Abstract:
The present invention relates to a process for the production of fuels with superior low temperature properties. Specifically, the present invention relates to the production of fuels that meet ASTM and military jet fuel kerosene specifications by heterogeneous, reactive distillation esterification of oils. The oils may be naturally high in unsaturation, such as whole plant oils, tall oil fatty acids, and rosin acids, or the oils may be from the unsaturated fraction isolated from less unsaturated seed oils such as palm olein, caprylic and caproic acids, or from mixtures thereof.

Description:
[0001]    This application claims priority under 35 U.S.C. 119(e) to U.S. provisional application 60/962,673, filed Jul. 31, 2007, the contents of which are incorporated by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a process for the production of fuels with superior low temperature properties. Specifically, the present invention relates to the production of fuels that meet ASTM and military jet fuel kerosene specifications by heterogeneous, reactive distillation esterification of oils. The oils may be naturally high in unsaturation, such as whole plant oils, tall oil fatty acids, and rosin acids, or the oils may be from the unsaturated fraction isolated from less unsaturated seed oils such as palm olein, caprylic and caproic acids, or from mixtures thereof. 
       BACKGROUND 
       [0003]    Interest in the production of fuels acceptable for use in airborne gas turbine engines from animal and vegetable fats has increased due to the possibility of global warming. Unlike petroleum-based fuels, fuels made from by-product animal fat and vegetable oils are both renewable and carbon dioxide neutral. However, a barrier to entry to the jet fuel market for fuels derived from animal fat and vegetable oil is the poor low temperature fluidity of the most popular derivatives such as biodiesel or alkyl esters of fatty acids. 
         [0004]    One method that has been proposed for avoiding the low temperature failings of biodiesel esters is to co-feed raw animal fat and vegetable oils along with diesel to a distillate hydrotreater making Ultra-Low Sulfur Diesel (ULSD) (see e.g., Marker et al., “Opportunities for Biorenewables in Petroleum Refineries,” American Chemical Society Symposium, National Meeting in Washington, D.C., Aug. 28-Sep. 1, 2005). However, this method presents several drawbacks that have made this practice unsuitable in practice. The process conditions for hydrotreating units for ultra-low sulfur diesel production favor hydrodeoxygenation (HDO), producing water that may adversely affect catalyst performance. Likewise, HDO increases hydrogen requirements. It has also been found that the heat released from these materials would substantially lower catalyst cycle lengths such that several days per year of production of both green and regular diesel would be lost. The high acidity of the lower cost feeds (from 2 to 200 total acid number) is well in excess of the accepted 0.5 to 0.6 TAN limit for which standard feed-section metallurgy, typically carbon steel, can be used. The cost of upgrading the unit just to run a small amount of animal or vegetable fat becomes an economic disincentive. Finally, it has been found that just 5% co-feeding could lead to a cloud point increase of 10-15° F. A diesel isomerization catalyst would be required to counteract the cloud point increase and preserve the green diesel molecules in the diesel pool. Unfortunately, such isomerization catalysts are typically noble metal catalysts and would require a two-stage hydroprocessing scheme, which is not typical for distillate hydrotreating units, necessitating major unit modifications. 
         [0005]    Previous research looked at stand-alone decarboxylation/hydrodeoxygenation as an option. Decarboxylation is favored at lower pressures while hydrodeoxygenation increases with increasing pressure. Decarboxylation results in odd number paraffin production and CO 2  formation whereas hydrodeoxygenation results in even carbon number paraffin production; therefore, the ratio of nC17 to nC18 is a measure of the DeCO 2 /HDO ratio. Standard hydrotreating catalysts of NiMo, CoMo and Pd all showed activity for both reactions. The main drawback to decarboxylation as demonstrated by Marker et al. is a much lower yield of diesel material due to the loss of a carbon to CO 2 . Again, a disadvantage of hydrodeoxygenation is that it produces two moles of water for every diesel-like molecule produced. Furthermore, it has been shown from product distributions high in saturated C16, C17, and C18 that the product would have excessively high cloud points. 
         [0006]    Another option that has been considered is specific decarboxylation followed by reforming. However, this process not only sacrifices a carbon to CO 2 , but also suffers from excessive cracking of C17 material to lower hydrocarbons as a result of the use of reforming technology for the final step. 
