5 cents per pound. Removing the water and methanol and bringing the glycer ol content up to 88% can generate a credit based on the va lue of crude glycerol. Salts, a nd particularly sulfur salts or potassium salts reduces the value of the crude glycerol produced.

Summary There are multiple operating options available for making biodiesel. Many of these technologies

can be combined under various conditions and feed stocks in an infinite number of ways. The technology choice is a function of desired capacity, feedstock type and quality, alcohol recovery, and catalyst recovery. The dominant factor in bi odiesel production is the feedstock cost, with capital cost contributing only about 7 % of the final product cost.

However, some reaction systems are capable of ha ndling a variety of feedstocks and qualities, while others are not. Also, the di fferent approaches to the esterification process result in quite different operating requirements, different wa ter use requirements, and different operating

modes.

In general, smaller capacity plants and variable feedstock quality suggest use of batch systems. Continuous systems generally lead the operation on a 24/7 basis, requiring larger capacities to justify larger staffing requirements and re quire a more uniform feedstock quality.41 

## Laboratory - Exercise 1

Investigations of Biodiesel Production Pitfalls

Equipment needed: Four 500 ml Erlenmeyer flasks containing 200 g of soybean oil Four 125 ml Erlenmeyer flasks, 3 containing 43 g of methanol, one containing 21.5 g of

methanol Four hot plate/stirrers Four separatory funne ls with ringstands Aluminum foil (for capping flasks) Small bottles containing preweighed sodium hydroxide, three with 2 g, one with 10 g.

Group 1: Too much water (How mu ch harm can a little water do?)

• Starting with 200 g of soybean oil in a 500 ml flask. Add 10 g of water.

• Add 2 g of sodium hydroxide pellets to 43 g of methanol and agitate until the pellets are

dissolved.

• Add the methanol/sodium hydroxi de solution to the soybean oil. Add a magnetic stirring bar and place on the hot plate. Heat to 60-65oC and agitate for 1 hour. Pour into a separatory funnel and allow to separate. Does separation occur? What happens when you add water?

Group 2: Too much catalyst (If a little bit is good, more must be better)

• Start with 200 g of soybean oil in a 500 ml flask.

• Add 10 g of sodium hydroxide pellets to 43 g of methanol and agitate until the pellets are dissolved. Heating may be required.

• Add the methanol/sodium hydroxi de solution to the soybean oil. Add a magnetic stirring bar and place on the hot plate. Heat to 60-65oC and agitate for 1 hour. Pour into a separatory funnel and allow to separate. Does separation occur? What happens when you add water?

Group 3: Not enough alcohol (Why should I use more than the reaction needs?)

• Start with 200 g of soybean oil in a 500 ml flask.

• Add 2 g of sodium hydroxide pellets to 21.5 g of methanol and agitate until the pellets are dissolved. Heating may be required.

• Add the methanol/sodium hydroxi de solution to the soybean oil. Add a magnetic stirring bar and place on the hot plate. Heat to 60-65oC and agitate for 1 hour. Pour into a separatory funnel and allow to separate. Does separation occur? What happens when you add water?

Group 4: High free fatty acid feedstock (Why tr eat this any differently than refined oil?)

• Start with 200 g of animal fat in a 500 ml flask.42 • Add 2 g of sodium hydroxide pellets to 43 g of methanol and agitate until the pellets are

dissolved.

• Add the methanol/sodium hydroxide solution to the animal fat. Add a magnetic stirring bar and place on the hot plate. Heat to 60-65oC and agitate for 1 hour. Pour into a separatory funnel and allow to separate. Does separation occur? What happens when you add water?43 

## 5. Basic Plant Equipment and Operation

The intent of the basic plant equipment and operati on section is to discuss the principles behind the primary plant equipment that would be used in a biodiesel production facility. Included will be discussion of reactors (both batch and contin uous types), pumps, centrifuges, and distillation columns. Although there will be additional equipment in the plant such as settlers, storage tanks, etc., the four classes of equipment discusse d here represen t the heart of the process.

Reactors

The reactor is the only place in the process wher e chemical conversion occurs. Therefore, it dictates what chemical species must be handled downstream of the reactor. Reactors can be placed into two broad categories, batch reactors and continuous r eactors. Two of reactors within the continuous reactor category are continuous stirred tank reactors (C STRs) and plug flow reactors (PFRs). In the batch reactor, the reactants are charged into the re actor at the determined

amount. The reactor is then closed and taken to the desired reaction conditions (temperature, pressure, and stir rates). The chemical compos ition within the reactor changes with time. Once the prescribed reaction time is complete, the ch emical contents of the reactor are removed and sent for subsequent processing. In contrast, continuous reactors have a steady flow of reactants into the reactor and products out of the reacto r. Once a continuous flow reactor reaches steadystate operation, the product com position leaving the reactor beco mes constant. For CSTRs, the

reactants are fed into a well-mixed reactor. The composition of the product stream is identical to the composition within the reactor. Hold-up tim e in a CSTR is given by a residence time distribution. For PFRs, the reactants are fed in to one side of the reactor. The chemical composition changes as the material moves in pl ug flow through the reactor. The residence time is defined by the length of time required fo r molecules to travel through the reactor.

The most important considerations within a reacto r are the extent of reac tion of the reactants, which is known as conversion, a nd the selectivity of the reaction to the desired products, which is known as the selectivity. Key reactor variables that dictate conversion and selectivity are temperature, pressure, reaction time (residen ce time), and degree of mixing. In general, increasing the reaction temperature increases the reaction rate and, hen ce, the conversion for a given reaction time. However, if more than one reaction is taking place, the selectivity to the desired products can be impacted by changing the reaction temperature. In the transesterification reaction, the selectivity of the reaction is not negatively impacted by increasing temperature. Increasing temperature in the transesterification reaction does impact the operating pressure. The reaction is a liquid phase reaction, so the pressure in the reac tor must be maintained at a level that keeps the methanol in the liquid phase. Ther efore, as the reaction temperature is increased the pressure must also be increa sed. Another means to increase th e conversion in a reactor is to increase the reaction time. For a given temperatur e, conversion will increase as the reaction time increases. Increasing the reaction time has th e negative effect of decreasing the chemical throughput in a given plant or increasing the reac tor size for a given chemical throughput. A final very important parameter in the reactor is th e degree of mixing or mass transfer. For batch reactors and CSTRs the degree of mixing is dire ctly related to the amount of energy introduced through the impeller. Increasing th e speed of the impeller, which increases the energy input to the reactor, increases the degree of mixing, whic h improves the performance of the reactor. A44 threshold exists where additional mixing will not provide any performance enhancement. For PFRs, the degree of mixing is dictated by the design of the reactor and/or catalyst used within the reactor, since no mixing device is present in the reactor that can be independently manipulated.

Batch reactors have several pos itive features including good mixi ng characteristics and relative ease of handling homogeneous catalysts as used in the biodiesel transesterification reaction. As mentioned above, the reactant concentrations cha nge with time in the reactor as does the product concentration. Therefore, the effect of r eaction inhibition by reaction products such as experienced with glycerol can be minimized. Howe ver, batch reactors are generally not used in the production of large volume chem icals since it is most effici ent to operate the subsequent separation and purification steps in a continuous mode. This batch to continuous mode can be overcome by using multiple reaction vessels sequen ced to give effectively continuous operation or by employing a large “surge” vessel after the reactor.

CSTRs, which obviate the primary problem with batch reactors, also provide good mixing characteristics. However, the uniform concentra tion present in the reactor gives a lower reaction driving force and can also maximize reaction inhibi tion effects. PFRs or tubular reactors are the

most common reactors in the chemical indus try. Despite being c ontinuous, the reactant concentration in these reactors changes with di stance down the bed to maximize the driving force for the reaction, which is similar to a batc h reactor. Achieving good mixing can be more problematic with PFRs and these type of reactors typically use solid catalysts.

Pumps

While the role of pumps in a chemical plant seems rather mundane, they play the key role in moving chemicals through the manufacturing plant. The most common type of pump in the chemical industry is a centrifugal pump. In ma ny applications, the flui d shear introduced by a centrifugal pump is not a point of concern. Howe ver, the shear created by a centrifugal pump can create emulsion problems for the product stream fr om the biodiesel reactor. The amount of fluid shear imparted by the pump can be significantly reduced by using a positive displacement pump.

There are a number of different types of positive displacement pumps including gear pumps (external and internal) a nd lobe pumps. External gear pumps ge nerally have two gears with equal number of teeth located on the outside of the gear s, whereas, internal gear pumps have one larger gear with internal teeth and a smaller gear with external teeth. The liquid is “pumped” from the action of being carried between the gear teeth followed by displacement as the teeth mesh. In most external gear pumps, one gear drives the othe r. The internal gear pu mp has the advantage of fewer moving parts, but it cannot handle li quid streams containing solids or abrasives.

Lobe pumps impart less shear to the fluid being pumped than gear pumps. The lobes in the pump are designed for no contact, so timing gears are required for each lobe. Therefore, the lobe pump has more moving parts than gear pumps. Lobe pumps are best for handling “fragile” and high45 viscosity fluids at low pressure s. As with gear pumps, lobe pumps work by carrying the fluid between lobes followed by displacement as the l obes come together. Since the lobes are designed for no contact, fairly high amounts of slip can oc cur if the fluid to be pumped has low viscosity.

Centrifugal pumps convert velocity into feet of head where the feet of head is independent of the density of the liquid being pumped. However, conve rsion of the feet of head to output pressure requires inclusion of the fluid density. For exampl e, 231 feet of water in a column, i.e., 231 feet of head, exerts a pressure of 100 psig at the ba se, whereas the same 231 feet of biodiesel with specific gravity of 0.88 would exert a pressure of 88 psig. Therefore, the discharge pressure of the pump when pumping water would be higher than with biodiesel.

The primary components of a centrifugal pump are (1) a shaft, (2) a coupling attaching the shaft to a motor, (3) bearings to support the shaft, (4) a seal around the shaft to prevent leakage, (5) an impeller, and (6) a volute, which converts the kinetic energy imparted by the impeller into feet of head. The role of the impeller is to increase the kinetic energy of the liq uid. The liquid enters and leaves the impeller at the same pressure. As the liquid flows through the volute, the velocity is decreased with the energy transferred to pressure.

While leaking pump seals or impeller wear can be operational issues with pumps, the most common issue with pumps is the loss of sucti on pressure. Suction pressure is the pressure supplied to the suction side of the pump by the li quid. During loss of suction pressure, which is known as cavitation, the pump disc harge pressure and th e discharge flow rates become erratic and low. Cavitation is created by va porization of liquid at the pump suction. This situation can be created be a plug in the suction piping or by starting the pump up too rapidly. In general, positive displacement pumps are more difficult to cavitate than centrifugal pumps.46 Centrifuges Centrifuges are most typically used to separate solids and liquids, but they can also be used to separate immiscible liquids of different densities. This type of separation can also be achi eved using a settling tank. While a settling tank may be cheaper, a centrifuge can

be used to increase the rate of separation relative to a settling tank. In a centrifuge the separation is accomplished by exposing the mixture to a centrifugal force. The more dense phase will be preferentially separated to the outer su rface of the centrifuge. A centrifuge generally consists of (1) a bowl containing the mixture, (2) a drive shaft and drive-shaft bear ings, (3) a drive mechanism, and (4) a casing to segregate the separated products. Centrifuges ar e very amenable to continuous operation. A range of different centrifuge configurations are available with a common form being disk centrifuges.

The performance of a centrifuge depends on the specifications of the cen trifuge as well as the characteristics of the mixture to be separated. While centrifuges are generally rated in terms of mixture handling capacity, the product quality specifications are the most important

consideration. The choice of a ppropriate centrifuge type and size are predicated on the degree of separation needed in a specific system. The viscos ity of the liquid has important implications on the choice of centrifuge size since higher viscosity fluids are more difficult to handle. If a sufficient density difference exists between immiscible fluids, centrifugation is a faster means of separation than settling.

Distillation

Another means of separating chemicals in a flui d mixture is by exploiting the differences in boiling points between the chemicals. If the boiling points are sufficiently different for the chemicals to be separated, such as with water an d biodiesel, an evaporator or flash vaporizer can be used for the separation. In the evaporator, th e liquid is heated to a temperature in which only the more volatile chemical species will vapori ze. As such the vapor stream leaving the evaporator will be enriched in the more vol atile species and the liquid stream from the evaporator will be enriched in the less volatile species. In an evaporator, the separation is accomplished by supplying heat while the mixture is held at a fixed pressure. In contrast, a flash vaporizer first heats the liquid, at an elevated pressure. Then, th e heated liquid is sent through a flash valve that decreases the pressure. The decr ease in pressure causes the more volatile portion of the liquid mixture to vaporize.

An important separation device for miscible fluids with similar boiling points (e.g., methanol and water) is the distillation column. Separation in a distillation column is predicated on the difference in volatilities (boiling points) between chemicals in a liquid mixture. In a distillation column the concentrations of the more volatile sp ecies are enriched above the feed point and the less volatile species are enriched below the feed point. The vaporization in the column is driven by heat supplied in the reboiler, which is subseq uently removed in the overhead condenser. The47 temperature in the distillation column is the highest at the bottom and decreases moving up the column. Distillation columns can use either trays or packing.

The degree of separation that can be achieved in a distillation column is dictated by the relative volatilities of the chemicals to be separated, the nu mber of trays or the height of the packing, and the reflux ratio. Chemicals with very different boiling points are easier to separate than those with similar boiling points. Increasing the number of trays or the height of packing can increase the amount of separation. Reflux ratio is the am ount of condensed overhead vapor that is fed back into the top of the column. Increasing the reflux ratio increases the degree of separation. Increasing the number of trays or height of pack ing increases the column height and increasing

the reflux ratio increases the column diameter. Ei ther of these increases in column dimensions increases the capital cost of the column.

Trayed distillation columns are the most common. A tray is where contacting occurs between the vapor stream and the liquid stream. The liquid ente rs the feed tray through the downcomer from the above tray, flows across the tray where is in teracts with the vapor, spills over a weir, and, finally, flows through the downcomer to the next tray. The vapor flows up through holes in the tray. It is important to have appropriate flow ra tes of the liquid and vapor to get optimal column operation. If the liquid flow rates become too hi gh, poor contacting occurs with the vapor and in the extreme flooding of the column can occur. The onset of flooding in a column is easy to observe since the column pressure drop will in crease markedly when flooding begins. If the vapor flow rate is too high, liquid can be entrai ned and carried over to th e above tray leading to poorer separation. In contrast, t oo low of a vapor rate can lead to weeping of the liquid through the tray, which diminishes the separation. The li quid and vapor flow rates are dictated by the operation of the reboiler and the condenser, which means they are controlled by the energy input

and removal.

The materials of construction is an important c onsideration for the reactor and storage tanks. The demands on the materials used for the storage ta nks downstream of the reactor are lower since these tanks contain nearly pH neutral chemicals. The materials of construction for the storage

tanks are discussed in the Tran sportation and Storage section. In contrast, the materials of construction required in the reactor must withst and basic conditions for the transesterification

reaction or acidic conditions if an esterification approach is used to convert free fatty acids. For the base-catalyzed transesterific ation reaction, stainless steel is the preferred material for the reactor. However, stainless steel will not be good choice for use in the acid-catalyzed

esterification reaction because stainless steel is subject to attack by acids. Under these conditions, an acid resistant material such as Hast elloy should be used fo r the reaction vessel.48 

## 6. Chemical Plant Controls

To produce quality products from a biodiesel production plant it is ne cessary to be able to control the production process. While a tremendously important control variable is the properties of the feedstock to the process, this section will only di scuss the control of chemical plants within the unit operation of the processes, such as those discussed in the previous section. Two general types of processes can be used in the production of biodiesel, either batch or continuous. For batch processes the process cont rol variables will be temperature, pressure, and levels. Temperature and pressure control are critical w ithin the reaction and separation sections of the plants. Level control dictates charging to th e reactor, operation of separation devices, and product storage. For continuous processes the pr ocess control variables include temperature, pressure, level, and flow rate.

