Abstract:
A method is disclosed for obtaining an alkyl ester fuel, i.e., a biodiesel, and alcohol from a feedstock mixture of free fatty acids and/or glycerides mixed with a simple alcohol, such as methanol. The method uses an electrostatic probe to provide an electrical field within the reactor. This causes the glycerin molecules to precipitate out of the feedstock mixture. A drain is provided in the reactor, to drain the glycerin from the reactor. When the precipitated glycerin reaches a level to make contact with the electrostatic probe, an electrical circuit is closed, which causes the drain to open. When the level drops, the drain closes. Glycerin absorbs water and draining glycerin from the reactor naturally dehydrates the fuel.

Description:
[0001]    This application claims priority from the provisional application 61/149,471, filed on Feb. 3, 2009, with the United States Patent Office. 
     
    
     BACKGROUND INFORMATION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to the production of alkyl esters, i.e., biodiesel. More particularly, the invention relates to a method of producing biodiesel with supercritical alcohol. More particularly yet, the method relates to a multi-step process that removes excess water from the feedstock. 
         [0004]    2. Description of the Prior Art 
         [0005]    Chemically, vegetable oil and animal fats are known as triglycerides. A triglyceride is a glycerin molecule (C3H5(OH)3) with three fatty acid molecules attached to three hydroxyl groups and glycerin is an alcohol with three hydroxyl groups. 
         [0006]    A fatty acid is a long chain hydrocarbon molecule, and is found in vegetable oils and animal fats. Stearic, palmitic, and oleic acids are examples of fatty acids. A fatty acid is also known as an ester. Sometimes one or more of the fatty acids are detached from the glycerin molecule. These are known as free fatty acids or FFAs. A glycerin molecule with a single fatty acid attached is known as a monoglyceride, while a glycerin molecule with two fatty acids is known as diglyceride. 
         [0007]    Biodiesel, properly known as an alkyl ester, is the result of transesterification of glycerides in which the fatty acid molecules are removed from the hydroxyl group of the glycerin and attached to the hydroxyl group of a single headed alcohol. Methanol, ethanol, propanol, and butanol are examples of single headed alcohols. The resulting biodiesel is known by the alcohol from which it was formed. Alkyl esters made with methanol are known as methyl esters; with ethanol as ethyl esters; with propanol as propyl esters; and with butanol as butyl esters. Alkyl esters can also be formed by esterification of FFAs, or transesterification of glycerides. An FFA is esterified when it combines with an alcohol molecule. A water molecule is formed when an FFA is esterified. Esterification is desirable for certain feedstocks, especially those with a high percentage of FFAs. 
         [0008]    The most common means of performing transesterification is with the use of a base catalyst. The most common catalysts are potassium or sodium based. Small-scale biodiesel production processes use sodium hydroxide or potassium hydroxide, commonly known as lye. Larger biodiesel producers use sodium or potassium methylate, which is essentially the metal dissolved in methanol. Base transesterification works best with feedstocks low in FFAs because each FFA molecule combines with a catalyst molecule and produces a molecule of soap, rather than an alkyl ester. The loss of yield to the production of soap is typically 5-10%. The transesterification process takes about 30-120 minutes. The soap then has to be removed from the product stream by means of some cleaning process and the catalyst in the byproduct neutralized. 
         [0009]    The most common means of esterification is with a strong acid catalyst such as sulfuric acid. The reaction is much slower than base transesterification, taking 4-8 hours, and because of that, generally requires the use of large capacity tanks. Furthermore, the reaction is self-limiting, because the esterification process itself produces water and water retards the reaction. Multiple stages are required if the feedstock is sufficiently high in FFA content. For example, waste oil that contains 20% FFA must go through at least two acid stages in order to be processed by the normal base stage. The first stage may reduce FFA content to 5%, the second to 1%. The first acid stage in a case like this produces a mixture of methanol, water, and acid that can be drained off, because of a combined density greater than that of the oil. At a minimum, the water must be removed before the next acid stage can take place. 