         [0007]    Another option that has been considered is hydrodeoxygenation of the methyl esters of the fatty acids from animal fat and vegetable oils. Senol et al. (2005) studied the reactions of methyl heptanoate and methyl hexanoate over commercial NiMo/g-Al2O3 and CoMo/g-Al2O3 hydrotreating catalysts in either oxide or sulphide form. The work involved hydrocracking to a substantial amount of gasoline range material such as hexenes and heptenes. 
         [0008]    In order to overcome the limitations of the of the prior art for the production of bioderived jet fuel, it is an object of this invention to provide a means of producing jet fuel grade material from plant and animal oils. 
       SUMMARY 
       [0009]    In one embodiment, whole plant oils naturally high in unsaturated fatty and rosin acids such as tall oil fatty acids and rosin acids isolated from pine trees during the Kraft pulping process are esterified. Whole plant oils such as tall oil and its derivatives represent a source of highly unsaturated oils with excellent low temperature properties that are further enhanced via esterification with alcohols. 
         [0010]    In another embodiment of the current invention, an unsaturated fraction is isolated from a less saturated oil and then esterified. For example, palm oil is regularly separated into and highly saturated (stearin) and unsaturated (olein) fractions via fractional crystallization. The olein fraction represents an excellent feedstock for producing jet fuel esters. The same separation technique can be applied to other vegetable or animal oils such that a highly unsaturated fraction is isolated and esterified. In one embodiment, the low temperature properties of some whole plant and seed oils such as tall oil fatty acids and rosins can be improved by separation of the C18 and C16 fractions by distillation. This is possible because the C16 fraction is normally saturated while the C18 fraction is highly unsaturated. 
         [0011]    It is another embodiment of the invention to esterify caprylic (C8) and caproic (C6) fatty acids. Palm and coconut oils as well as cow butter are rich in these carboxylic acids. Obtaining these carboxylic acids in pure form may involve first hydrolyzing the whole oil to yield glycerin and fatty acids, and then fractionating the fatty acids by distillation to yield a caprylic and caproic fatty acid concentrate. 
         [0012]    It is a further embodiment of the invention to avoid low temperature problems caused by contamination associated with other wet chemical methods used to produce fuel esters. For example, transesterification reactions catalyzed with bases such as NaOH and esterification reactions catalyzed with acids such as sulfuric acid tend to leave behind a certain amount of soap contamination in the fuel. This soap readily forms solid particles at temperatures well in excess of jet fuel freezing points making these processes unsuitable for use even with tall oil derivatives. 
         [0013]    While it is possible to “winterize” or “fractionally crystallize” esters produced from a variety of feedstocks using a variety of wet chemical transesterification or esterification processes such that the low freeze point fraction is suitable for use as jet fuel, such processes not only increase capital costs and complexity substantially, but also suffer from yield loss. 
         [0014]    Thus, high yields of fuels that are equivalent or otherwise superior to petroleum-derived jet fuel can be produced with acceptable capital cost by starting with suitably unsaturated fatty acids such as tall oil acids, by fractionally crystallizing whole oils, by distilling out C16 fractions, by recovering a caprylic/caproic acid concentrate, or by a mixture of the above, and utilizing the esterification process of the current invention to thereby avoid soap or glycerin contamination of the product. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0015]      FIG. 1  demonstrates the sequence of operations according to one embodiment of the invention as applied to whole plant oils such as tall oil fatty acid and rosin. 
           [0016]      FIG. 2  demonstrates the sequence of operations according to another embodiment of the invention where tall oil is first distilled to remove the C16 fraction. 
           [0017]      FIG. 3  demonstrates the sequence of operations according to another embodiment of the invention where glyceride-containing feed is first split and then distilled to remove the C16 fraction and concentrate the unsaturated C18 fraction molecules. 
           [0018]      FIG. 4  demonstrates the sequence of operations according to another embodiment of the invention where caprylic and caproic acids are purified from palm or coconut oil prior to esterification. 
       