It is important to note that pr ocess variables, which are monito red and controlled, are not the same as the product specifications. Specifications are properties that are required of the final product such as those given in ASTM D 6751 for biodiesel. Control variables are those process variables that are monitored and adjusted to meet the desired specifications. As such, specifications are generally measured off-line in an analytical laboratory, while the process variables are process conditions that are measured in real time. The process variables are related to the specifications through some type of process model. The process model will typically be provided by the process licensor with subsequent refinement from plant operation experience. Some manufacturing plants will develop their own statistical or empirical models. It should be noted that the process models ar e typically feedstock dependent.

In discussing process variable measurement inst ruments, the performance of the instruments can be characterized by their accuracy and/or precision. Accuracy is the ability of an instrument to

measure the correct or “true” value of a process variable. In contrast , precision is the ability of an instrument to reproduce a process variable value within a certain interval. As such, it is possible to have either “accurate and imprecise” or “inacc urate and precise” instruments. For day-to-day operation the ability of a measurement instrument to be precise is generally more important than the need to be accurate. However, when the measurement instrument needs to be replaced, accuracy becomes an important issue.

When considering an instrumental control system for a chemical plant there are a number of factors that should be considered; cost, precis ion, reliability, and operator interface. It is desirable to have the lowest cost monitoring sy stem that will allow appropriate control of the process. Precision is important to assure that th ere is reproducibility of measurements. Reliability insures that the system is not prone to failures. Finally, it is desirable to have a monitoring and control system that provides an easy interface with the plant operators.49 The most common process variable monitored in a chemical plant is the temperature. The temperature at a specific point is generally measured using thermocouples, which are electrical devices. Thermocouples are made of two different metallic wires. These wires are connected at a junction. An electrical current is established when the junction is

heated. The electrical curre nt is proportional to the temperature at the junction. For this reason, the thermocouple must be calibrated to correlate temperature to current. Thermocouples are categori zed by the metals used in their junctions, with different metal junction combinations being more appropriate for different temperature ranges. Individual thermocouples are typi cally not calibrated by the supplier. Instead, manufacturers will typically test a representativ e portion of the thermocouples they produce. Due to the manufacturing procedure, thermocouples are usua lly precise (or completely don’t work) but can be inaccurate. The thermocouple is typically enclosed within a thermowell. The thermocouple must be fully inserted into the thermowell with the thermowell extended appropriately into the process to obtain a correct reading. If the temperat ure of a process liquid is being measured it is generally adequate for the thermo well to extend about 2-3 inches in to the liquid. Due to inferior heat transfer, the thermowell should exte nd about 6 inches into a process vapor.

Thermocouples are typically used in processes for both monitoring and controlling the process. When a thermocouple is used as part of a proce ss control loop, the electri cal current output from the thermocouple is compared with the current e xpected for the set point temperature. Deviation of the actual temperature from the set point temper ature will cause a change in the output to a

heater or cooler that is included in the control loop.

Pressure values can be measured using several di fferent types of devices such as liquid column, elastic element, and electrical sensing. The mo st common form of a liquid column device is a manometer in which the liquid height can be obser ved visually and correlated to the pressure through the density of the liquid. Liquid column de vices are rarely used in chemical processes. Elastic element devices measure pressure by determ ining deformation of an elastic material. This elastic material is usually metallic. One type of elastic element device uses bellows elements. The displacement of the bellows can be correlated to the pressure value. The most common pressure measurement instruments used in chem ical processes are electrical sensing devices, which are known as strain gauges. Strain gauges are predicated on the fa ct that the electrical resistance of conducting solids changes with length and diameter of the solid. The dimensional changes of the solid induced by a pressure causes

an increase in the electri cal resistance of the conducting solid. An elastic element or electrical sensing device used to measure pressure is called a pressure transducer. The pressure tap for a transducer should be located in the vapor phase of a process. A transducer located in the liquid phase will detect a different pressure at different dept hs in the liquid.50 To use a pressure transducer for process control or in continuous monitoring, the transducer must produce an electrical signal. For the elastic elemen t device, the direct pr essure reading due to displacement must somehow be converted to an el ectrical signal. An advant age of the electrical sensing approach is that these devices dire ctly provide an electrical signal. As with thermocouples, pressure transducers can be used for process control by comparing the measured pressure to a desired set point pressure.

Level measurement can be used to yield mass ba lance information across the process or loading in a batch reactor. Historically, level measurement devices have typically been displacer-based meters or differential pressure meters. The displacer-based meter is based on buoyancy. The displacer is immersed in a displacer chamber that is located as a side chamber. The displacer is restrained by an elastic element whose motion is proportional to the buoyant force, so the level can be determined by the vertical location of the displacer. Differential pressure meters, which are the most common level indicating devices, measure the difference in pressure between two pressure taps in a vessel. It is important to note that both of these level measurement indicators are liquid density de pendent, so that changes in liquid densities can affect their readings. This effect can be particularly important if seve ral liquid phases are present in the device. For example, the heig ht of liquid in a gauge glass

connected to a vessel is not a direct measurement of the liquid level! Recently, leve l measurement devices have been developed that use signal reflectance from an ultrasonic transducer or radio frequency, which measures the change in impedance between two capacitor electrodes.

In addition to density, there are a number of factors that can infl uence the reliability of a level measurement. Plugged taps can occur without causing apparent discrepancies in the level measurement device. Excursions to high liquid le vels can impact the reliability of the level measurement as can the presen ce of foam in the vessel.

The measurement and control of level in a vessel ge nerally requires two taps into the vessel. It is important that these taps are properly valved. Sin ce the level indicator can be used to monitor the amount of material in a vessel, it can be used to track chemical inventor y within the process as well as to control the charging of chemicals into a batch process. Level measurements can be converted to electrical signals that can be used for control. The most common control loop that includes level measurement is with operation of pumps.

The final process variable that will be discussed is flow rate. Flow rate measurement will generally only be needed if a c ontinuous flow process is used. Th ere are a number of methods for measuring flow rates, but the most commonly used approach in chemical processes are51 differential pressure flowmeters and positive disp lacement flowmeters. Flowmeters can be used in monitoring and control. The most common co ntrol loop that includes flow rate is with operation of pumps.

Differential flowmeters measure the difference in pressure between the two sides of a restriction

in a confined stream. These flowmeters are base d on restrictions imposed by a venturi tube, an orifice plate or a flow nozzle with the orifice plate meter being the most common. The orifice plate hole should be appropriately sized for the flow rate range of interest. If the hole is too large the resulting pressure change and, therefore, accuracy of the flow measurement will be inaccurate. In contrast, if the hole is too small, th e flow rate measurement is at the cost of too high of a pressure drop. While or ifice plate meters are typically used for flow rate measurement and subsequent control, it shoul d be noted that the meters do have some potential problems. First, this type of flowmeter is highly dependent on the densit y and viscosity of the liquid. Orifice plates can erode or have blockage, wh ich will lead to inaccurate measurement. The pressure taps can become plugged leading to erroneous flow values.

Positive displacement flowmeters measure flow rates using the fluid to displace a measuring device. Most common are rotary displacement flowmeters. In these meters, flow rate is measured by the amount of rotation created by flow of the fluid past a rotary device. An example of a rotary flowmeter is a vane-liquid

flowmeter in which a set of vanes mounted on a rotor with opposing pairs rotated in a cylindrical chamber.

The overall monitor and control sy stem for a process can be achi eved by individual monitors and controllers located directly at the process. Howeve r, it is most common in the chemical processes to have a centralized system for monitoring and controlling. This can be readily accommodated for most processes by a PC based system. A numb er of different software options are readily available for these types of systems.52 

## 7. Pretreatment of High Free Fatty Acid Feedstocks

Many low cost feedstocks are available for biodi esel production. Unfortunately, many of these feedstocks contain large amounts of free fatty acids (FFAs). As discussed elsewhere, these free fatty acids will react with alkali catalysts to produce soaps that inhibit the reaction.

The following ranges of FFA are commonly found in biodiesel feedstocks:

Refined vegetable oils < 0.05 % Crude vegetable oil 0.3 – 0.7% Restaurant waste grease 2 – 7%

Animal fat 5 – 30% Trap grease 40 – 100%

Generally, when the FFA level is less than 1%, a nd certainly if it is less than 0.5%, the FFAs can be ignored. Common catalyst amounts are:

Sodium hydroxide: 1% of triglyceride weight Potassium hydroxide: 1% of triglyceride weight Sodium methoxide: 0.25% of triglycerides weight

Soaps may allow emulsification that causes the sepa ration of the glycerol a nd ester phases to be less sharp. Soap formation also produces water that can hydrolyze the triglycerides and contribute to the formation of more soap. Further, catalyst that has been converted to soap is no longer available to accelerate the reaction.

When FFA levels are above 1%, it is possible to add extra alkali catalyst. This allows a portion of the catalyst to be devoted to neutralizing th e FFAs by forming soap, while still leaving enough

to act as the r eaction catalyst.

Since it takes one mole of catal yst to neutralize one mole of FFA, the amounts of additional catalyst can be calculated by the following formulas:

Sodium hydroxide: [%FFA](0.144) + 1 % Potassium hydroxide: [%FFA](0.197)/0.86 + 1 % Sodium methoxide: [% FFA](0.190) + 0.25 %

For example, when adding sodium methoxide to a feedstock with 1.5 % FFA, the amount of catalyst would be: (1.5)(0.190) + 0.25 % = 0.54 % of the triglycerides weight

Note that a factor of 0.86 ha s been included with the potassium hydroxide calculation to reflect that reagent grade KOH is only 86% pure. If other grades of catalyst are used, this factor should be adjusted to their actual purity.53 This approach to neutralizing the FFAs will sometim es work with FFA levels as high as 5 – 6%. The actual limit can depend on whether other types of emulsifiers are present. It is especially important to make sure that the feedstock contai ns no water. 2-3% FFA may be the limit if traces of water are present.

For feedstocks with higher amounts of FFA, th e addition of extra catalyst may create more problems than it solves. The larg e amount of soap created can ge l. It can also prevent the separation of the glycerol from the ester. More over, this technique conve rts the FFAs to a waste product when they could be converted to biodiesel.

When working with feedstocks that contain 5-30% FFA or even higher, it is important to convert the FFAs to biodiesel or the proc ess yield will be low. There are at least four techniques for converting the FFAs to biodiesel:

1. Enzymatic methods : These methods require expensive enzymes but seem to be less affected by water. At the present time, no one is using these methods on a commercial

scale.

2. Glycerolysis : This technique involves adding glycerol to the f eedstock and heating it to high temperature (200oC), usuall y with a catalyst such as zinc chloride. The glycerol

reacts with the FFAs to form mono- and diglycerides. Figure 21 shows the rate of decrease of the fatty acid level in a batch of animal fat. This technique produces a low FFA feed that can be processed using traditional alkali-catalyzed techniques.

Figure 21. Reduction of Fatty Acids by Glycerolysis
(100 g animal fat, 13 g glycerol, 0.1 g ZnCl, 200oC, 11 psi vacuum)54 The drawback of glycerolysis is the high temperature and that the reaction is relatively slow. An advantage is that no methanol is ad ded during the pretreatment so that as water is formed by the reaction

FFA + glycerol → monoglyceride + water

The water immediately vaporizes and can be vented from the mixture.

3. Acid Catalysis : This technique uses a strong acid su ch as sulfuric acid to catalyze the

esterification of the FFAs and the transester ification of the triglycerides. The reaction does not produce soaps because no alkali metals are present. The esterification reaction of

the FFAs to alcohol esters is relatively fast, proceeding substa ntially to completion in one hour at 60oC. However, the tran sesterification of the trigly cerides is very slow, taking several days to complete. Heating to 130o C can greatly accelerate the reaction but

reaction times will still be 30-45 minutes. Anothe r problem with acid cat alysis is that the water production from the following reaction

FFA + methanol → methyl ester + water

stays in the reaction mixture and ultimately stops the reaction, usually well before reaching completion.

4. Acid catalysis followed by alkali catalysis : This approach solves the reaction rate problem by using each technique to accomplish th e process for which it is best suited. Since acid catalysis is relative ly fast for converting the FFAs to methyl esters, it is used as a pretreatment for the high FFA feedstocks. Then, when the FFA level has been reduced to 0.5%, or lower, an alkali catalyst is added to convert the triglycerides to methyl esters. This process can convert hi gh free fatty acid feedstocks quickly and effectively. Water formation is still a pr oblem during the pretreatment phase. One approach is to simply add so much excess methanol during the pretreatment that the water produced is diluted to the level where it does not limit the reac tion. Molar ratios of alcohol to FFA as high as 40:1 may be needed. The disadvantage of this approach is that more energy is required to recover the ex cess methanol. Another approach would be to let the acid-catalyzed esterification proceed as far as it will go un til it is stopped by water formation. Then, boil off the alcohol and water. If the FFA level is still too high, then additional methanol and, if necessary, acid cata lyst can be added to continue the reaction. This process can be continued for multiple steps and will potentially use less methanol than the previous approach. Again, the disadvantage is the large amount of energy required by the distillation process.

A less energy intensive approach is to let th e acid-catalyzed reaction mixture settle. After a few hours, a methanol-water mixture will rise to the top and can be removed. Then,

additional methanol and acid can be added to continue the reaction. [Patent pending, Earl Hammond, ISU]. It is also possible to use flui ds such as glycerol and ethylene glycol to

wash the water from the mixture.55 Procedure for High FFA Feedstocks

1. Measure FFA level 2. Add 2.25 g methanol and 0.05 g sulfuric acid for each gram of free fatty acid in the oil or

fat. Sulfuric acid and methanol should be mixe d first and then added slowly to the oil. 3. Agitate for one hour at 60-65oC.

4. Let mixture settle. Methanol-water mixture will rise to the top. Decant the methanol, water, and sulfuric acid layer.

5. Take bottom fraction and measure new FFA level. 6. If FFA is > 0.5%, return to step 2 with ne w FFA level. If FFA is < 0.5%, proceed to step

7. 7. Add an amount of methanol equal to 0.217 x [grams of unreacted triglycerides] and an amount of sodium methoxide equal to [0. 25 + (%FFA)0.190]/100 x [grams of unreacted triglycerides]. Mix the sodium methoxide with the methanol and then add to the oil. This corresponds to a 6:1 molar ratio of methanol to oil for the unreact ed triglycerides. It ignores any methanol that may have carried over from the pretreatment. 8. Agitate for 1 hour at 60oC.

Example : 100 g of 12% FFA animal fat

Pretreatment: 2.25 x 12 g = 27.0 g methanol 0.05 x 12 g = 0.6 g H 2 SO 4 (sulfuric acid)

Mix acid with methanol. Then add mixture to fa t. Agitate for one hour at 60oC. Let settle and separate bottom phase. Acid valu e should decrease substa ntially, to at least 5-6 mg KOH/g. There fore, FFA = 2.5%

Second step of pretreatment: 2.25 x 2.5 g = 5.6 g of methanol 0.05 x 2.5 g = 0.13 g H 2 SO 4

Mix with oil, agitate at 60oC for 1 hour. FFA should be < 0.5%. Removal of upper phase is usually optional at this point.