         [0010]    A troublesome fluid byproduct that needs to be removed is produced in this step of the process. Methanol dissolves in water, as does sulfuric acid. The acid esterification process mixes dry oil, methanol, and sulfuric (or another very strong) acid. It&#39;s circulated for some hours to keep it well mixed, then left to stand. Over some additional hour(s), if enough FFAs were esterified to produce enough water, the mix of water/methanol/acid will sink to the bottom where it can be drained off. The acid must then be neutralized or saved for later use in neutralizing the base catalyst and the methanol must be recovered from the water. It&#39;s always problematic; the water must be removed, because it will cause soap formation in the base catalyzed phase, and removing the water also removes the alcohol. Furthermore, acid esterification incurs greater expense at each step of the process: tanks much larger than the transesterification tanks are needed; acid must be added and later neutralized; and methanol has to be recovered from the water produced by the esterification. 
         [0011]    An alternate means of esterifying FFAs and transesterifying glycerides is to put the FFAs or glycerides in the presence of an alcohol in a supercritical phase. A phase is supercritical when the pressure and temperature are above the critical point for the alcohol. For methanol, the critical pressure is 81 bar, or approximately 1190 psi, and the critical temperature is 239.6 degrees C., or approximately 463 degrees Fahrenheit. In a supercritical phase, liquid disperses evenly throughout its environment, as a gas does, thus eliminating the need to emulsify the alcohol in the feedstock. 
         [0012]    Keiichi Tsuto, et al, in U.S. Pat. No. 6,288,251 teach a supercritical method of transesterifying glycerides, in which a virtually complete conversion of glycerides occurs in several minutes, without a catalyst, at molar ratios of 40 to 1 or more alcohol to glycerin (for glycerides), and at temperatures of 350 degrees C. (662 degrees Fahrenheit) and pressures of 40 MPa (6,000 psi). 
         [0013]    Commercial acid esterification processes use strong acids, such as sulfuric acid, and an alcohol, usually methanol. The esterification process produces water, which stops the reaction. Multiple stages are required if the FFA percentage is high, and the water/acid/methanol has to be drained between stages. Additional acid and methanol must be added to complete the esterification process. This acid esterification process is much slower than the base esterification process, normally taking a minimum of 4 hours. 
         [0014]    Production of biodiesel with supercritical alcohol is known. Using a supercritical process for the esterification of FFAs or transesterification of glycerides avoids many of the problems of acid esterification and base transesterification. No base catalyst is used, so there is no saponification of FFAs, and the byproducts are nearly pure glycerin and excess alcohol with traces of water. Shiro Saka discloses in U.S. Pat. No. 7,227,030 that the supercritical process tolerates higher levels of water in the feedstock than catalytic processes. Cleaning the fuel after processing is greatly simplified because there is no soap to remove. The process yield approaches 100%, because FFAs are esterified rather than saponified. Any feedstock can be used, even 100% FFAs, because the FFAs are esterified without a catalyst. 
         [0015]    The conventional supercritical process must run at temperatures from 500 to 700 degrees Fahrenheit and pressures from 3000 to 6000 psi. These operation parameters require the use of expensive equipment. The ratio of alcohol to feedstock is extremely high. For example, approximately 1.5 gallons of methanol must be mixed with each gallon of FFAs or glycerides that are fed through the process. The processor must be large enough to hold 2.5 gallons of the mixture for each gallon of alkyl esters to be produced, and, thus, must be approximately twice the size of a reactor used with the normal ratios of alcohol to feedstock in the catalytic process. Furthermore, the alcohol must be separated out at the end of the process. It is, of course, much more expensive to remove 1.4 gallons of alcohol, rather than 0.1 gallon. The ratio of alcohol to feedstock is necessarily a molar ratio of oil molecules to alcohol molecules. Ethanol/propanol/butanol molecules are larger than methanol molecules, and thus, an even greater volume of higher alcohols is required when using these alcohols. 