    
    
     DETAILED DESCRIPTION  
       [0019]    The present invention provides a method of producing bio-derived fuel that meets jet fuel kerosene standards. Jet fuel standards are set by standards organizations, pipelines, and various militaries. In general, the key specification that separates jet fuel kerosene from regular kerosene and diesel is freeze point. For the most part, the esters of plant and animal oils fail freeze point tests by a large margin. According to the invention, certain whole plant oils and especially tall oil fatty acids and rosin acids, the unsaturated fraction of seed oils, and caprylic and caproic acid represent exceptions to this rule. Furthermore, according to the invention, utilizing heterogeneous reactive distillation instead of wet chemical methods of esterification produces reduces contamination of the fuel with higher melting substances such as soap and glycerin. Therefore, one object of this invention is to employ heterogeneous reactive distillation to esterify suitably unsaturated whole plant oils, especially tall oil acids and rosins, the unsaturated fraction of other plant and animal oils, and/or caprylic and caproic acid to produce specification jet fuel. 
         [0020]    There are numerous specifications for jet fuel issued by standards organizations, pipelines, government entities, and militaries. A summary of these has been published by ExxonMobil (ExxonMobil,  World Jet Fuel Specifications With Avgas Supplement,  2005, incorporated herein by reference). The most difficult specification to meet when attempting to apply bio-fuel technology is freeze point. Freeze point is similar to the Cloud Point and Cold Flow Plugging point specifications used in biodiesel specifications. To measure freeze point, ones takes a sample of fluid down in temperature past the point at which crystals form and then slowly warms the fluid back up until all crystals disappear. The temperature at which all crystals disappear is the freeze point. 
         [0021]    Other specifications may be met or exceeded by the products of this invention except for viscosity, density, energy content, and distillation range, depending on the particular embodiments employed. Unlike freeze point, however, these specifications can be reached by blending in relatively small amounts of regular specification jet fuel. By small amounts is meant less than about 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1% of regular specification jet fuel. Freeze point is little affected by blending because given time, crystals will form, while the specification is not for the temperature at which the crystals form, but that at which they melt. Therefore, the crystals that form from regular jet fuel in a jet fuel/bio-fuel blend may act as nucleation points for the biofuel crystals. Upon heating, the biofuel part of the crystals may then persist to higher temperatures. 
         [0022]    As an example, an ester product made from tall oil fatty acids will exceed maximum boiling point, density, and viscosity. While it will also have lower heat content than specification jet fuel, a 10% tall oil ester blend with jet fuel will meet all specifications. On the other hand, caprylic and caproic acids have the advantage of meeting all specifications except energy content. Because they pass more tests, higher concentration blends are possible with caprylic and caproic acids. 
         [0023]    It is an embodiment of the invention to provide a method for obtaining an alkyl esters based biofuel which forms crystals that melt at lower temperatures than most alkyl ester biofuels. In one embodiment, the invention encompasses five additional methods that together or alone lead to fuels with superior low temperature properties. 
         [0024]    The first method utilizes tall oil fatty acids that may or may not be contaminated with tall oil rosin acids as the feedstock to the esterification method of the invention. Tall oil rosin-contaminated tall oil fatty acids are not only less expensive but they also produce bio-jet fuel that has a lower freeze point up to a certain level of contamination. This can be seen in Table 1, where increasing rosin content leads to lower titers up to a level between 1.8 and 4.5%. Titer is another word for a measure of crystallization temperature but not necessarily freeze point. However, the analogy is consistent. By starting with these materials, and esterifying them according to the invention using various alcohols, one can prepare esters that have exceptionally low freeze point. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Typical Properties 
               
               
                             Fatty Acids 
               
             
          
           
               
                 Product 
                 Fatty 
                 Rosin 
                   
                 Color, 
                 Acid 
                 Iodine 
                 Titer, 
               
               
                 Family 
                 Acids, % 
                 Acids, % 
                 Unsaponifiables, % 
                 Gardner 
                 Number 
                 Number 
                 ° C. 
               