Then add 0.217 x (88) = 19.1 g methanol [0.25 + (0.5)(0.190)]/100 x 88 = 0.30 g sodium methoxide

Agitate at 60oC for one hour. If glycerol and ester do not separate , add 50 g of warm distilled water to encourage separation. Wash 3-4 times.56 

## 8. Patent Discussion

Although transesterification is an old and well-known process in organic chemistry, much of the modern chemical techniques were described in patents issued to chemists for DuPont and Colgate-Palmolove-Peet during a nd immediately after World War II. A careful examination of these patents shows that most of the techniques used since that time were anticipated and covered by these patents.

Prior to the 1940s, most soap was made followi ng a traditional process of saponifying fats through the addition of sodium or potassium hydr oxide and water. The glycerol freed by this process usually remained with the soap and provided some moisturizing benefit. Techniques were available to separate the glycerol but they provided a water-contaminated crude product and were not suitable for use with potassium so aps. Because of the high demand for glycerol to produce explosives for the war effort, there wa s a strong motivation to remove the glycerol during soap production and provide a high-quality, dry product. George Bradshaw of E.I duPont received a patent (#2,271,619) for a process that added about 1.6 times the theoretical amount of an alcohol, such as methanol, which contained 0. 1 to 0.5% sodium or potassium hydroxide, to an oil or fat. When performed at 80oC, this pr ocess provided 98% convers ion to a high-quality, water-free glycerol and alcohol es ters. The esters were then sa ponified with additional caustic soda to produce soap and the alcohol was r ecovered by heating. The residual glycerol and

methanol in the alcohol esters could be remove d by two or three washes with a small amount of

water.

Bradshaw recognized that this proc ess could be conducted in stages: “ ... the amount [of alcohol] can be reduced substantially by working in steps. For instance, if the theoretical quantit y of methyl alcohol is used, the conversion will be about 80 per cent. The glycerine can then be removed and more alcohol and catalyst added. If th e alcohol is added in three steps the amount required can be reduced, for inst ance, from one and six-tenths times theory to one and two-te nths times theory.”

In his second patent (#2,360,844) Bradshaw says that with three or four steps the amount of alcohol can be reduced to 1.1 times theory.

Bradshaw noted that other alcohols beside meth anol could be used for the process including ethanol, propanol, isopropanol, th e butanols, and the pentanols. Bradshaw further describes how fats containing small amounts of water and free fa tty acids could still be processed with this technique by adding additional catalyst. He mentions that a drawback of this approach is that soap will be formed that can inhibit the separatio n of the glycerol. He suggests acidification of the reaction mixture to split the soaps back to free fatty acids and salts to facilitate the separation. His suggestions for acceptable acids include fo rmic, acetic, propionic, hydrochloric, sulfuric, sulfamic, and phosphoric.

Bradshaw’s work was followed by a extensive set of patents from researchers at the ColgatePalmolive-Peet company. These patents describe the same essential process as Bradshaw but add57 special features to improve the quality or quantity of the product. Each of these patents will be discussed with emphasis given to the unique contributions they make to the art.

H.D. Allen described modifications to Bradshaw’s process that were intended to make it easier to implement as a continuous flow process. He raised the temperat ure of the reaction to the range 80oC to 160oC under enough pressure to keep the alcohol in the liquid phase. Allen noted that either acid or alkali catalysts could be used and that acid catalysts were better suited to feedstocks with high free fatty acid levels since the free fatty acids would be esterified during the process. At the higher temperatur es, the reaction times were shortened to 3 to 4 minutes at 135oC with an alkali catalyst and 45 minutes with an acid catalyst. Allen’s proposed system for transesterification is shown in Figure 1 and incl udes a reaction coil (plug flow reactor) to provide the time needed to complete the reaction.

Although previous researchers had noted the prob lem with using alkali ca talysts with high free fatty acid feedstocks, and some had commented on the suitability of acid catalysts for this purpose, G.I. Keim appears to have been the first to propose a sequence of an acid catalyzed reaction to convert the free fatty acids to esters followed by an al kali catalyzed step to convert the triglycerides. Keim notes that the alkali cata lysts are 10 to 50 times faster with triglycerides than the acid catalysts. However, the alkali reacts with the free fatty acids to form soap, which causes gelling problems and difficu lty separating the glycerol. By starting with the acid to

convert the FFAs to esters until th e FFA level is at a tolerable am ount for the alkali catalyst, then switching to the alkali catalyst , the reaction time is kept s hort without undesirable soap formation. Keim recognizes that the conversion of the FFAs to esters produces water. He says that removal of this water might be necessary for the reaction to proceed to completion and he says this can be accomplished by absorption with sodium sulfate or by heating. He also observed

that transesterification reactions with higher alcohols, beyond meth anol, are especially sensitive

to water.

Arrowsmith makes a number of additional observati ons about the alkyl este r production process.

He notes that it is desirable to minimize the amount of alkali catalyst used because the amount of soap formed increased with increasing catalyst. He also observed that increasing the amount of

excess alcohol could decrease the amount of cat alyst required. However, excess alcohol can cause problems with separation of the esters and glycerol. This probl em is particularly severe for ethanol and higher alcohols. The separation process can be enhan ced by adding water, which will pull the glycerol out of the este r phase. However, now the lower pha se is a mixture of glycerol, alcohol, and water. Considerable extra energy is required to se parate and purify these three compounds, especially if they involve azeotrope s as is the case with ethanol and water. Arrowsmith proposes to solve this pr oblem with a water-free process.58 Figure 22. Process Flow Diagram by H.D. Allen, Patent No. 2,383,579.59 Instead of using a washing pro cess to purify the esters, he r ecommends a vacuum distillation. According to Arrowsmith, the esters and glycer ol form an azeotrope with a boiling temperature that is lower than the boiling temperature of ei ther pure component. Distillation is performed at pressures of 1 to 15 mm Hg. Arrowsmith states that the transesterification reaction can be conducted at temperatures as high as 250 C, but that at temperatures above about 140-160 C, the soap can catalyze the reverse reaction and break up the esters to free fatty acids and alcohol. By staying at 130 C, it was possible to attain 30 second reaction times and achieve 97% complete

reaction.

Trent also noted the tendency of excess alcohol to serve as a cosolvent an d inhibit the separation of the ester and glycerol. Tren t developed an apparatus for re moving the alcohol and separating the esters and glycerol in a single device. Two variations of the device are shown in Figure 23. The first device (left side) shows a system wher e the oil and alcohol/catalyst mixture are mixed and passed through a coil of sufficient length to allow the reaction to reach completion before entering a chamber that is heated by a steam coil. The entire process is at or near atmospheric pressure. The temperature of the chamber is hi gh enough that the alcohol boils off leaving the mixture of esters and glycerol. Acid is added to neutralize the mixture and split the soap into free fatty acids and salts. Then, the ester/glycerol mixt ure passes through a tube into the lower half of the chamber where it separates. Esters are removed from the top of the lower chamber and the glycerol is removed from the bottom.

The second variation (right side) is similar except that the reac tion is conducted at higher temperature and pressure. Instead of a steam coil to boil off the alcohol, the pressurized mixture is sprayed into the upper half of the container, which acts as a flash chamber. Vaporized alcohol is removed and the ester/glycerol enters the bo ttom half of the container where it separates as

with the other device.

Trent mentions other novel aspects of the alcohol ysis process: Trent says that esters produced with higher alcohols (ethanol a nd above) have a greater tendency to gel from soap formation than with methanol. He claims that better yields are found if reactants are mixed cold and then heated, instead of preheated and th en mixed. He says that early increases in temperature increase saponification and recommends using a step-w ise increase in temperature from 40-50 o C then up

to 110 to 123 o C. He also mentions water washing using either batch, concurrent, or countercurrent flow systems. In addition to wa ter washing, Trent claims that a liquid-liquid extraction process using a selective solvent that is at least partially immiscible with the esters can be used. He specifically suggests polar solvents su ch as furfural, sulfur dioxide, nitromethane, methanol, ethanol, ethylene glycol, allyl alc ohol, ethyl sulfate, acetaldehyde, acetamide, dichlorodiethyl ether, and methyl carbitol. Trent notes that al cohol can be used as a washing agent for the oil to remove free fatty acids and water.

Percy points out that since alcohol is immiscible with the triglyceride feedstock and the final glycerol product is immiscible with the ester la yer, this requires a high level of agitation to prevent saponification (soap formation) and sepa ration of the glycerol layer and consequent removal of the alcoholysis catalyst. However, when mono- and diglycerides are present, the reaction mixture will be homogeneous. Percy advo cates a process where only enough alcohol or60 Figure 23. Trent’s Combined Reactor and Glycerol Separator61 catalyst is added to form mono- and diglycerid es. This reaction mixture will be homogeneous. This partially reacted mixture can be held and a stream bled off to which more alcohol and catalyst are added to complete the reaction. Percy claims that if the reaction is conducted at 10 atm and 175o C, the reaction will proceed to completion without catalyst. When using higher alcohols, if ester/glycerol separation is a pr oblem, adding methanol can help encourage the separation.

Dreger observes that ethanol and higher alcohol s allow the use of higher reaction temperatures but he says that this increases the tendency to ward soap formation. He advocates lower reaction temperatures of around 40o C. In addition, he re commends the use of small amounts of methanol to ensure complete reaction and to promote sepa ration of glycerol when using ethanol and higher alcohols. Adding 10-30% methanol to whatever al cohol is being used will encourage complete reaction and ester/glycerol separation.

Patents Referenced:

Bradshaw (1942, 1944): E.I. duPont de Nemours & Company, 2,271,619 and 2,360,844 Allen (1945): Colgate-Palmolive-Peet, 2,383,579 Arrowsmith (1945): Colgate-Palmo live-Peet, 2,383,580 and 2,383,581 Percy (1945): Colgate-Palmolive-Peet, 2,383,614 Keim (1945): Colgate-Palmolive-Peet, 2,383,601 and 2,383,602 Trent (1945): Colgate-Palmolive-Peet, 2,383,632 and 2,383,633 Dreger (1945): Colgate-Palmolive-Peet, 2,383,59662 

## 9. Patent List for Biodiesel

The following patents relate to the producti on, processing, and utilization of biodiesel.

Arrowsmith, C.J. and J. Ross, "Treating Fa tty Materials," US Patent 2,383,580. 1945.

Assmann, G., G. Blasey, B. Gutsche, L. Jero min, J. Rigal, R. Armengaud and B. Cormary, “Continuous process for the producti on of lower alkyl esters”, U. S. Patent # 5,514,820, May 7,

1996.

Bam, N., Drown, D.C., Korus, R., Hoffman, D.S., Johnson, T.G., and Washam, J.M., "Method for Purifying Alcohol Esters," US Patent 5424467, June 13, 1995.

Barnhorst, J. A., M D. Staley and D. A. Oest er, “Transesterification process”, U.S. Patent 6,489,496, December 3, 2002.

Basu, H.N. and M.E. Norris, "Process for Produc tion of Esters for Use as a Diesel Fuel Substitute Using a Non-Alkaline Catalyst," US Patent 5525126, June 11, 1996.

Bayense, C. R., H. Hinnekens and J. Martens, “Esterification process”, U. S. Patent 5,508,457, April 16, 1996.

Billenstein, S., B. Kukla and H. Stuhler, “Process for the preparation of fatty acid esters of shortchain alcohols”, U. S. Patent 4,668,439, May 26, 1987.

Bollman, H. 1921. Process for the Removal of Fa tty Acids, Resins, Bitter and Mucilaginous Substances from Fats and Oils. U.S. Patent 1,371,342.

Bradin, D.S., "Biodiesel Fuel," US Patent 5578090, November 26, 1996.

Bradshaw, G.B., “New Soap Process,” Soap, V. 18, May 1942, pp. 23-24, 69-70.

Bradshaw, G.B. and W.C. Meuly. 1942. Process of Making Pure Soaps. U.S. Patent 2,271,619.

Bradshaw, G.B. and W.C. Meuly, “Preparation of Detergents,” U.S. Patent No. 2,360,844 Awarded Oct. 24, 1944.

Buxton, L.O., "Process for Treating Fa tty Oils," US Patent 2,345,097. 1944.

Colbeth, I. M., “Process of r eacting glyceride oils”, U. S. Patent 2,469,371, May 10, 1949.

Connemann, J., A. Krallmann, and E. Fischer, “Process for the continuous production of lower alkyl esters of higher fatty acids”, U. S. Patent 5,354,878, October 11, 1994.

Demmering, G., C. Pelzer and L. Friesenhagen, “Process for the production of fatty acid lower alkyl esters”, U. S. Patent 5,455,370, October 3, 1995.63 Feldman, J. and J.M. Hoyt, "Extractive Disti llation of Alcohol-ester mixtures," US Patent 4,473,444. 1984.

Foglia, T.A., L.A. Nelson, W.N. Marmer, "Produc tion of biodiesel, lubr icants and fuel and lubricant additives," US Patent 5,713,965. 1998.

Glossop, G. A., “Treating Fatty Glycerides ”, U.S. Patent 2,383,599, August 28, 1945.

Granberg, E. P., R. G. Schafermeyer and J. A. Letton, “Method for purifying an inert gas while preparing lower alkyl esters”, U. S. Patent 5,844,111, December 1, 1998.

Haas, M. J., S. Bloomer and K. Scott, “Process fo r the production of fatty acid alkyl esters”, U.S. Patent 6,399,800, June 4, 2002.

Hay, R. G., J. G. McNulty, W. L. Walsh, “Pro cess for preparing esters”, U.S. Patent 3,692,822, September 19, 1972.

Hunt, T. K., L. Jeromin, W. Johannisbauer, B. Gutsche, V. Jordon and H. Wogatzki, “Recovery of tocopherols”, U. S. Patent 5,646,311, July 8, 1997.

Jeromin, L., E. Peukert and G. Wellmann, “Process for the pre-esterification of free fatty acids in fats and oils”, U. S. Patent 4,698,186, October 6, 1987.

Jeromin, L., E. Peukert, B. Gutsche, G. Wollma n, and B. Schleper, “Process for the continuous transesterification of fatty acid lower alkyl es ters” U. S. Patent 4,976,892, December 11, 1990.

Johnson, L.A. and E.G. Hammond, “Soybean Oil Es ter Fuel Blends,” U.S. Patent 5,520,708, May 28, 1996.

Kawahara, Y. and T. Ono, "Process for Producing Lower Alcohol Esters of Fatty Acids," U.S. Patent 4,164,506. Awarded Aug. 14, 1979.

Keim, G.I., “Treating Fats and Fatty Oils,” U.S. Patent 2,383,601, Awarded Aug. 28, 1945.

Kesling Jr., H. S., L. J. Karas, and F. J. Liotta Jr., “Diesel fuel”, U. S. Patent 5,308,365, May 3,

1994.

Klok, R and H. H. Verveer, “Process for producin g fatty-acid lower-alkyl mono-esters”, U. S. Patent 5,116,546, May 26, 1992.

Kuceski, V. P., “Purification of alpha, omega alka nedioic acids by partial esterification”, U. S. Patent 2,824,122, February 18, 1958.64 Lepper, H. and L. Friesenhagen, “Process for th e production of fatty acid esters of short-chain aliphatic alcohols from fats and/ or oils containing free fatty acids”, U. S. Patent 4,608,202, August 26, 1986.