         [0016]    What is needed, therefore, is a method of producing biodiesel that requires less time than conventional methods and is less expensive to implement. What is further needed is a process that prevents FFAs from binding with glycerin during processing. What is yet further needed is a process that reduces the ratio of alcohol to feedstock required to obtain full conversion of glycerides to alkyl esters. 
       BRIEF SUMMARY OF THE INVENTION 
       [0017]    The invention is a method of and apparatus for creating alkyl esters by isolating glycerin molecules from a feedstock-alcohol mixture. The feedstock may be a glyceride, such as a triglyceride, or an FFA. Examples of feedstock include cooking oils and fats, i.e., vegetable oils, animal fats, and combinations of oils and fats. The oils and fats may be also be waste products, i.e., cooking oils and fats that have outlived their usefulness for human consumption. Free Fatty Acids (FFAs) are fatty acids, such as palmitic or stearic acid, that are no longer chemically attached to a glycerin molecule. Another name for an alkyl ester is “biodiesel” and these two terms are used herein interchangeably. The apparatus according to the invention comprises a reactor with an anode, which, together with a grounded housing, generates an electric field within the reactor itself. An electrostatic probe is provided within the reactor to establish an electric circuit that controls a drain valve for glycerin. Glycerin is a polar molecule with a much higher density than alcohol, water, or the feedstock itself. When glycerin molecules align within the electric field, the molecules are electrically attracted to each other and coalesce very quickly into small droplets that readily drop out of suspension because of their density. The reactor is placed at an angle and the feedstock inlet placed at the lower end of the reactor. The glycerin settles at the bottom of the reactor, above an electrically controlled drain. When the level of the glycerin is high enough to cover the end of the probe, a conducting circuit is created, which opens a drain valve. As the level of the glycerin drops below the probe, the circuit is interrupted and the drain valve is closed. 
         [0018]    An electric field strength of 100-300 volts per inch in the reactor has experimentally been demonstrated to be adequate to process the feedstock, though higher or lower strengths may also function well. 
         [0019]    An example of suitable parameters for processing the feedstock is as follows: a residence time of 6 to 8 minutes has been experimentally determined to achieve adequate transesterification of glycerides with a molar ratio of alcohol to triglycerides of 10 to 1, a pressure of 1500 psi, and a temperature of 330 degrees C. This is merely an example of a system that functions well. It is understood that other parameters for temperature, pressure, residence time, and ratios will also work, as will other alcohol types. Higher temperatures and pressures speed the reaction but require more expensive equipment. The ratio of alcohol to glycerides must be adjusted, based on the FFA content of the feedstock. For feedstocks with less than 10% FFAs, molar ratios of 8 to 1 glycerides to alcohol will suffice. For feedstock with an FFA content of up to 30%, molar ratios of 15 to 1 will suffice. 
         [0020]    The method according to the invention naturally dehydrates the fuel throughout the process because of the electrostatic precipitation. Water is also a polar molecule, and, when esterifying nearly pure FFAs, the electrostatic field will coalesce and precipitate the water, just as happens with the glycerin. Any glycerin in the feedstock will absorb a large portion of the water and carry it off. The method of producing alkyl esters according to the invention has a naturally high tolerance for water, whether it is in the feedstock or produced by esterification. Optionally, glycerin may be reintroduced into the reactor to absorb excess water and be electrostatically precipitated. This step is only necessary when excess water is present in the process. 