               
                   
               
             
          
           
               
                 Tall Oil 
                 97.5 
                 0.8 
                 1.7 
                  2+ 
                 194 
                 128 
                 9 
               
               
                 Fatty Acids 
                 96.5 
                 1.5 
                 2.0 
                  3+ 
                 193 
                 135 
                 0 
               
               
                   
                 96.0 
                 1.8 
                 2.2 
                 4 
                 192 
                 135 
                 −7 
               
               
                   
                 92.0 
                 4.5 
                 3.5 
                 6 
                 191 
                 131 
                 −5 
               
               
                   
                 91.0 
                 0.5 
                 8.5 
                 5 
                 180 
                 120 
                 6 
               
               
                 Distilled 
                 86 
                 11.5 
                 2.5 
                 8 
                 191 
                 — 
                 3 
               
               
                 Tall Oils 
                 63 
                 29 
                 8 
                 8 
                 182 
                 — 
                 2 
               
               
                   
                 70 
                 28 
                 2 
                 7 
                 187 
                 — 
                 11 
               
               
                   
                 49 
                 48 
                 3 
                  7+ 
                 174 
                 — 
                 — 
               
               
                 Tall Oil 
                 76 
                 2.5 
                 21.5 
                 — 
                 165 
                 — 
                 45 
               
               
                 Light Ends 
               
               
                 Tall Oil 
                 51 
                 28 
                 — 
                 — 
                 145 
                 — 
                 — 
               
               
                 Pitch 
                 3 
                 31 
                 — 
                 — 
                 68 
                 — 
                 — 
               
               
                   
               
               
                             indicates data missing or illegible when filed 
               
             
          
         
       
     
         [0025]    The second method involves removing the C16 fraction from various whole oils via distillation such as those derived from tall oil, palm, soybean, canola, and rapeseed in order to leave behind the a higher concentration of the highly unsaturated C18:1, C18:2, and C18:3&#39;s. This method is suited to tall oils which may be manufactured by distillation such that removing the C16 fraction may involve a modification of existing distillation conditions. Fractional distillation may also be used for the ester product itself. 
         [0026]    The third method involves the choice of alcohol used to esterify the fatty acids (and rosin acids in the case of tall oils) according to the method of the invention. Multi-functional polyols, glycols, iso-butanol, and tert-butanol can each produce bio-jet esters with exceptionally low freeze points as well as higher energy contents than methyl, ethyl, and propyl esters. 
         [0027]    The fourth method of the invention involves esterification of a caprylic (C8) and/or caproic (C6) acid concentrate. In one embodiment, obtaining the concentrate begins with hydrolysis of whole palm, palm kernel, or coconut oil. The free fatty acids that result are then fractionated by distillation to yield a distillate rich in caprylic and caproic acid. As can be seen from Table 2 (adapted from Graboski, M. S. et al., “Combustion of Fat and Vegetable Oil Derived Fuels in Diesel Engines,” Prog. Energy Combust. Sci., Vol. 24, pp. 125-164, 1998), the esters of caprylic and caproic acid have excellent freeze points. 
         [0000]                                                                                            TABLE 2                           Fatty acids   Methyl esters                        Melting   Boiling   Melting   Boiling       Acid   No. of       point   point   point   point       chain   carbons   Structure   (° C.)   (° C.)   (° C.)   (° C.)                    Caprylic   8   CH 3 (CH 2 ) 6 COOH   16.5   239   −40   193       Capric   10   CH 3 (CH 2 ) 8 COOH   31.3   269   −18   224       Lauric   12   CH 3 (CH 2 ) 10 COOH   43.6   304   5.2   262       Myristic   14   CH 3 (CH 2 ) 12 COOH   58.0   332   19   295       Palmitic   16   CH 3 (CH 2 ) 14 COOH   62.9   349   30   338       Palmitoleic   16   CH 3 (CH 2 ) 5 CH═CH(CH 2 ) 7 COOH   33   —   0   —       Stearic   18   CH 3 (CH 2 ) 16 COOH   69.9   371   39.1   352       Oleic   18   CH 3 (CH 2 ) 7 CH═CH(CH 2 ) 7 COOH   16.3   —   −19.9   349       Linoleic   18   CH 3 (CH 2 ) 4 CH═CHCH 2 CH═CH(CH 2 ) 7 COOH   −5   —   −35   366       Linolenic   18   CH 3 CH 2 CH═CHCH 2 CH═CHCH 2 CH═CH(CH 2 ) 7 COOH   −11   —   —   —       Arachidic   20   CH 3 (CH 2 ) 18 COOH   75.2   —   50   —       Eicosenoic   20   CH 3 (CH 2 ) 7 CH═CH(CH 2 ) 9 COOH   23   —   −15   —       Behenic   22   CH 3 (CH 2 ) 20 COOH   80   —   54   —       Erucic   22   CH 3 (CH 3 ) 7 CH═CH(CH 2 ) 11 COOH   34   —   —   —                    
These esters also have lower densities and viscosities than the esters of tall oil meaning that they can be blended into jet fuel at a higher percentage without the blend going out of specification.
 