Lepper, H. and L. Friesenhagen, “Process for th e production of fatty acid alkyl esters”, U. S. Patent 4,652,406, March 24, 1987.

Matsukura, T. and Y. Nakagawa, “Method for Ma nufacturing High Quality Fatty Acid Esters,” US Patent 4,371,470 February 1, 1983.

Matsumoto, W., E. Nakai, T. Nezu, and K. Suzuki , "Process for the Transesterification of fat and oil," US Patent 5,089,404. 1992.

Mittelbach, M., "Method for the Preparation of Fatty Acid Al kyl Esters," US Patent 5849939, Dec. 15, 1998.

Nourreddini, H., “Process for Producing Biodiesel Fuel with Reduced Viscosity and a Cloud Point Below 32 deg. Fahrenheit,” U.S. patent 6,015,440, January 18, 2000.

Noureddini, H., “System and pro cess for producing biodiesel fuel with reduced viscosity and a cloud point below thirty-two (32) degrees fahrenheit”, U. S. Patent 6,174,501, January 16, 2001.

Peter, S., R. Ganswindt and E. Weidner, “Method for producing fatty acid esters”, U. S. Patent 6,211,390, April 3, 2001.

Reierson, R.L., "Process for Removing Glycerin," US Patent 4,360,407. 1982.

Schmerling, L., "Transesterification of Ca rboxylic Acids," US Patent 4,112,235. 1978. (tin halide catalyst)

Sprules, F. J. and D. Price, “Production of fatty esters”, U.S. Patent 2,494,366, January 10, 1950.

Stage, H., "Process for Deodorizing and/or Phys ical Refining of high-Boiling Organic Edible Oils, Fats, and Esters," U.S. Patent 4,599,143, July 8, 1986.

Stage, H., "Process for Degassing, Dehydratin g, and Precut Separation in Straight-run Distillation of Crude Fatty Acids," U.S. Patent No. 4,680,092, July 14, 1987.

Stern, R., G. Hillion, P. Gateau, and J.-C. Gu ibet, "Process for Manufacturing a Composition of Fatty Acid Esters Useful as Gas Oil Substitute Motor fuel with Hydrated Ethyl Alcohol and the Resultant Esters Composition," U.S. Patent 4,695,411. Awarded Sept. 22, 1987.

Stern, R., G. Hillion, M.N. Eisa, "Process for the Production of Ethyl Es ters," U.S. Patent 6,013,817, Awarded Jan. 11, 2000.65 Stern, R., G. Hillion, J. J. Rouxel, and S. Le porq, “Process for the production of esters from vegetable oils or animal oils alcohols”, U. S. Patent 5,908,946, June 1, 1999.

Stern, R., G. Hillion and J. J. Rouxel, “Improve d process for the production of esters from fatty substances having a natural origin”, U. S. Patent 5,424,466, June 13, 1995.

Stidham, W. D., D. W. Seaman, M. F. Danzer, “Method for preparing a lower alkyl ester product from vegetable oil”, U. S. Patent 6,127,560, October 3, 2000.

Tanaka, Y., A. Okabe and S. Ando, “Method for th e preparation of a lower alkyl ester of fatty acids”, U. S. Patent 4,303,590, December 1, 1981.

Trent, W.R., "Process of Treating Fatty Glycerides," US Patent 2,383,632. 1945.

Trent, W.R., "Process for Treating Fatty Glycerides," US Patent 2,383,633. 1945

Venter, J. J. and C. McDade, “Transesterificati on catalyst”, U. S. Patent 5,183,930, February 2,

1993.

Wimmer, T., "Process for the Production of Fatty Acid Esters of Lower Alcohols," US Patent 5399731, March 21, 1995.

Wimmer, T., "Process for Preparing Fatty Acid Esters of Short-Chain Monohydric Alcohols," US Patent 5,434,279. 1995.

Wu, W. T. and J. W. Chen “Method of preparing lo wer alkyl fatty acids este rs and in particular biodiesel”, U. S. Patent 6,398,707, June 4, 2002.

Yeh, L. I., R. H. Schlosberg, R. C. Miller, J. R. Bateman, R. F. Caers, “Diesel fuel composition”, U.S. Patent 6,447,557, September 20, 2002.

Yeh, L. I., R. H. Schlosberg, R. C. Miller, J. R. Bateman, R. F. Caers, “Diesel fuel composition”, U.S. Patent 6,458,176, October 1, 2002.

Yeh, L. I., R. H. Schlosberg, R. C. Miller, J. R. Bateman, R. F. Caers, “Diesel fuel composition”, U.S. Patent 6,447,558, September 10, 2002.66 

## 10. Post Reaction Processing

Introduction

The objective of this section is to describe in more detail the steps in processing the ester phase resulting from the transesterification. This section will discuss the recovery of the esters from the reaction mixture, and, then, the refining needed to meet the requireme nts of ASTM D 6751-2. The topics include: ester/glycerol separation, ester washing, este r drying, other ester treatments,

and additization.

Ester/Glycerol Separation

The ester/glycerol separation is typically the firs t step of product recovery in most biodiesel processes. The separation process is based on the f acts that fatty acid alcohol esters and glycerol are sparingly mutually soluble, and that there is a significant difference in density between the ester and glycerol phases. The presence of metha nol in one or both phases affects the solubility of ester in glycerol and glycerol in ester.

The ester washing step is used to neutralize a ny residual catalyst, to remove any soaps formed during the esterification reaction, and to remove residual fr ee glycerol and methanol.

Ester drying is required to meet the stringent limits on the amount of wate r present in the final biodiesel product. In addition, there may be othe r treatments used to reduce color bodies in the fuel, remove sulfur and/or phosphorus from the fuel, or to remove glycerides.

Additization is the addition of materials the have a specific functionality that modifies one or more fuel properties. Examples include cloud poi nt/pour point additives, antioxidants, or other stability enhancing agents.

Fatty acid alcohol esters have a density of about 0.88 gm/cc, while the glycerol phase has a density on the order of 1.05 gm/cc, or more. The glycerol density depends on the amount of methanol, water, and catalyst in the glycerol. This density difference is sufficient for the use of simple gravity separation techniques for the two phases.

However, the rate of separation is affected by several factors. Most biodiesel processes use relatively intense mixing, at least at the beginni ng of the reaction, to in corporate the sparingly soluble alcohol into the oil phase. If this mixing c ontinues for the entire r eaction, the glycerol can be dispersed in very fine droplets throughout the mixture. This dispersion requires from one hour to several hours to allow the droplets to coalesce into a distinct glycerol phase. For this reason, mixing is generally slowed as the reaction begi ns to progress, to re duce the time required for phase separation.

The more nearly neutral the pH, the quicker the glycerol phase will coalesce. This is one reason to minimize the total catalyst use. In some batc h systems the reaction mixture is neutralized at the beginning of the glycerol /ester phase separation step.67 The presence of significant quant ities of mono-, di-, and triglycerides in the final mixture can lead to the formation of an emulsion layer at th e ester – glycerol interface. At best, this layer represents a net loss of product, unless it is recovered and separate d. At worst, the ester phase will not meet the biodiesel specif ication and will have to be re-r un. If problems with mono, di, and triglycerides occur, you shoul d reevaluate the entire reacti on to see where improvements can be made to improve process yields in the preceding steps.

The esterification process is run with an excess of alcohol to ensure complete reaction and to attain higher reaction rates. The residual alcohol is distributed between the ester and glycerol phases. The alcohol can act as a dispersant for the es ter into the glycerol phase and for the glycerol into the ester phase. The result can be a need for additional proc essing of the products to meet specifications. Other people claim that meth anol aids in phase separation, which is one reason that product is generally phase se parated before methanol recovery.

Process Equipment for the Ester/Glycerol Separation

Decanter Systems: There are three categories of equipmen t used to separate the ester and glycerol phases. Decanter systems rely solely on the density difference and residence time to achieve the separation. For re latively small throughput, or batc h processes, the 1 to 8 hours required for complete separation of the phases may be acceptable. However, a separation that requires a residence time of 1 hour requires a deca nter with a volume of at least 700 gallons to affect separation for a 5,000,000 gallon per year, c ontinuous plant. For lower extent of reaction, the separation is slower, and the decanter will have to be much larger.

The primary determinant for designing a decan ter for biodiesel production is the desired residence time. This, plus the product mixture flow rate determines the size of the unit.

Decanter units should be rather tall and narrow to allow physical separation between the ester and the glycerol withdrawal points. L/D ratios of 5 to 10 can work well.

The temperature in the decanter affects the solu bility of the alcohol in both phases, and the viscosity of the two liquids. Too high a temperatur e in the decanter can cause residual alcohol to flash, potentially restricting the flow of the es ter phase out of the tank. On the other hand, too low a temperature increases the viscosity in both phases. The increased viscosity will slow the coalescence rate in the system.

The presence of an emulsion layer is indicativ e of mono- and di-glycerides. The emulsion layer will form between the phases. In continuous ope ration, there must be a provision for removing the emulsion, so it does not fill the decanter.

Centrifuge System: Many of the continuous plants use a centrifuge for the phase separation. The centrifuge creates an artif icial, high gravity field by spinning at very high speeds. The separation can be completed rapidly and effectively. The di sadvantage of the centrifuge is its initial cost, and the need for considerable and careful maintenance. However, centrifuges are used extensively in the food processing, rendering, and biodiesel industries.68 Centrifuges are high-speed , rotating devices. The artificial gravity of the centrifuge results from the rotation. Speeds of 2000 to 5000 rpm are not unc ommon. The same high- speed rotation that creates the centrifugal effect also creates a de vice that requires careful and rather frequent maintenance. Although centrifuges are relatively expensive, multiple units to ensure on-line availability are advisable.

At smaller capacities, either a batch or a continuous centrifuge can be used. The use of a batch centrifuge in a continuous proce ss requires a surge tank to match the batch cycle time with the continuous processing rate.

Hydrocyclone: An intriguing, relatively new device that has been considered for use in biodiesel plants is the hydrocyclone. A li quid – liquid hydrocyclone uses an inverted conical shape and the incompressibility of the liquids to accelerate the liquid entering the cyclone. The effect is similar to a centrifuge, with the heavier material being forced towards the wall and downward, and the lighter material forced to the center and upward. The result is a density-based separation. Although they are now used for oil – water sepa rations, hydrocyclones are at the experimental stage for application in biodiesel production.

A hydrocyclone operates on the Bernoulli principle of trading pressure for speed in an incompressible flowing system. The relative density of the fluids determines the separating force applied, while the relative viscosity determines the resistance to separa tion.The liquid mixture enters the hydrocyclone at a moderately high pr essure (about 125 psig). The pressure decreases and the velocity increases as the liquid passes for the wider to the narrower part of the inverted cone. The more dense liquid is accelerated towa rds the outside wall, while the less dense liquid

concentrates in the center. The result is a separation based upon induced g-forces.

It would appear that the presence of volatiles creates a problem in a hydrocyclone. The rapid reduction of pressure in the device will induce fl ashing of the volatile liq uid (alcohol), disrupting or stopping the separation process. Excess methanol should be removed from the system before introducing the reaction mi xture to a hydrocyclone.

Ester Washing

The primary purpose of the ester washing step is the removal of any soaps formed during the

transesterification reaction. In addition, the water provides a medium for addition of acid to neutralize the remaining catalyst a nd a means to remove the product salts. The residual methanol should be removed before the wash step. This prevents the addition of methanol to the wastewater effluent. However, some processes remove the methanol with the wash water and

remove it from the wash water.

The use of warm water (120 to 140 oF) prevents precipitation of saturated fatty acid esters and

retards the formation of emulsions with the use of a gentle washing action.

Softened water (slightly acidic) eliminates calcium and magnesium contamination and neutralizes remaining base catal ysts. Similarly, removal of ir on and copper ions eliminates a69 source of catalysts that decrease fuel stability. Gentle washing prevents the formation of emulsions and results a rapid a nd complete phase separation.

The phase separation between esters and water is typically very clean and complete. However, the equilibrium solubility of water in esters is higher than the specified water content for B100. Therefore, after the washing step there will be more than the equilibrium amount of water

present.

Vacuum driers can either be batch or conti nuous devices for removing water. The system is operated at a highly reduced pressure, which a llows the water to evaporate at much lower temperature. A variation that also allows for rather high heating and evaporation rates is the falling film evaporator. This devi ce operates at reduced pressure . As the esters pour down the inside wall of the evaporator the direct contact with the heated wa ll evaporates the water rapidly. Care should be taken with high temperature evapor ators to avoid darkenin g the fuel which is a sign that the polyunsaturated met hyl esters are polymerizing.

Because the total water burden in th e esters is low, molecular sieves , silica gels, etc. can also be used to remove the water. An advantage of thes e systems is that they are passive. However, a disadvantage is that these units mu st be periodically regenerated.

Other Ester Treatments

There are absorbents on the market that sel ectively absorb hydrophylli c materials such as glycerol and mono- and di-glycer ides (i.e Magnesol from the Dallas Group). This treatment, followed by an appropriate filter, has been shown to be effective in lowering glycerides and total glycerol levels.

Some vegetable oils and many yellow greases and brown greases leave an objectionable color in the biodiesel. There is no color specification in ASTM D 6751, but an activated carbon bed is an effective way to remove excessive color. The fats and oil industry literature has other bleaching technologies that may also be e xplored for biodiesel producers.

The European specification for sulfur content is much tighter than the U.S. requirement. As a result, a number of producers in Europe are reso rting to the use of vacuum distillation for the removal of sulfur compounds from the biodiesel product. By 2006, all U.S. biodiesel must meet new sulfur standards of 15 ppm or less. Therefor e, biodiesel producers needs to be aware of the sulfur content of their fuel, and must incorpor ate sulfur reduction techno logy if needed before

that date. Vacuum distillation has the added be nefit of deodorization and the removal of other minor contaminants, which may provide a benef it to those firms that use highly degraded feedstocks such as trap grease.

Filtering is an essential part of all biodiesel production. While feedstocks entering the plant

should be filtered to at least 100 μm, biodiesel leaving the plant should be filtered to at least 5 μm to ensure no contaminants are carried with the fuel that could damage the engine. At has been suggested that the fuel could be cooled be fore filtering to capture some of the saturated70 esters as they crystallize and thereby lower the cloud point of th e fuel. The crystallized esters could be melted by heating and used within the plant as boiler fuel.

Additization of Esters

Petroleum-based diesel fuels are treated with a wide range of additives to improve lubricity, detergency, oxidative stability, corrosion resi stance, conductivity, and many other properties. Additive technology for biodiesel is less advanced, so fewer additives are available to enhance performance.

Figure 24. Oxidation behavior of vege table oils and animal fats [1].

One area where the biodiesel producer needs to give serious consideration is oxidative stability. Biodiesel, because it contains large numbers of molecules with double bonds, is much less oxidatively stable than petroleum-based diesel fuel. Fortunately, stability-enhancing additive technology is well-developed in the food industry and many of these additives can be carried over to stabilizing biodiesel.