         [0021]    The combination of temperatures and pressures mentioned above produces a fuel with an FFA content between 1% and 5%. Specifications for biodiesel require no more than approximately 0.25% FFA. One of several options may be employed to complete the esterification process. First, the fuel so obtained may be dehydrated so esterification may be completed with less alcohol. Dehydration may be accomplished by several methods, such as distilling off the water and alcohol, removing the water with an adsorbent or molecular sieve, or adding glycerin back into the fuel, mixing thoroughly so the glycerin absorbs most of the water, and removing the water, glycerin, and some of the alcohol through electrostatic precipitation. Temperature and pressure in the reactor may also be controlled so as to maintain the alcohol in a supercritical state while changing water from a liquid to vapor. Liquid water during the transesterification stage accelerates breakdown of glycerides, but can be vaporized and removed in the final esterification stage, which allows more complete esterification of FFAs. The steam rises to the top of the esterification reactor, where it may be detected and removed. Lastly, the water may be electrostatically coalesced and precipitated as described above and drained as a liquid. 
         [0022]    Non-catalytic esterification is more effective at lower temperatures and pressures than are ideal for transesterification. In particular, temperatures that are just short of supercritical and pressures between 1600 and 1800 psi have been experimentally shown to be optimal. The molecular energy available at higher temperatures and pressures breaks apart alkyl-esters more quickly, just as it breaks glycerides into fatty acids and glycerin. A heat exchanger may be used to extract heat from the fuel coming out of the processor and use it to heat the feedstock being pumped into the processor. In a typical installation, the volume of the heat exchanger is approximately equal to the volume of the processor, so the residence time of the fuel in the heat exchanger and in the processor is approximately the same. The feedstock entering the heat exchanger is gradually warmed and partially esterified as it enters the final heater and processor. The temperature of high FFA fuel leaving the processor at supercritical temperatures is gradually cooled off as it flows through the heat exchanger and transfers its heat to the incoming feedstock. The time spent cooling in the heat exchanger constitutes a natural esterification environment, so the fuel has a lower acid content than it otherwise would. 
         [0023]    The FFA content of the fuel can be brought within fuel-use specifications by taking the fuel from the output of the heat exchanger and passing it over a solid acid catalyst such as Dowex DR2030. The time the fuel must be in contact with the resin varies with the FFA content, alcohol type and amount, temperature, and pressure. The FFA levels can optionally be controlled with a solid catalyst placed within either the processor or heat exchanger. This is a known process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. The drawings are not drawn to scale. 
           [0025]      FIG. 1  is a schematic illustration of the reactor with electrostatic separator for supercritical production of alkyl esters with continuous electrostatic removal of glycerin. 
           [0026]      FIG. 2A  is a schematic illustration of the removal of glycerin and water in a multi-stage process. 
           [0027]      FIG. 2B  is a schematic illustration of the removal of glycerin and water in a multi-stage process, with an additional process for water removal via addition of glycerin as a dehydrating agent. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The present invention will now be described more fully in detail with reference to the accompanying drawings, in which the preferred embodiments of the invention are shown. This invention should not, however, be construed as limited to the embodiments set forth herein; rather, they are provided so that this disclosure will be complete and will fully convey the scope of the invention to those skilled in the art. The same reference designations in different embodiments indicate that the elements are functionally the same. In all of the embodiments shown, the wavy lines in the reactors indicate a feedstock mixture, the small circles at the lower end the reactors indicates glycerin or a glycerin and water. 
         [0029]      FIG. 1  illustrates a first method  1000  and reactor  100  for supercritical production of alkyl esters according to the invention. The reactor  100  comprises a housing  110  that encloses a reaction chamber  112 , the housing having an inlet port  140  near a lower end  110 B of the housing, a discharge port  170  at an upper end  110 A, and a drain  160  at the lower end  110 B. An anode  120  extends into the chamber  112 . The housing  110  is grounded. An electrostatic probe  130  that serves to control the opening and closing of the drain  160  is provided near the lower end  110 B, extending in the direction of the drain  160 . Feedstock FD comprising FFAs and/or glycerides is mixed with a simple alcohol A, such as methanol. The mixture is heated to a temperature above the critical point for the alcohol A and pumped into the reactor  100  through the inlet port  140  at a pressure above the critical pressure of the alcohol. Free fatty acids and/or glycerides are esterified and transesterified, respectively, in the chamber  112  over the course of several minutes into glycerin GL and alkyl esters. An electric field is maintained in the chamber  112  between the anode  120  and the grounded housing  110 . The electric field polarizes the glycerin molecules GL, causing the glycerin to coalesce into droplets, which then drop out of the mixture because of the higher density of glycerin. 