         [0028]    The fifth method according to the invention, which in combination with the use of highly unsaturated fatty acids as feed stocks leads to fuels with superior low temperature properties, includes a method of reactive distillation (such as disclosed in U.S. Pat. No. 5,536,856, incorporated herein by reference). The method according to the invention involves both the application of reactive distillation and slurry reactor technology. The esterification method itself is superior to other wet chemical methods because it does not lead to soap formation. Soap contamination always leads to poor freeze points. In one embodiment, the esterification method is particularly applicable to tall oil fatty acids and rosin and caprylic and caproic acids because these materials may be recovered in their fatty acid state. The glycerides in other starting materials must be “split” to a relatively high degree (for example, to about 90% or greater) prior to esterification in order to obtain optimum catalyst life. 
         [0029]    Referring to  FIG. 1 , tall oil fatty acid, or other whole plant oils, that may contain some amount of tall oil or other rosin acids, is esterified by a reactive distillation, slurry reactor method of the invention. This method is described in detail in U.S. Pat. No. 5,536,856 incorporated herein by reference. 
         [0030]    Referring to  FIG. 2 , tall oil fatty acid is manufactured such that the C16 fraction is separated from the C18 fraction of whole plant, seed, and animal oils during distillation. The C18 fraction is then esterified according to the esterification method of the invention. 
         [0031]    Referring to  FIG. 3 , the glycerides in less suitably unsaturated whole plant, seed, and animal oils, especially rapeseed and canola, are first split into fatty acids and glycerin either by hydrolysis or saponification. The C16 fraction is then distilled from the resulting fatty acids prior to esterification of the C18 fraction. 
         [0032]    Referring to  FIG. 4 , palm or coconut oil is first subjected to hydrolysis. The resulting free fatty acids are then fractionated in order to obtain a caprylic/caproic concentrate. The concentrate may contain minor amounts of C10, C12, C14, C16, and C18. However, the lower the contamination with these higher carboxylic acids, the better the low temperature performance of the fuel. The caprylic/caproic concentrate is then subjected to esterification according to the esterification method of the invention. 
         [0033]    In each of the methods, the choice of alcohol will affect the freeze point of the resulting bio-jet fuel esters. For example, tert-butanol produces esters with a much lower freeze point for the same feedstock than methanol. The use of all C1 to C8 alcohols, polyols, glycols, and glycol ethers can be envisioned according to the invention. U.S. Pat. No. 5,536,856, incorporated herein by reference, teaches different configurations depending on the boiling point of the alcohol. 
         [0034]    In one embodiment, the process is performed on an industrial scale. For example, in a preferred embodiment, production occurs from 500 kg or more of feedstock per day. Alternatively, production may occur on batches of 1,000 kg, 5,000 kg, 10,000 kg or more feedstock per day. Global jet fuel and kerosene production is estimated at several million tons per year. 
         [0035]    It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as reflux drums, pumps, vacuum pumps, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks, and the like may be required in a commercial plant. The provision of such ancillary items of equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice. 
         [0036]    Modifications and variations of the present invention relating to a the selection of reactors, feedstocks, alcohols and catalysts will be obvious to those skilled in the art from the foregoing detailed description of the invention. Such modifications and variations are intended to come within the scope of the appended claims. All numerical values are understood to be prefaced by the term “about” where appropriate. All references cited herein are hereby incorporated by reference in their entirety.