Figure 24 shows typical oxidation beha vior for vegetable oils and animal fats. In this test, the oil and fat were exposed to heat and aeration to accele rate the oxidation process. The progress of the oxidation is monitored by measuring the peroxide va lue, or the fraction of the oil that has been converted to a peroxide molecule, the first step in the oxidation process. Both the vegetable oil and animal fat have no conversion for a period of time (called the induction period), but then oxidize quickly. In this case, th e lard actually oxidizes earlier th an the vegetable oil, probably due to the presence of natural anti oxidants in the vegetable oil. Lipi d oxidation is an autocatalytic process so that when it starts, it progresses at eve r-increasing rate.71 Figure 25. Antioxidants for oils and fats [2].72 Figure 26. Comparison of tert -butylhydroquinone (TBHQ) with other anioxidants by the active oxygen method (AOM). (From Sherwin, E. R. and Thompson, J. W., Food Technol. (Chicago), 21(6) , 106, 1967.

Figure 25 shows some of the most common antioxidant additives used in food products: BHA, BHT, PG, and TBHQ. The propertie s of these additives are well known and they are considered

safe.

Figure 26 shows a comparison of the effectiveness of the different antioxida nts in three different

oils. This data was obtained from oils and not biodiesel, however, since the oxidation processes

are similar for the two classes of compounds, the relative effectiven ess of the additives should be similar. Figure 26 clearly shows that TBHQ is th e most effective additive for equivalent treat

rates.

Table 8 shows additional comparisons between the additives for variable treat rates. Again, TBHQ is almost twice as effective as the nearest other additive.

Since metals, especially copper, can act as catalysts for oxidation, compounds called chelators are frequently added to the oils and fats along with the antioxidants. Chelators bind up metals into stable molecules that do not participate in promoting oxidation. Citric acid is a common chelator used in the food industry [1]. Figure 4 shows the effect of adding citric acid to

antioxidants. It extends the oxidative induction period and improves the st ability of the oil.73 Table 8. Comparison of TBHQ with BHA, BHT, and PG in Vegetable Oils by AOM and Oven Storage Tests.74 Figure 27. Effect of Citric Acid on Antioxidant Effectiveness

References

1. Furia, T.E., Editor, Handbook of Food Additives, CRC Press, Cleveland, OH, 2 nd Edition,

1972.

2. Furia, T.E., Editor, Handbook of Food Add itives, Vol. 2, CRC Press, Cleveland, OH, 2 nd

Edition, 1980.75 

## 11. Treatment and Recove ry of Side Streams

Introduction

There are three non-ester side stre ams that must be treated as a part of the overall biodiesel process. These streams are:

1) The excess alcohol that is recycled within the process; 2) The glycerol co-product, and; 3) The wastewater stream from the process.

In this module it is assumed that methanol is the alcohol used in the process. Similar comments would apply to other alcohols.

Methanol recycle is necessary because an exces s of methanol is required for an effective

transesterification reacti on system. The recovery of the unused methanol saves input costs for the process and essentially eliminates the emissions of methanol to the su rroundings. The emissions

reduction is needed because methanol is highly flammable and is toxic.

Glycerol is recovered and part ially refined as a co-product fr om biodiesel production. About 10 percent by weight of the input reactants are converted to gl ycerol in the transesterification reaction. On both a weight basis a nd a volume basis, partially refine d glycerol is worth more than the biodiesel product.

Wastewater constitutes an operating cost for the plant, both because of the water consumption

and because of the water trea tment costs to the plant.

Methanol Management

There are several physical paramete rs that are important to the r ecovery and recycle of methanol. Methanol’s relatively low boiling point, 64.7 °C, mean s that it is fairly volatile and can largely be removed from the oil, ester and aqueous st reams by flash evaporati on and re-condensation. The low boiling point, along with a low flash point, 8 ° C, also mean the methanol is considered to be highly flammable.

Methanol is fully miscible with water and with gl ycerol. However, it has a low solubility in fats and oils (approx. 10 % wt/wt at 65 °C in tallow). Methanol is more soluble in esters, but it is not fully miscible. The solubility in glycerol and water means that methanol will prefer these phases when there is a two-phase system present. The low solubility in fats and oils is the reason for the solubility-limited phase of the overa ll transesterification reaction.

When the two phases present are esters and glycer ol, the methanol will distribute between the phases. At 90:10 % wt/wt ester and glycerol, the methanol distributes approximately 60:40 wt % between the phases. This fact is important, si nce the reaction is complete at 90:10 wt %.76 If the methanol is allowed to remain in the system during phase separation, the methanol acts as a phase stabilizer, retarding the rate of gravity separation. It is advantageous to remove the methanol before phase separation.

Methanol can be recovered using distillation, either conventional or vacuum, or partially recovered in a single stage flash. An alternative to distillation is a falling-film evaporator.Residual methanol in the ester phase can be removed in the water wash step in ester postprocessing. Product esters are typi cally washed with warm (140 °F), softened water to remove soaps and residual methanol.

Glycerol Refining

The recovered glycerol from the transesterific ation reaction contains residual alcohol, catalyst residue, carry-over fat/oil and some esters. Th e glycerol from rendered feedstocks may also contain phosphatides, sulfur compounds, proteins, aldehydes and ketones, and insolubles (dirt, minerals, bone, or fibers)

Chemical Refining: There are several factors that are important in the chemical refining or glycerol. First, the catalyst tends to concentr ate in the glycerol phase where it must be

neutralized. The neutrali zation step leads to the precipitation of salts.

Also, the soaps produced in the esterification mu st be removed by coagul ation and precipitation

with aluminum sulfate or ferric chloride. The removal may be supplemented by centrifuge separation.

The control of the pH is very important because low pH leads to dehydration of the glycerol and high pH leads to polymerization of the glycer ol. The glycerol may then be bleached using activated carbon or clay.

Physical Refining: The first step in physical refini ng is to remove fatty, insoluble or precipitated solids by filtration a nd/or centrifugation. This removal may require pH adjustment. Then the water is removed by evaporation. All p hysical processing is typically conducted at 150 – 200 °F, where glycerol is less viscous, but still stable.

Glycerol Purification: The final purification of glycerol is completed using vacuum distillation with steam injection, followed by activated carb on bleaching. The advantages of this approach

are that this is a well-establis hed technology. The primary disadva ntage is that the process is capital and energy intensive. Vacuum distillation of glycerol is best suited to operations > 25 tons per day.

Ion exchange purification of glycerol is an a ttractive alternative to vacuum distillation for smaller capacity plants. The ion exchange system uses cation, anion, and mixed bed exchangers to remove catalyst and other impurities. The glycerol is first diluted with soft water to a 15 to 35 percent glycerol-in-water solution. The ion excha nge is followed by vacuum distillation or flash drying for water removal, often to an 85 percent partially refined glycerol.77 The advantage of this process is the fact that all purif ication takes place in the resin vessels so the system is suited to smaller capacity operations. The disadvantages are that the system is subject to fouling by fatty acids, oils and soaps. The system also requires regeneration of the beds producing large quantities of wastewater. Regeneration requires parallel systems to operate and regenerate simultaneously.

Wastewater Considerations

Ester washing produces about 1 gallon of water pe r gallon of ester per wash. All process water

must be softened to eliminate calcium and ma gnesium salts and treated to remove iron and copper ions. The ester wash water will have a fa irly high BOD from the residual fat/oil, ester, and glycerol.

The glycerol ion exchange systems can produce larg e quantities of low salt waters as a result of the regeneration process. In addition, water so ftening, ion exchange and cooling water blowdown will contribute a moderate dissolved salts burden.

The aggregate process waste waters should meet local municipal waste treatment plant disposal requirements, if methanol is fully recovered in the plan t and not present in the wastewater. In many areas, internal treatment and recycle of th e process water may lead to cost savings and easier permitting of the process facility.

Summary

Methanol affects all product recovery operations. The methanol must be fully recycled for best economy and for pollution prevention. Glycerol is an economically signifi cant co-product that should be as fully refined as practicable. Prope rly managed, wastewaters can be treated in a municipal sewer system, but internal treat ment and recycling should be considered.78 

## Laboratory – Exercise 2

Pretreatment of High Free Fatty Acid Feedstocks

Equipment needed:

Sulfuric acid

Methanol Four 500 ml Erlenmeyer flasks One liter each of four feedstocks of varying FFA level Four hot plate/stirrers Scale (reads to 0.1 g)

Divide the class into four groups . Each group should have different feedstocks with varying free fatty acid levels.

Procedure:

• Measure the acid value of the feedstock. Reme mber that FFA% is approximately equal to

half the acid value.

• Put 200 g of the feedstock into a 500 ml flask, heat to 60-65oC.

• Measure out in a 125 ml flask an amount of methanol equal to 2.25 g of methanol for every gram of FFA in the flask. Add an amount of concentrated sulfuric acid equal to 0.05 g for every gram of FFA in the flask.

Remember to always add acid to the alc ohol and not the other way around. Why?

• Gently agitate the methanol-acid mixture until well-mixed. Add to the feedstock. Agitate

at 60-65oC for one hour. Let cool.

• Measure the acid value of the fluid.

You might not notice any separation of a methanol-water layer. This can take some time to occur

because the densities of th e two layers are close.

Reminder: Acid Value Measurement 1) Prepare two 600 ml beakers by adding 125 ml of solvent. The solv ent consists of 50% isopropyl alcohol/50% toluene and s hould be available in the lab. 2) Add about 5 grams of sample (weigh to get exact amount) to one of the beakers. 3) Add 2 ml of phenolphthalein indicator to both beakers. 4) Titrate both beakers with 0.1 N KOH to the first permanent pink color in each. 5) Calculate the acid value from: AV = (A-B) x N x 56.1/W

where A = number of ml of KOH to neutralize sample beaker

B = number of ml of KO H to neutralize blank beaker N = normality of KOH solution (0.1 in this case) W = weight of sample in grams

Remember that %FFA ≈ 1⁄2 (AV)79 

## 12. Feedstock Preparation

It has been mentioned earlier that biodiesel can be produced from a wide variety of feedstocks including vegetable oil, animal fats, restaurant waste oils, and trap grease. Another way to categorize these feedstocks is to identify those coming from oilseeds and those that originate from rendering plants. This section will discu ss the processes used to produce these two categories of feedstocks and the imp lications for biodiesel processing.

Production of oil from oilseeds

Soybean oil is the largest source of vegetable oil in the United Stat es and rapeseed oil (Canola) is the largest source in Eu rope. Most of these oils are used as food products but they require considerable processing before they are consid ered edible. Table 9 shows the composition of crude soybean oil compared with a fully refined oil. Fully refined oils are also known as RBD oils where RBD strands for refined, bleached, and deodorized. Table 9 shows how a crude soybean oil typically contains 1.5 to 2.5% phos phatides, 1.6% unsapoifiable matter including 0.15 to 0.21% tocopherol (Vitamin E), and 0.3 to 0.7% free fatty acids. These compounds contribute color, odor, and strong flavors to the oil that are usually unde sirable for cooking. Some of the compounds also contri bute to poor storage stability.

It has been shown that high quality biodiesel can be produced even from crude soybean oil [8]. However, use of partially or fully refined oil si mplifies the biodiesel production process so some producers have chosen to use degummed oils or even RBD oils. The following discussion describes current practice for processing soybean o il in the U.S. Similar processes are used for rapeseed oil in Europe.

Table 9. Average Composition for Crude and Refined Soybean Oil [7, p. 14]

Crude Oil Refined Oil Triglycerides, % 95 – 97 >99 Phosphatides, % a,b 1.5 – 2.5 0.003 – 0.045 c Unsaponifiable Matter, % 1.6 0.3 Plant sterols, % d 0.33 0.13 Tocopherols, % e 0.15 – 0.21 0.11 – 0.18 Hydrocarbons, % f 0.014 0.01 Free fatty acids, % 0.3 – 0.7 < 0.05

Trace Metals a
Iron, ppm 1 – 3 0.1 – 0.3 Copper, ppm 0.03 – 0.05 0.02 – 0.06

a Evans, C.D., G.R. List, R.E. Beal, L.T. Black. 1974. JAOCS. 51:444-448.

b List, G.R., A.J. Heakin, C.D. Evans, L.T. Black, T.L. Mounts. 1978. JAOCS. 55:521-522.

c Corresponding to 1-15 ppm of phosphorus

d Weihrauch, J.L., J.M. Gardiner. 1978. J. Am. Diet. Assoc. 73:39-47.

e Bauernfeind, J.C. 1977. Crit. Rev. Food Sci. 8:337-382.

f Gutfinger, T., A. Letan. 1974. J. Sci. Food Agric. 25:1143-1147.80 Oil is removed from soybeans by two techniques : mechanical crushing and solvent extraction. Mechanical crushing is prefered for smaller pl ants because it requires a smaller investment. Typically plants that process less than 100,000 kg/day use mechanical crushing and plants that process more than 300,000 kg/day us e solvent extraction [4, p. 113].

Before the oil can be extracted, the seeds must be prepared. This involves removing stems, leaves, stones, sand, dirt, and weed seeds. The st ems and stones can be screened out because they are usually larger than the seeds. Weed seeds, sand, and dirt are usually smaller and can be removed by finer screens. Magnets can remove iron particles.

After cleaning, the seeds are often dehulled. The hulls are abrasive and c ontain very little oil (less than 1%). Removing the hulls reduces the w ear on the screw press. It can also increase the

extraction of oil because the hulls absorb oil. On oilseeds with thick sh ells, dehulling typically involves cracking the shells and then separating the shells by sc reening or aspiration. With soybeans, dehulling generally includes removal of residual fines.

Oilseeds that are to be processed by solvent extrac tion are usually flaked to increase the exposure

of the oil to the solvent. Hexa ne in a percolation extractor is the most common technology used today. This type of extractor drips the solvent down on the flaked soybeans so it can dissolve the oil in a manner similar to a coffee pe rcolator. The oil-laden solvent, or miscella , is then filtered

and removed from the extractor. To remove the so lvent, the miscella is heated to vaporize the solvent. This gets the the mis cella to about 5% solvent [4]. Then the remaining solvent is stripped by injecting steam into the oil. The st eam and hexane vapors are condensed and, since the two fluids are insoluble, they can be separa ted in a settling tank. Figure 1 shows a schematic of a typical solvent extracti on plant that includes several stages of solvent recovery.

The alternative to solvent extraction is mech anical extraction or crushing. The beans are generally preheated to destroy enzymes that cause problems with using the meal as animal feed. One popular way of cooking the bean is to use an expanding extruder. This device compresses the beans to very high pressure so that the temperature rises to 150o - 160oC, which is high enough to deactivate the enzymes. The extrusion al so frees the oil and produces a frothy, liquid that quickly solidifies as the water flashes to vapor and reduces the moisture level from 10-14% to 6-7%. After cooking or extrusion, the oil seeds are usually fed to a screw press where they are compressed to high pressure. Draina ge slots are provided for the oil to leave the press. Moisture control is important to maintain proper temperature. If the moisture level is too high, then beans

will be soft and the oil will contai n excessive solids. If the moisture level is too low, then the oilseeds will overheat. Ideally, the moisture shou ld be 2-3% for full pressing and 4-6% for prepressing [4]. If the beans have not been extruded , then pre-pressing may be used to break the cell

walls and release the oil from the seed. It can preceed both mechanical and solvent extraction.