         [0030]    Glycerin GL is much more electrically conductive than the feedstock FD. When glycerin GL accumulates at the lower end  110 B of the housing  110  in a sufficient amount to cover a lower end  132  of the electrostatic probe  130 , a conducting circuit is established between the electrostatic probe  130  and ground. The drain  160  has a valve  162  that is controlled by this circuit. The valve  162  opens when the circuit is conducting, thereby allowing the glycerin GL to drain from the reactor  100 . When the level of the glycerin GL is below the lower end  132  of the electrostatic probe  130 , the circuit is interrupted and the valve  162  closed. Ideally, the length of the electrostatic probe  130  is dimensioned such that a small amount of glycerin GL remains in the reactor  100 , to prevent feedstock FD from draining from the reactor  100 . Fuel FL, along with traces of glycerin GL, water, and excess alcohol are pumped out of the reactor  100  at the discharge port  170 . A heat exchanger  180  may be incorporated into the production process to extract heat from the exiting fuel FL and to apply the heat to incoming feedstock FD. A fuel line  172  leads from the discharge port  170  through the heat exchanger  180  and a feed line  142  leads through the heat exchanger  180  into the inlet port  140 . FFAs that were not esterified under the supercritical conditions of processor  112  are more completely esterified in the heat exchanger  180  as the process temperature gradually declines. Water introduced within the feedstock FD or produced during initial esterification is absorbed by the glycerin GL and/or coalesced by the electrostatic field and is drained off with the glycerin GL via drain  160 . 
         [0031]      FIG. 2A  illustrates a second method and apparatus  2000  according to the invention of producing alkyl esters, wherein excess water is removed from the process in the form of steam. The apparatus in  FIG. 2A  uses a pair of pressure regulators  2012  to maintain different pressures in successive reaction chambers. In the embodiment shown, the method is a multi-stage process that includes a transesterification reactor  2100  and an esterification reactor  2200 . Note that while reactor  2100  is labeled as “transesterification reactor” some initial esterification occurs within it, particularly with high FFA content feedstock. The transesterification reactor  2100  has a housing  2110  that encloses a chamber  2112 , the housing having an inlet port  2140 , a discharge port  2170  and a drain  2160 . The esterification reactor  2200  also has a housing  2210  that encloses a chamber  2212 , an inlet port  2240 , a fuel discharge port  2270 , and a steam discharge port  2280 . A fuel conduit  2010  transports intermediate fuel FL INT  from the transesterification reactor  2100  to the esterification reactor  2200 . The pressures and temperatures of the reactors  2100  and  2200  are controlled at  2012 , so as to vaporize the water into steam in the esterification reactor  2200 . The steam is then discharged through the steam discharge port  2280 . 
         [0032]    By way of example, a mixture of feedstock FD and alcohol A are pumped into the transesterification reactor  2100  at a temperature of 345 degrees C. and a pressure of 2400 psi. A voltage of +300 V per inch is applied to the mixture by anode  2120 . Glycerin GL precipitates out of the mixture and collects at the lower end of the reactor  2100 , where it drains off through the drain  2160 . This step takes generally 6 to 10 minutes. The intermediate fuel FL INT  is discharged through the discharge port  2170  and carried via the fuel conduit  2010  and pumped through the inlet port  2240  into the esterification reactor  2200 . Here again a voltage of +300 V per inch is applied to the mixture. Water is converted to steam and discharged through the steam-discharge port  2280 . Means for converting water to steam are well known and are not described in any detail herein. Common methods include heating the water to the boiling point or decreasing the pressure, so that the water flashes over to steam. Glycerin GL collects at the lower end of the reactor  2200  and drains off through drain  2260 . The finished product, i.e., the alkyl ester fuel FL, is discharged through the fuel-discharge port  2270 . The electrostatic precipitation and the drain control  2130  in the reactor  2100  and  2230  in the reactor  2200  are described above with reference to the electrostatic probe  130  shown in  FIG. 1 . 