After extrusion from the seed, crud e soybean oil contains impurities that can be classified into two categories: oil insoluble and oil soluble [1]. The oil-insoluble impurities consist of seed fragments and meal fines, free water, and long-chain hydrocarbons or waxes that cause cloudiness when the oil is refrigerated. Most of this material can be removed by filtration. As described in Table 1, the oil-soluble materials include free fatty acids, phosphatides, gummy or mucilaginous substances, color bod ies, protein, tocopherols, ster ols, hydrocarbons, ketones, and81 aldehydes. Some of these compounds, particularly the ketones and aldehyd es, can cause the oil to have an unpleasant taste and smell. Other compounds are actually desirable, such as the tocopherols which act as antioxida nts. The sterols are relatively in ert and can remain in the oil. The food-grade oil typically contains 0.5 - 1. 5% non-oil compounds that are known collectively

as unsaponifiable matter

.

Processing of crude soybean oil to food-grade o il generally consists of three steps: refining (degumming and caustic refining), bleaching, and de odorization. As mentioned earlier, these oils are known as RBD oils (refined, bleanched, and deodorized).

Refining

The refining step is designed to remove the phos pholipids and free fatty acids from the crude oil. Crude soybean oil typically contains 500-700 ppm of phosphorus which corresponds to 1.5% to 2.1% phospholipids [1]. The main reason to rem ove these phospholipids is that some of the compounds, particularly the calcium and magne sium salts of phosphatidic and lysophatidic acids, are strong emulsifiers. If these com pounds are still present during the later alkali neutralization step, they will inhibit the separation of the soaps and lower the yield of neutral oil. Phospholipids (also called phosphotides) will also r eact with water to form insoluble sediments

that are not desirable when cooking with the oil.

Refining may consist of two steps. The first step is degumming. In this st ep, crude soybean oil is

mixed with 1% - 3% of water and the mixture is agitated mechanically for 30-60 minutes at 70o C (158o F). This hydrates the phosph olipids and gums and these hydrat es are insoluble in the oil. They can be separated by settling, filtering, or centrifuging. The phosphorus content can be lowered to 12-170 ppm by this procedure. The byproduct of water degumming has value as a feedstock for lecithin production.

Table 1 shows the profile of phospholipids from lecithins produced from various oilseeds. It shows that there are significant differences in the distribution of compounds between the

different feedstocks.

A portion of the phosphatides are not hydratable by contact with water alone. The addition of citric or phosphoric acid will hydrate those rema ining. Citric acid is pr efered if the byproduct is to be used for lecithin. The acid hydrat ion is accomplished by adding 0.05-0.2 wt% of concentrated (85%) phosphoric acid to crude oi l, which has been heated to 60 - 85 C. The

residence time varies from a few seconds to 1-2 minutes depending on the type and quality of the oil. The extent of gum removal by these tec hniques is related to the strength of the acid treatment. Figure 29 shows a schematic diagram of the processes involved to degum soybean oil and produce edible lecithin.82Figure 28. Soybean oilextraction plant [7].83 Table 10. Phospholipid Composition [wt%] of Commercial Lecithins [2, p. 52]

Phospholipid Soy Rapeseed Sunflower seed Phosphatidylcholine 21.9 24.6 25.4
Phosphatidylethanolamine 13.6 22.1 11
phosphatidylserine 0.8
phosphatidylinositol 12 14.7 19.4

PG + DPG* 2.3 1.2
Phosphatidic Acid 5.8 3.3

N -Acyl-phosphatidylethano

lamine 2.8 1

Lyso-phospholipids 2.9 19.4

Others 3.6 19.2

* PG + DPG = Phosphatidylglycerol and Diphosphatidyl glycerol. Note: some comm ercial lecithins contain glycolipids (phosphorus-free polar lipids) that are not included in the totals given in the table.

Figure 29. Flow diagram for soybean oil degumming [7].

The second phase of refini ng is neutralization, or caustic refining . This process is to remove the free fatty acids present in the cr ude oil. An alkali solution, us ually sodium hydroxide, is added that reacts with the free fatty acids to produce so aps. The soaps are insoluble in the oil and easily84 separated by water washing. Th e alkali solution also neutralizes any acid remaining from the degumming stage.

The alkali will react with the trig lycerides in the oil also, so th e neutralization parameters (type or alkali, solution strength, temperature, agitati on, and time) must be optimized to minimize the yield loss. There can be additional losses from emulsification and suspension of oil droplets in the soap solution. A byproduct of caustic refini ng is a mixture of soap, water and oil known as soapstock. This has been considered as a feedstoc k for biodiesel as its cost is low, but its high water content and converting the soaps to methyl esters are significant obstacles to cost-effective utilization. Figure 30 shows a diagra m of the caustic refining process.

Figure 30. Flow Diagram for Causti c Refining of Soybean Oil [7].

Bleaching

The primary purpose of bleaching is to remove the co lor pigments from the oil. It also helps to remove remaining soap, trace metals, phosphatid es, and sulfur compounds. Hydroperoxides, the initial products of oxidation are broken down duri ng the bleaching process and some of the final oxidation products, the carbonyl compounds, are removed.

Bleaching involves mixing bleaching clays, consis ting of natually occurring bentonite and montmorillonite clays, with the oil and agitating for 10-30 minutes. The oil is usually heated to 90o - 120o C and the process occu rs under a slight vacuum to ex clude oxygen. A consequence of the vacuum and high temperature is that resi dual water from the neutralization and water washing is removed. This is not always desirable because the effectiveness of the clays is usually enhanced by the presence of a small amount of water. The clays are usually activated before use85 with a mineral acid such as sulfuric acid. This removes some of the minerals from the clay and produces a larger volume of micropores and smaller clay particles.

The bleaching clays are mixed with the oil, either directly or by premixing with a small amount of oil to produce a slurry and then adding the sl urry to the oil. After the adsorption processes have reached equilibrium, the clay is filtered out by self-cleaning filters.

The peroxide value of the oil is very low after bleaching becau se the hydroperoxides have been eliminated but the free fatty acid level may be slightly higher. This may be due to some of the sulfuric acid leaching out of the clay or to hydrolys is of the oil. Hydrolysis is a chemical reaction between the oil and water that produc es free fatty acids and glycerin.

Deodorization

Deodorization is the final processing stage before the oil is ready for use in food products. The

deodorization removes the trace components that give the o il an unpleasant taste and odor. Deodorization is essentially a dist illation process that occurs at high temperature (200o - 260o C) and low pressure (2 - 7 torr, 2.5-9.2 mbar). The odor causing compounds are more volatile than the oil and will be removed by extended heat ing at high temperature and low pressure.

The first step of deodorization is deaeration. This step removes all the dissolved oxygen so that the oil does not oxidize at the high temperatures used in deodorization. In batch deodorizers, the deaeration occurs naturally while th e oil is heated from the typical inlet temperature of 40o - 80o C to the deodorization temperatur e range of 200o - 260o C. If adequate vacuum is available, the oil should be deaerated by the time the oil reaches 100o - 120o C.

Preheating is sometimes identified as the sec ond step of deodorizati on. In continuous flow deodorization processes, a portion of the heat required for heating the oil comes from cooling the deodorized oil. As much heat as possible is tran sfered from the final pr oduct to incoming oil. After preheating, the oil is heated to its final temperature by steam or another heating fluid such

as Dowtherm A.

After the oil has been held at high temperature for sufficient time (2 - 5 hours in a batch deodorizer), it is cooled to 120 o C where it is common practice to add 50 - 100 mg/kg of citric

acid. The citric acid is added to chelate trace meta ls in the oil so that they can be removed by filtration. Chelation is a chemical process wh ere a compound, known as a chelating agent, binds to a metal compound and makes it unavailable for furt her reaction. Then the oil is cooled further to its storage or bottling temperature. One final stage of filtration, known as polish filtration, occurs immediately before the oil leaves the plant.

As mentioned earlier, biodiesel can be produced from oil at any stage in the various processes. The compounds removed during refining, bleachi ng, and deodorization are mostly removed by water washing or collect in th e glycerol phase. Biodiesel produced from crude soybean oil

retains somewhat more color than biodiesel fr om refined oil but phosphorus and free fatty acid levels seem to be equivalent [8].86 Table 11. American Fats and Oils Associatio n Specifications for Tallows and Greases* Grades Titre, min

oC FFA, max FAC, color, max R&B, max MIU

Edible tallow 41.0 0.75 3 None **** Lard (edible) 38.0 0.50 ** None   Top white tallow 41.0 2 5 0.5 1 All beef packer

tallow 42.0 2 None 0.5 1

Extra fancy tallow 41.0 3 5 None 1 Fancy tallow 40.5 4 7 None 1 Bleachable fancy

tallow 40.5 4 None 1.5 1

Prime tallow 40.5 6 13-11B None 1 Special tallow 40.0 10 21 None 1

No. 2 tallow 40.0 35 None None 2

A tallow 36.0 15 39 None 2

Choice white

grease 36.0 4 13-11B None 1

Yellow grease ** * 15 39 None 2

* from the National Renderers Association ** Lovibond color 5-1/4 inch cell – max 1.5 red. Lard peroxide value 4.0 ME/K max. *** Titre minimum, when required, to be negotiated between buyer and seller on a

contract basis. **** Moisture maximum 0.20%. Insoluble impurities maximum 0.05%.

Rendering

Rendering is the process whereby by-products of the food industry, including animal fat, bone, offal, hides, feathers, and blood are recycled in to usable products. In the case of rendering operations that may be attached to slaughtering facilities, the products may be edible fats and proteins that are suitable for human consumption. These edible fats may be refined, bleached and

deodorized in a manner similar to the described above for vegetable oils. This provides a product that is suitable for food, cosmetics, and other high value uses. Table 11 shows the specifications for the wide variety of edible and inedible ta llows and greases produced from rendered fats.

Larger volumes of material are processed in plan ts that produce so-called “inedible” products, although the products are suitable for use in animal feeds. Thes e inedible rendering plants may process fat and bone trimmings, meat scraps, restaurant grease, fa llen animals, and other sources of fat and protein. Table 12 shows the compositi on and tallow and grease yield of various raw materials for inedible rendered products.

Figure 31 shows a typical rendering operation. Dead stock carcasse s and other raw materials are received by the plant and fed in a continuous flow to a crusher that grinds the material to a uniform paste-like consistency.87 Table 12. Composition of Raw Materials for Inedible Rendering Products* Source Tallow/grease

wt% Protein solids

wt% Moisture

wt% Packing house offal and bone

Steers 30-35 15-20 45-55

Cows 10-20 20-30 50-70

Calves 10-15 15-20 65-75 Sheep 25-30 20-25 45-55 Hogs 25-30 10-15 55-65 Dead stock (whole animals)

Cattle 12 25 63

Calves 10 22 68 Sheep 22 25 53 Hogs 30 28 42 Butcher shop fat and

bone 31 32 37

Blood None 16-18 82-84 Restaurant grease 65 10 25 Poultry offal 10 25 65 Poultry feathers none 33 67

* from National Renderers Association

This material is then fed to a cooker that heats the material to 121-135oC (250-275oF). The high temperature encourages the removal of water and separates a portion of the fat from the solids. The fat and solids are fed to a drain pan where th e fat is drained off and the solids are fed to a screw press where the remaining fat can be s queezed out. The dry solids are ground to a fine powder and sold as meat and bone meal. The fat is centrifuged or f iltered to remove particulates. Since the fat may still contain some water, it is common to reheat the fat with steam injection to encourage separation of the water. After heating, the dry fat will rise to the top, the water will

settle to the bottom and an inter phase layer will appear that consists of emulsified water and oil and possibly protein and minerals. This material is separated and retu rned to the cooker for reprocessing.

Inedible fats with free fatty acid levels below 15% and MIU (moisture, insolubles, and unsaponifiables) less than 2% are generally sold as yellow grease . Fats with higher free fatty acid

levels are sold as brown grease . Although their products are refe rred to as inedible, most

rendered material is reintroduced to the food chai n as animal feed. Meat and bone meal are used to add protein and minerals to feed and grease is used to increase calories or energy content. Renderers are coming under increasing scrutiny becau se of concerns about disease transmission. Most renderers will no longer accept sheep carca sses in the United States because of concerns about scrapies. The same concern also exists with deer and elk carcasses, due to chronic wasting disease, a relative of Bovine88 Figure 31. Rendering plant operations

Spongiform Encephalapathy (BSE – Mad Cow Diseas e). Products from ruminants(cattle) are not fed to other cattle. This requirement has been reinforced by recent events in England where BSE has spread to large herds of ca ttle through use of infected animal products in the feed. The risks of disease transmission are thought to be minimal in the United States and use of animal products across species is generally considered safe. Ho wever, concerns about BSE may cause future legislation further restricting th e use of animal byproducts in an imal feed and if this occurs, biodiesel may become the most significant market for animal fat.

References

1. Hui, Y.H., Editor, Bailey’s Industrial Oil a nd Fat Products, V. 2. Edible Oil and Fat Products: Oils and Oilseeds, Chapter 11 Soybean Oil, Fifth Edition, Wiley-Interscience, New York.

2. Schneider, M., “Phospholipids,” Chapter 3 from Lipid Technologi es and Applications , Edited by F.D. Gunstone and F.B. Padley, Marcel Dekker, New York, 1997.89 3. Allen, D.A., “Refining,” Chapter 6 from Lipid Technologies and Applications , Edited by F.D. Gunstone and F.B. Padley, Marcel Dekker, New York, 1997.

4. Williams, M.A., “Extraction of Lipids fr om Natural Sources,” Chapter 5 from Lipid Technologies and Applications , Edited by F.D. Gunstone and F. B. Padley, Marcel Dekker, New York, 1997.

5. Peterson, C.L., M. Feldman, R. Korus, a nd D.L. Auld, “Batch Type Transesterification Process for Winter Rape Oil,” Applied Engineeri ng in Agriculture, Vol. 7, No. 6, Nov. 1991, pp.

711-716.

6. Peters, Richard, Process Chemical E ngineer for Proctor and Gamble, Personal

Communication.

7. Erickson, D.R., E. H. Pryde, O.L. Brekke, T.L. Mounts, and R.A. Falb, Editors, “Handbook of Soy Oil Processing and Utilization,” Published jo intly by the American Soybeasn Association, St. Louis, MO, and the American Oil Chemists” Society, Champaign, IL, 1980.

8. Van Gerpen, J.H. and B. Dvorak, “The Effect of Phosphorus Level on the Total Glycerol and Reaction Yield of Biodiesel,” pr esented at Bioenergy 2002, The 10 th Biennieal Bioenergy Conference, Boise, ID, Sept. 22-26, 2002.

9. Franco, D.A. and W. Swanson, The Original Recyclers , Animal Protein Producers Industry,

1996.90 

## 13. Feedstock Quality Issues

As a general rule, the closer a feedstock is to c ontaining pure triglycerides, the easier it will be to convert it to biodiesel, and the hi gher will be its cost. The lower th e cost of the feedstock, the more difficult and more expensive it is to convert to biodiesel.

Biodiesel feedstocks will contain many possible contaminants such as water, free fatty acids, particles, and phospholipids. There is potential for each of these contaminants to impact the quality of the final biodiesel product.

Water

It is important that water be kept out of the biodiesel production process. While most processes can tolerate up to 1% water, even this low le vel will increase soap production and measurably affect the completeness of the transesterification reaction. Wa ter can be removed by several techniques. Heating to break any emulsions be tween the water and oil and then allowing the

water to settle to the bottom can remove water. Ce ntrifuges can also be used to separate water and oil. Heating the oil under pr essure and then spraying into a vacuum chamber can bring the water content to a very low level.

Crude soybean oil typically contai ns about 0.3% water. This can be removed easily by a flash vaporization process. Yellow grease (and usually brown grease) will contain about 0.5% water because it has been dewatered during previous pr ocessing steps, although the sum of the water, insolubles, and unsaponifiables (MIU) is allowed to be as high as 2%. Restaurant grease and trap grease that have not been rendered should always be checked for water content because it may be very high.

Water not only impacts the transesterification pro cess, it can impact the effective cost of the feedstock. Analyses of two actual samp les of trap grease are shown below.

Sample A Sample B

Water 81.5% 40.5%

Solids 6.34% 10.2% Lipids 12.2% 49.3%

Trap grease is a very inexpensive feedstock. In most cases, restaurants will pay to have the material removed. However, even if the cost to have the material brought to the plant were as low as 2 cents/lb, the actual cost for the lipid material in sample A would be more than 8 times higher, or over 16 cents/lb. This is a pretty high price for material that is almost all free fatty acids. This price also does not include the added cost of disposing of the water after it is removed or the cost of landfilling the solids.

Solids

Feedstocks used for biodiesel should always be filtered to ensure no particles enter the processing scheme. This can be a particular problem with recycled products such as trap grease,91 which typically contains fine particles that may require a fine filter. A 100 μm filter will suffice to remove particles from most f eedstocks although all fuel leavi ng the plant should be filtered to 5 μm.

Free Fatty Acids

The impact of free fatty acids (FFAs) on the bi odiesel production process have been discussed extensively elsewhere. While FFAs can impact the production process by deactivating the catalyst, creating soap, and by releasing water wh en they are converted to esters, they do not seem to impact the final acid value of the bi odiesel. Biodiesel produced using a normal alkalicatalyzed process that meets the ASTM specifica tion for total glycerol will usually have 0.2- 0.3% FFAs regardless of the initial acid content of the feedstock. The alkali catalyst also serves as a caustic stripper and rem oves the FFAs by converting them to soap that is removed during washing.

Phosphorus

Crude soybean oil contains from 600-900 ppm of phosphorus in the form of phospholipids or gums. This material must be removed because the ASTM specification from biodiesel limits the phosphorus content to 10 ppm. Fort unately, even if the oil is not degummed to remove this material, the usual biodiese l production process removes th e phosphorus and it is either transferred to the glycerol fraction or removed by water washing. There does not seem to be any effect of the initial phosphorus content of the feedstock on th e final phosphorus content of the

biodiesel.

Sulfur

One of the frequently-touted advantages of biodiesel is that it contains very low levels of sulfur. The Environmental Protection Agency has mandate d that sulfur levels for petroleum-based diesel fuel must drop to 15 ppm from 500 ppm by 2006. Most biodiesel is already below this level. However, some samples of biodiesel from yellow grease and animal fat have shown sulfur levels as high as 40-50 ppm. The source of this sulfur is still not comp letely understood, but it may be originating from food preservatives, protei ns, the hair on the animal hides and in those cases where poultry are being processed, from chicke n feet. Additional research is needed in this

area.

Iodine Value

The iodine value is a very crude but commonly used indicator of the level of saturation of an oil. While saturation and fatty acid profile do not seem to have much impact on the transesterification process, they do affect the properties of the biodiesel. Saturated fats produce a biodiesel fuel with superior ox idative stability, a hi gher cetane number, but poor low temperature properties. Biodiesel from saturated fats is more likely to gel at ambient temperatures than biodiesel from vegetable oils , especially soybean oil.92 

## 14. Plant Safety

Introduction

The responsibility for the safe operation of any chemical process facility, including a biodiesel plant, begins with the CEO of the company a nd extends to every employee and visitor on the plant site. Safety is a result of training, operating, maintaining, and preparing to prevent unplanned and dangerous events in the plant. It is a way of thinking and acting during routine activities and in response to extraordinary situations.

There are numerous regulatory requirements associated with training and housekeeping to monitor and improve safety in the workplace. Regulations and procedur es dictated by OSHA, EPA, and other agencies apply to biodiesel plan ts of any size. Some of these requirements are covered in other modules. In addition, every pl ant should have one person with the designated responsibility for overseeing the traini ng and inspections in the workplace.

Given this introduction, this module will focus on some of the key elements of a plant safety program, safe design, and safe operation. The empha sis is on elements of safety associated with the operation of a chemical process facility. This module is intended to be a reminder for what must be known and taught, more than detailed instru ction in all of the elements of plant safety.

Basic Definitions

• Safety : The prevention of accidents by the use of appropriate technologies to identify the hazards of a chemical plant and to e liminate them before the accident occurs.

• Hazard: Anything with the potentia l for producing an accident.

• Risk: The probability of a hazard resulting in an accident.

Keys to a Successful Safety Program

•Safety Knowledge – Every person associated with the plant must know what the materials used in the plant are and what their hazards are. The people must be trained in the correct and safe operation of the processing equipment, and know how to respond in an emergency situation.

•Safety Experience – Training is more than a le cture, a video module, or reading a manual. Personnel must have hands-on training in pr oper operation of equipment and develop an understanding of how to recogni ze and correct dangerous situations. The use of process simulators is needed, as is training with actual safety equipment.

•Technical Competence – The ability to use tr aining in proper operation and proper safety precautions must be demonstrated and documented on a regular basis.93 •Safety Management Support – Safety must be modeled and monitored throughout the management chain.

•Commitment – Demonstrated commitment to safety in the workplace and beyond must begin with the CEO and permeate all employees. Breaches of safe procedures must be clear dismissal grounds and enforced.

Types of Chemical Plant Accidents

A biodiesel facility is first and foremost a chemi cal process facility. As such, the plant is subject to the same dangers as any other chemical pro cess plant. Table 13 lists categories of accidents that are uniquely associated with chemical plan ts. Note that the likeli hood of an event and its potential impacts are not closely related.

Table 13. Types of Chemical Plant Accidents

Probability Potential

Fatalities Potential $ Loss

Fire High Low Moderate

Explosion Moderate Moderate High

Toxic Release Low High Low

For a biodiesel plant, the greatest risk is asso ciated with methanol. It presents a fire and explosion hazard as well as being toxic th rough oral ingestions of skin contact.

In order to better understand how acc idents occur in a chemical plan t, it is useful to examine the causes for loss with respect to the source of the loss event. These causes are listed in order of frequency in Table 14.94 Table 14. Causes for Loss in a Chemical Plant Accident (in order)

•Mechanical Failure •Operational Error

•Unknown – Miscellaneous •Process Upset

•Natural Hazard •Design Error •Arson – Sabotage

Since the most common cause for loss is mechanical failure, it is useful to examine the specific equipment items responsible for the failures. Th ese items are listed in order in Table 15.

Table 15. Hardware Associated with Losses

•Piping Systems

•Miscellaneous of Unknown •Storage Tanks •Reactor Piping Systems •Process Holding Tanks •Heat Exchangers

•Process Towers •Compressors •Pumps •Gages

Accident Sequence

It is rare that there is a single causal event that leads to a loss accident. Indeed, one of the most important ways to prevent accidents is to examine carefully the chain of ev ents that result in an accident and that bring the accident sequence to an end. These lesson’s learned provide the basis for anticipating potential accident situations and planning interventions that halt the accident process before a loss occurs. The accident sequence can be described as follows:

•Initiation – event that begins the accident

•Propagation – event(s) that sustain or expand the accident

•Termination – event(s) that stop the accident or diminish it is size

The propagation steps in an accident sequence ar e generally associated with potential hazards that exist – accidents ready to ha ppen. In many cases these steps are the results of intentional acts with unintended consequences. For example, tu rning off alarms and sensors during servicing makes the service personnel vulnerable to accidents that would have been prevented if the alarms were operational. The chemical process industr y has a special category of hazards present95 because the industry is based on th e utilization and tran sformation of chemical raw materials into new chemical products. These hazards are linke d to the physical and chemical reactivity properties of the various chemicals present. The categories of chemical process hazards the may be present in a chemical facility are listed in Table 16.

Table 16. Categories of Chemical Process Hazards

•Explosion •Gases – toxic, flammable, compressed •Flammable Liquids

•Flammable Solids •Oxidizers and Organic Peroxides •Poisons, Pesticides, Carcinogens

•Radioactives

•Corrosives

Identifying Hazards

The most fundamental tasks in developing a safety program for a chemical process facility are summarized in the sequence shown below.

•What are the hazards? – Identification •What can go wrong and how? – Scenario Building

•What are the chances? – Risk •What are the consequences? - Results

There are a number of formalized procedural tool s for hazard identificatio n. Any given facility may choose to use one or more of these tools. The tools include:

• Process Hazards Checklist – Use a standard checklist that covers possible hazardous conditions in the operation. When a hazard is noted, the action step is to remove the hazard, or to mitigate its effect.

• Hazard Survey – Use an individual or a team to inspect a specific process or operational area, looking for hazardous condition s. Again, the action step is to eliminate or mitigate the hazard.

• Hazard and Operability Studies HAZOP – This is a formalized approach that examines every piece of equipment in a process on the ba sis of its function. The questions of cause and effect for each operation are framed in terms of the item not functioning, under functioning and over functioning (NOT, LESS, MORE, etc.). The effect of each condition is estimated, along with the likelihood of the cause being present. This process helps identify likely, significant faults for correction.96 • Safety Review – An independent, experi enced team reviews operations and safety procedures in use at a facility. The review includes simulated incidents and monitors the response to the incidents.

Toxic Hazards

In the chemical process industry there is the hazard of exposure to chemicals that are toxic. Toxicity is defined as the ability of a chemi cal substance to cause physical harm to a living organism. The harm may range from a temporary irritation to permanent loss of function, to death. The toxic effect may be the result of a lo ng term, cumulative effect (chronic toxicity) or a rapid response to the presence of the toxicant (acute toxicity).

In either case, a key factor in the response is the amount of material pr esented in the exposure (dose) and the degree of toxicity exhibited by th e material. The dose is typically measured in terms of amount per kg of body weight. The degrees of toxicity based upon the dosage required to cause death in 50 percent of the ex posed subjects is shown in Table 17.

Table 17. Degree of Toxicity LD 50 (dose per kg body weight)

•< 1.0 mg Dangerously toxic •1.0 – 50 mg Seriously toxic •50 – 500 mg Highly toxic •0.5 – 5 gm Moderately toxic •5 – 15 gm Slightly toxic •> 15 gm Extremely low toxicity

When working with and around various chemical s, there are a number of paths for toxic materials to enter the body. These routes include:

•Ingestion – mouth or stomach

•Inhalation – mouth or nose •Injection – skin cuts •Dermal absorption

The toxic hazards can be eliminated from th e body by excretion (kidneys, liver, lungs, other), detoxification through internal biotransformation, or by storage, typically in fatty tissue or bones. Methanol is the primary issue for toxic exposure in a biodiesel plant.

Because of the risks from chronic exposure, th ere are limits to allowable exposures to most

chemicals. These limits are based on a combinati on of time and concentration. these limits are

termed “threshold limit values” and are described below.97 Threshold Limit Values

•TLV-TWA: Time weighted average, 8 hr Day

•TLV-STEL: Time weighted average, short-term exposure, < 15 min

•TLV-C: Ceiling limit, do not exceed at all

A required source of information on the hazards associated with chemicals in use in the workplace is the Material Safety Data Sheet. Th ere must be copies of the MSDS for every

chemical in use in each area of the chemical plant and associated laboratories available in a public, easily located place. The hazard prope rties and recommended methods of handling each compound are included in the MSDS. All employees must be familiar with the chemicals that they are working with and must follow the recomme ndations for their handling, use and disposal.

Designing for Safety

There are a number of safety enhancement stra tegies –that can be applied during the design phase for any chemical plant. They include:

•Substitution – Less hazardous chemicals and equipment •Attenuation – Less hazardous ope rating conditions = “less” •Isolation – Isolate equipment from sources of hazard or from risk of harm to personnel

•Process intensification – smaller/ higher throughput; smaller inventory

Similarly, there are safety enhancement strategies to be applied during th e operating phase of a chemical plant:

•Enclosures

•Local Ventilation

•Dilution Ventilation •Wet Methods and Cleaning •Good Housekeeping

•Personal Protection

Summary

•Safety is PREPARED

•Safety is PREPLANNED

•Safety is PREVENTION

•Safety is PREEMINENT98 

## 15. Biodiesel Transportation and Storage

There are several key properties that need to be considered in the trans portation and storage of biodiesel. These properties include exposure temperature, oxidative stability, fuel solvency, and material compatibility. Exposure te mperature refers to the temperat ure at which the biodiesel is being stored or transported. Oxidative stability co ncerns the ability of the biodiesel to be stored for extended periods without degradation. Fuel so lvency refers to the solvent properties of the

biodiesel. The interaction of the biodiesel with ma terials of construction for storage tanks, seals, and gaskets is the material compatibility. Each of these properties will be discussed in more detail and within the context of storage.

All types of diesel fuels can gel at low temperat ures. The temperature required to induce gelling (freezing) in biodiesel is genera lly higher than that of conventi onal diesel fuel and depends on the composition of the biodiesel. In particular, sa turated methyl esters will have higher freezing points than unsaturated methyl esters. The compos ition of the biodiesel is, of course, dictated by the oil feedstock used for its production.

As with gelling, all diesel fuels have the potential for oxidative stability issues. Oxidative stability concerns the degradation of diesel fu el due to reaction of the fuel with oxygen and catalysts. As such, the reactivity of the fuel is important. This reactivity can be related to the presence of C=C (olefinic) bonds in the fuel w ith increased content of the C=C bonds correlated to decreased oxidative stability of the diesel fuel. The increase in instability of a given diesel fuel molecule is generally directly proportional to the increase in the number of C=C bonds in the molecule (i.e., a molecule containing two C=C bonds has half the stability of a molecule containing one C=C bond). The oxidativ e stability of a diesel fuel can be estimated using the iodine number (ASTM D 1510). The longer term stabil ity of a diesel fuel can be evaluated using an accelerated stability test (AST M D 2274). Oxidative degradation of diesel fuel is manifest by higher acid numbers, increased viscosity, a nd the formation of gums and sediments.

While biodiesel is only a mild solvent, it does ha ve higher solvency properties than diesel fuel. Due to this property, residual sediments in dies el storage tanks or vehi cle fuel tanks can be solvated by biodiesel. The solvent properties of biodi esel are mitigated by the use of biodiesel in blends with diesel fuel. Typically, 20% or less blends of biodiesel in diesel will nearly completely dilute the solvency effect. However, th is solvency effect should be considered if pure

biodiesel is stored in a tank that was previously used for standard diesel.

The material compatibility is also an important pr operty to consider with bi odiesel. It is best to use stainless steel or aluminum equipment in th e processing of biodiesel. Oxidation and sediment production in either biodiesel or diesel can be initiated by contact with brass, bronze, copper, lead, tin and zinc. Biodiesel can have compatibil ity issues with some polymers. Shown in Table 18 is compatibility of biodiesel with a nu mber of polymers relative to the polymer’s compatibility with standard diesel. Biodiesel is compatible with the seals, gaskets and adhesives commonly used today. However, there may be issu es with pre-1993 seals, gaskets and adhesives.99 Table 18: Biodiesel material compatibility.