         [0033]      FIG. 2B  illustrates a method of removing water generated through esterification or introduced with the feedstock. Because water is fully miscible in glycerin and not miscible at all in oil or biodiesel, glycerin mixed with wet biodiesel will absorb most of the water. In the presence of an electrostatic field, the glycerin and water mixture will coalesce and precipitate from the feedstock. As shown in  FIG. 2B , a mixture of feedstock FD, which includes oil and FFAs, and alcohol A is introduced into the transesterification reactor  2100  through an inlet port  140 . The feedstock-alcohol mixture is preheated under pressure to 345 degrees C. prior to injection into the transesterification reactor  2100 . 
         [0034]    The FFAs are esterified and the glycerides undergo transesterification for approximately 2 to 4 minutes. The electric field between the anode  120  and the housing wall of the reactor  2100  causes the free glycerin to rapidly coalesce and precipitate from the feedstock FD. The glycerin GL is drained from the reactor  2100  through the drain  2160 . The remaining mixture contains partly transesterified glycerides, water produced through esterification, water from the feedstock, and mostly esterified FFAs. This mixture is pumped through the outlet port  2170  into a static mixer  2400 , where it is again mixed with glycerin GL. The glycerin GL recovered from the drains  2160  and  2260  may also be re-introduced into the static mixer  2400 . The glycerin GL mixes with the feedstock mixture, during which process it absorbs water from the feedstock FD. The glycerin-feedstock-water-alcohol mixture then flows into a glycerin removal chamber  2300 , in which an electrostatic field between the anode  2320  and the chamber wall  2310  causes the coalesced glycerin-water droplets to precipitate out, thereby removing water from the feedstock FD. The glycerin-water droplets are removed through the drain from the chamber  2300  via the drain control  2330  and the corresponding drain  2360 . The remaining feedstock-alcohol mixture, i.e., the biodiesel, glycerides, and alcohol, flows into a second reaction chamber, a transesterification reactor  2200 , where the mixture undergoes final esterification. In this reactor  2200 , the glycerin GL and water are electrostatically precipitated out and removed via the drain control  2230  and drain  2260 . A pressure regulator valve  2012  maintains the necessary pressure in the reactor chamber  2200 , to ensure that the alcohol A remains in a supercritical state. Alkyl esters, in this case, methyl esters, and excess alcohol, i.e., alcohol A, exit the reactor chamber  2200  via an exit valve  2014 . The fuel FL and alcohol A are cooled and cleaned for use. 
         [0035]    The process of converting water to steam and removing it from the reactor via a steam discharge port, as described with reference to  FIGS. 2A and 2B , may also be implemented with the apparatus shown in  FIG. 1 . The reactor  100  would then be equipped with a steam discharge port and the necessary pressure regulator or heater to convert the water. 
         [0036]    The fuel product FL obtained from the methods  1000  and  2000  according to the invention may be processed through a solid catalyst to bring the fuel into compliance with ASTM or other fuel quality standards. Under some combinations for temperature, pressure, and residence time, the fuel may be free of bound glycerin GL, but still contain some unesterified FFAs, enough to fail ASTM or other standard tests for fuel quality. Prior to separating out the excess alcohol A, these FFAs may be esterified in a final esterification process  2018  by passing the fuel with the excess alcohol over a wide variety of conventional solid acid catalysts. An example of a suitable catalyst is Dowex DR2030. 
         [0037]    It is understood that the embodiments described herein, including the operating parameters of temperature, pressure, and time, are merely illustrative of the present invention. Variations in the construction of the method and apparatus for producing biodiesel may be contemplated by one skilled in the art without limiting the intended scope of the invention herein disclosed and as defined by the following claims.