Polymer Relative to Standard Diesel Fluorosilicon Hardness little change, swell +7% Nitrile Hardness –20%, swell +18% Nylon 6/6 Little Change Polypropylene Hardness –10%, swell +8-15% Polyurethane Hardness little change, swell +6% Polyvinyl Much worse Teflon Little change Tygon Worse Viton A401-C Little change Viton GFLT Little change

When storing biodiesel, the temperature that is experienced by the biod iesel must be closely monitored so as to avoid the formation of crystals. These crystals are formed by the solidification (freezing) of the biodiesel. Crystal formation is undesirable since the crystals can plug fuel lines and fuel filters. To avoid the formation of crystals , biodiesel should be stored at a temperature at

least 15  ̊ F higher than the pour point of the fuel. For most pure biodie sel storage temperatures of

45-50  ̊ F are generally adequate. Even in most co ld climates, underground storage of pure biodiesel commonly provides the ne cessary storage temperature to prevent crystal formation. If pure biodiesel is stored above ground in cold climates, the storage sy stem needs to include additional features such as insulation, agitation capability, and/or heating capability. Blends of biodiesel with standard diesel can frequently be stored at lower temperatures. These blends still

need to be stored at least 15  ̊ F above pour point temperature fo r the blend. The formation of crystals is reversible as heating of the biodiesel can be used to melt the crystals. Alternatively, the crystals can be filt ered from the biodiesel.

Stability of the biodiesel is an important attribut e if the biodiesel is to be stored for a prolonged period. Poor stability can lead to increasing ac id numbers, increasing fuel viscosity, and the formation of gums and sediments. Information about the stability of the stored biodiesel can be achieved by monitoring the acid number and viscosity. Storage stability of bi odiesel has not been extensively examined relative to its composition. Therefore, the current best practice involves not storing biodiesel or biodiesel blends for more than six months. The stability of stored diesel fuels can be enhanced by using antioxidants. Ex amples of acceptable antioxidants include t-butyl hydroquinone (TBHQ), Tenox 21, and tocopherol. Wa ter contamination must be minimized in the stored fuel as it can lead to biological gr owth in the fuel. If water contamination occurs, biological growth can be mitigated by using biocides.

When selecting tanks for the storage of biodi esel several factors should be considered. Acceptable materials of construction for biodiesel storage tanks include aluminum, steel, Teflon and fluorinated polyethylene or po lypropylene. If the tanks to be used have been previously used for the storage of standard diesel fuels, the ta nks should be cleaned prior to use for biodiesel storage. Tanks should be selected to minimi ze the possibility of water contamination.100 Appropriate choice of storage tanks must also be considered for feedstock storage tanks. As with biodiesel storage tanks, the acceptable materials of construction for the feedstock storage tanks are aluminum, steel, Teflon and fluorinated pol yethylene or polypropylen e. Biodiesel plants based on a feedstock of fats must provide heated storage tanks to keep th e feedstock liquid. For vegetable oil feedstocks, the need for heated storage tanks is dictated by the geographical location of the biodiesel plant.

Several factors should be considered when ha ndling biodiesel. While biodiesel contains no hazardous materials, it should still be stored a nd used in well-ventilated areas. Storage of

biodiesel should not be near heat , spark, or flames sources. If stored in drums, proper drum handling procedures should be followed, wh ich includes no drum puncturing, dragging, or sliding. It is recommended that PVC-coated glov es as well as safety glasses or goggles should be used when handling biodiesel.

Typical forms of transportation vessels for biodie sel are rail cars, tank trucks, and drums. Choice of vessel is dictated by the amount of biodiesel being transported. When transporting biodiesel, the transportation vessels should be clean and dry. The preferred transportation vessels would be

ones dedicated to biodiesel. While dedicated biod iesel vessels may be possible with drums, it is less likely with tank trucks or rail cars. When us ing tank truck or rail cars, it is important to specify that the vessel sh ould be steam cleaned and blown dry prior to use if the vessel is not routinely used for biodiesel transport. In addition, the vessels must conform to the same materials of construction used in storage. Seals, gaskets, and adhesives present in the transfer systems should comply with the compatible materials discussed above. Due to its pour point, pure biodiesel is difficult to ship in cold weather. Methods to transport biod iesel in cold weather

include hot biodiesel in tank cars that are rapidly delivered, solidified biodiesel in tank cars that are equipped with steam coils, 20 % biodiesel blends w ith winterized diesel, and 50% biodiesel blends with kerosene (diesel No . 1), which has a pour point of 0  ̊ F.

Biodiesel is commonly used in blends with stan dard diesel fuel. As discussed above, use of biodiesel blends can improve th e thermal tolerance of the fuel by decreasing the pour point. Therefore, blending may be considered at the bi odiesel manufacturing site prior to storage. Several blending techniques can be employed. Biodi esel has a higher specific gravity than diesel fuel (0.88 versus 0.85), so blending can be ach ieved by splashing the biodiesel on the top of diesel. This blending technique is known as spla sh blending. Alternatively, agitated blending can be used. Agitated blending uses direct mechan ical agitation or blending induced by pump the

biodiesel/diesel fuel mixture back and forth in to a tank truck. While water contamination is undesirable due to the potential to promote biolog ical growth in stored blends, biodiesel/diesel fuel blends will not separate upon exposure to water.101 

## 16. Product Quality

Generally, the fuel quality of biodiesel can be influenced by several factors: · The quality of the feedstock. · The fatty acid composition of the parent vegetable oil or animal fat. · The production process a nd the other materials used in this process. · Post-production parameters.

The best current measure for biodiesel quality is the ASTM standard, ASTM D 6751: Standard Specification for Biodiesel Fuel (B 100) Blend Stock for Distillate Fu els. This standard specifies the properties required for a biodie sel fuel to be used in an engine without problems. Even in blends with conventional diesel fuel, most pe ople in the industry expe ct that the biodiesel blending stock will meet the standard before being blended. While some properties in the standard, such as cetane number and density, re flect the properties of the chemical compounds that make up biodiesel, other properties provide indications of the quality of the production process. Generally, the parameters given in the standard ASTM D6751 are defined by other ASTM standards. However, other test methods, such as those developed for the American Oil Chemists’ Society, (AOCS) may also be suitable (or even more suitable as they were developed for fats and oils and not for petroleum-derived ma terials addressed in the ASTM standards). This discussion will focus on the most important issues for assuring product quality for biodiesel as it is related to production as well as some post-production parameters.

Production Process Factors

Completion of reaction

The most important issue during biodiese l production is the completeness of the

transesterification reaction. Th e basic chemical process that occurs during the reaction is indicated by the following sequence of events:

Triglyceride → Diglyceride → Monoglyceride → Glycerol ↓ ↓ ↓

methyl methyl methyl

ester ester ester

The triglycerides are converted to diglycerides, which in turn are converted to monoglycerides, and then to glycerol. Each step produces a mol ecule of a methyl ester of a fatty acid. If the reaction is incomplete, then ther e will be triglycerides, diglycerides, and monoglycerides left in

the reaction mixture. Each of th ese compounds still contains a gl ycerol molecule that has not been released. These compounds are referred to as bound glycerol . When the bound glycerol is added to the free glycerol, we get the total glycerol . The ASTM specification requires that the total glycerol be less than 0.24% of the final biodiesel product. The ASTM method requires that the total glycerol be measured using a gas ch romatographic method described in ASTM D 6584. Other methods can be used such as a high performance liquid chromatograph (HPLC; for example AOCS Recommended Practice Ca 14b-96: Qu antification of Free Gl ycerine in Selected102 Glycerides and Fatty Acid Methyl Esters by HP LC with Laser Light-Scattering Detection) or a chemical procedure such as that described in AOCS Official Method Ca 14-56 (Total, Free and Combined Glycerol Iodometric Method). All of th ese techniques require a fairly high level of training and some laboratory equipment.

Free Glycerol

Free glycerol refers to the amount of glycerol that is left in the finished biodiesel. Glycerol is

insoluble in biodiesel so almost all of the glycerol is easily rem oved by settling or centrifugation. Some glycerol may remain either as suspended dr oplets or a very small amount that is dissolved in the biodiesel, and is known as free glycerol. Most of this gl ycerol should be removed during the water washing process. Water-washed fuel is gene rally very low in free glycerol, especially if hot water is used for washing. Distilled biodies el tends to have a greater problem with free glycerol due to glycerol carry-over during disti llation. Fuel with excessive free glycerol will usually have a problem with glycerol settling out in storage tanks, creating a very viscous mixture that can plug fuel filters and cause combustion problems in the engine.

Residual Alcohol and Residual Catalyst

Since methanol (and ethanol) and the alkaline catal ysts are more soluble in the polar glycerol phase, most will be removed when the glycerol is separated from the biodiesel. However, the biodiesel may still contain 2-3% methanol after th e separation, which may constitute as much as

40% of the excess methanol from the reaction. Mo st plants will recover this methanol by heating the biodiesel. Any methanol rema ining after this hea ting process should be removed by the water washing process. Therefore, the residual alcohol level in the biodi esel should be very low. The allowable alcohol level is specified in European biodiesel standards (give number), but is not

included in the ASTM standard. Tests have shown that even as little as 1% methanol in the biodiesel can lower the flashpoint of the biodie sel from 170C to less than 40C. Therefore, by including a flashpoint specification of 130 C, the AS TM standard limits the amount of alcohol to a very low level (<0.1%). Residual alcohol left in the biodiesel will not affect its use in the engine. The amount is too small to negatively imp act the fuel’s performance. However, lowering the flashpoint presents a potential safety hazard as the fuel may need to be treated more like gasoline, which also has a low flashpoint, than diesel fuel.

Most of the residual catalyst is removed with the glycerol. Like the alcohol, remaining catalyst should be removed during the water washing. Al though a value for residual catalyst is not included in the ASTM standard, it will be limited by the specification on sulfated ash. Excessive ash in the fuel can lead to engine deposits and high abrasive wear levels.

Equipment and expertise requirements fo r quality control of production factors

All biodiesel production fac ilities should be equipped with a la boratory so that the quality of the final biodiesel product can be monitored. It is also important to monitor the quality of the

feedstocks.103 One strategy used by many producers is to draw a sample of the oil (or alcohol) from each delivery and use that sample to produce biodiesel in the laboratory. This test can be fairly rapid (1 or 2 hours) and can indicate whether seri ous problems are likely in the plant.

Measuring feedstock quality can us ually be limited to acid value and water content. These are not too expensive ($500 for the acid value equi pment and $5,000 for the water measurement equipment) and can be operated by less experienced technicians.

To monitor the completeness of the reaction accord ing to the total glycer ol level specified in ASTM D 6751 requires the use of a gas chromatogr aph and a skilled operator. Large producers will find that having this equipment on-site is necessary. Commercial laboratories (i.e. Williams Laboratory Services) are availabl e that can analyze the samples but the cost is $100-$150/test and the time required may be several days. Sm aller producers will need to use a more robust production process involving extra methanol and probably multiple reaction steps. Then the product quality can be monitored through peri odic testing by an outside laboratory.

Other possibilities for monitoring the transesterif ication reaction and asse ssing fuel quality are methods based on spectroscopy (such as near-infrare d spectroscopy) or physical properties (such as viscometry). These methods, although they ar e not (yet?) ASTM methods, are usually faster and easier to use than gas chromatography. Howeve r, some of them require somewhat extensive calibration. They also cannot accurately quantify glycerol at the low levels called for in the ASTM standard. To circumvent this, comparis on to a reaction and product known to meet

ASTM standards is needed.

Post-production Factors

Water and Sediment

Water and sediment contamination are basically housekeeping issues for biodiesel. Water can be present in two forms, either as dissolved water or as suspended water droplets. While biodiesel is generally considered to be insoluble in water, it actually takes up considerably more water than

diesel fuel. Biodiesel can cont ain as much as 1500 ppm of diss olved water while diesel fuel usually only takes up about 50 ppm . The standards for diesel fuel (ASTM D 975) and biodiesel (ASTM D 6751) both limit the amount of water to 500 ppm. For petroleum-based diesel fuel, this actually allows a small amount of suspended water. However, biodiesel must be kept dry. This is a challenge because many diesel st orage tanks have water on the bottom due to condensation. Suspended water is a problem in fu el injection equipment because it contributes to the corrosion of the closely fitt ing parts in the fuel injection system. Sediment may consist of suspended rust and dirt particles or it may or iginate from the fuel as insoluble compounds formed during fuel oxidation. Some biodiesel us ers have noted that switching from petroleumbased diesel fuel to biodiesel may cause an increa se in sediment that comes from deposits on the walls of fuel tanks that had previously containe d diesel fuel. Because it s solvent properties are different from diesel fuel, biodiesel may loosen sediments and cause fuel filter plugging during the transition period.104 Storage Stability

Storage stability refers to the ability of the fu el to resist chemical changes during long term storage. These changes usually consist of oxidati on due to contact with o xygen from the air. The fatty acid composition of the biodiesel fuel is an important factor in determining stability towards air. Generally, the more unsaturated fatty acids (C18:2, linoleic acid; C18:3 linolenic acid) are more susceptible towards oxidation. The changes can be catalyzed by the presence of certain metals (including those making up the storage contai ner) and light. If water is present, hydrolysis can also occur. The chemical changes in the fuel associated with oxidation usually produce hydroperoxides which can, in turn, produce short chain fatty acids and aldehydes. Under the right conditions, the hydroperoxide s can also polymerize. Therefor e, oxidation is usually denoted by an increase in the acid value and viscosity of the fuel. Ofte n these changes are accompanied by a darkening of the biodiesel color from yellow to brown and the development of a “paint”

smell.

When water is present, the este rs can hydrolyze to long chain fr ee fatty acids which also cause

the acid value to increase.

There is currently no generally accepted method for measuring the stability of biodiesel. The techniques generally used for petroleum-based fuel s, such as ASTM D 2274, have been shown to be incompatible with biodiesel. Other procedures, such as the Oil Stability Index or the Rancimat apparatus, which are widely used in the fats and oils industry, seem to be more appropriate for use with biodiesel. However, the engine i ndustry has no experience with these tests and acceptable values are not known. Also, the validity of accelerated testing methods has not been

established or correlated to act ual engine problems. If biodies el’s acid number, viscosity, or sediment content increase to the point where they exceed biodiesel’s ASTM limits, the fuel should not be used as a transportation fuel.

Additives such as BHT and TBHQ (t-butylhydroquinone) are common in the food industry and have been found to enhance the storage stability of biodiesel. Biodiesel produced from soybean oil naturally will contain some antioxidants (t ocopherols, i.e., vitamin E), providing some protection against oxidation (some tocopherol is lost during refining of the oil prior to biodiesel production). Any fuel that will be stored for more than 6 months, whether it is diesel fuel or biodiesel, should be treated with an antioxidant additive.105 

## Laboratory – Exercise 3

Design of a Process for an Arbitrary Feedstock

Divide the class into four groups. Have each gr oup develop a process flow diagram for a plant that would use one of the following feedst ocks and produce fuel-grade biodiesel.

1. Crude soybean oil 2. Refined soybean oil 3. Yellow grease

4. Untreated restaurant waste 5. Trap grease

After the groups have completed their discussions, have them present and defend their designs to

the entire class.F1146-E(05/2004) REPORT DOCUMENTATION PAGE Form Approved
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5b. GRANT NUMBER 4. TITLE AND SUBTITLE Biodiesel Production Technology

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BB044630 6. AUTHOR(S) J. Van Gerpen, B. Shanks, and R. Pruszko (ISU) D. Clements (RPDL), and G. Knothe (USDA/NCAUR)

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14. ABSTRACT (Maximum 200 Words)
Biodiesel is an alternative fuel for diesel engines that is gainign attention in the United States after reaching a considerable level of success in Europe. The purpose of th is book is to describe and explain the process and issues involved in producing this fuel.

15. SUBJECT TERMS biodiesel; production; transesterfication

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