Patent Publication Number: US-2013247452-A1

Title: Bacterial Production of Jet Fuel and Gasoline Range Hydrocarbons

Description:
This application claims priority from U.S. provisional application No. 61/614,714 filed on Mar. 23, 2012 and is incorporated herein in its entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention is generally in the area of bacterial production of fatty acids, and conversion of the fatty acids to hydrocarbons in the jet, diesel, or gasoline ranges. 
     BACKGROUND OF THE INVENTION 
     There is currently a strong interest in alternative fuels. These fuels predominantly come from two feedstocks, vegetable oils and sugars. Biodiesel is formed from vegetable oil, and ethanol comes from sugar. 
     Vegetable oils are mostly comprised of triglycerides, esters of glycerol, and three fatty acids. Fatty acids are, in turn, aliphatic compounds containing 4 to 24 carbon atoms, ideally between 10 and 18 carbon atoms, and having a terminal carboxyl group. Diglycerides are esters of glycerol and two fatty acids, and monoglycerides are esters of glycerol and one fatty acid. Naturally occurring fatty acids, with only minor exceptions, have an even number of carbon atoms and, if any unsaturation is present, the first double bond is generally located between the ninth and tenth carbon atoms. The characteristics of the triglyceride are influenced by the nature of their fatty acid residues. 
     Biodiesel fuels are fatty acid ethyl and/or methyl esters. These esters are typically prepared by transesterifying triglycerides, the major component in fats and oils, with ethanol and/or methanol, in the presence of an acid or base catalyst. Biodiesel fuels are associated with some limitations. For example, some research indicates that they cause higher emissions of nitrogen oxides (NO.sub.x), increased wear on engine components, and fuel injector coking (“Progress in Diesel Fuel from Crop Oils,” AgBiotechnology, (1988)). Also, biodiesel fuel does not provide as much power as petroleum-based diesel is burned (See, for example, Jori, et al., Hungarian Agricultural Engineering, 6:7, 27-28 (1993)), and the diesel engines may need to be retuned in order to run efficiently on biodiesel. 
     Another effort at producing a renewable fuel has involved the thermal conversion of animal carcasses to a liquid oil product and a water-soluble inorganic product. When animal carcasses are heated, at around 250 C, the triglycerides hydrolyze into glycerol and free fatty acids, and at around 500 C, the free fatty acids decarboxylate to form a mixture of products that relate to the hydrocarbon chains in the original fatty acids. This process is known as thermal decarboxylation. The rate of this process can be accelerated by the addition of various catalysts, though the end game is still the same—removal of a carboxylic acid moiety from the end of the fatty acid. 
     Most automobiles run on gasoline, and airplanes run on jet fuel, not diesel. It would be advantageous to provide a method for forming alternative fuel sources from vegetable oil feedstocks that have tunable molecular weights and octane or cetane ratings, so that a variety of gasoline, jet fuel, or diesel fuel compositions can be prepared as desired. The present invention provides such methods, as well as renewable gasoline, jet fuel, and diesel fuel compositions. 
     Allowed U.S. Publication No. 20080229654 to Bradin discloses a process for converting fatty acids or triglycerides to gasoline, jet, or diesel fuel, and provides a list of animal and vegetable sources for the triglycerides. U.S. Pat. No. 7,816,570 to Roberts et al. also discloses a process for converting triglycerides to jet fuel. The difference between the Roberts and the Bradin process is that the Roberts process burns the glycerol derived from the hydrolysis of triglycerides to produce the energy to run the overall process, whereas, in other patent applications, Bradin discloses converting glycerol to glycerol ethers and using the glycerol ethers as fuel additives. The contents of these references are incorporated herein in their entirety. 
     The limitation associated with using triglycerides is that the production of the raw materials is somewhat limited. Bacteria and algae are potential feedstocks for producing fatty acid and triglycerides, respectively, that are believed to hold great promise. It would therefore be desirable to provide methods for converting bacterial products to fuels. The present invention provides such methods. 
     SUMMARY OF THE INVENTION 
     Fuel additives and fuel compositions, and methods for their preparation and use directly as fuels, or as blends with conventional gasoline, jet, and/or diesel fuel, are disclosed. 
     Fatty acids are used to prepare the fuel additives and/or fuel compositions, and these are subjected to thermal decarboxylation to remove the carboxylic acid group. The resulting decarboxylated products comprise hydrocarbons in the C 10-20  range, and, depending on the starting materials, include one or more double bonds. If desired, the double bonds can be hydrotreated to produce linear paraffins. 
     Gasoline predominantly includes hydrocarbons in the molecular weight range of C 5-9 , for example, C 6-8 . Gasoline tends to include isoparaffins, so while intermediate C 5-9 -containing fractions, for example, C 6-8  fractions, can advantageously be isolated for direct use or sale, they can also be subjected to additional processing steps, such as isomerization and hydrotreatment. 
     Diesel fuel has a preferred molecular weight range of C 10-20 , ideally at the lower end of this range. By selecting appropriate thermal decarboxylation products, yields of products in the diesel range can be maximized. As the products include carbon-carbon double bonds, an optional hydrotreatment step may be performed. 
     There are many types of jet fuel, including kerosene-type jet fuel and wide-cut jet fuel. Kerosene-type jet fuel has a carbon number distribution between about 8 and 16 carbon numbers; wide-cut jet fuel, between about 5 and 15 carbon numbers. By using appropriate distillation conditions to separate the products of the thermal decarboxylation and hydrocracking steps, hydrocarbons in either jet fuel range can be provided. These can be used as is, or hydrotreated to hydrogenate the double bonds and/or isomerized to provide isoparaffins. 
     The thermal decarboxylation products are subjected to hydrocracking to provide products in a desired molecular weight range, isomerization to provide products with a desired degree of branching, and/or hydrotreating/hydrofinishing steps. The desired molecular weight range and degree of branching will, of course, depend on whether it is desired to provide a diesel, gasoline, or jet fuel composition, or additive for including in such compositions. 
     Alternative fuel compositions including the resulting products can be prepared by blending the products with gasoline, diesel fuel, or jet fuel, as appropriate. In one embodiment, the resulting alternative fuel contains between approximately 25 and 98 percent petroleum-based gasoline, diesel or jet fuel and between approximately 2 and 75 percent of the products from the molecular averaging or post-treatment steps. 
     In one embodiment, once the fatty acids are decarboxylated, they are used directly as diesel fuel, or as an additive in diesel fuel, without further steps such as hydrocracking or isomerization. In one aspect of this embodiment, double bonds in the decarboxylation product are hydrogenated. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention uses a two-stage process for producing jet fuel, diesel fuel, and/or gasoline from food and non-food feedstocks. First, a genetically-engineered microorganism is used to convert the feedstock into fatty acids. This organism is ideally produced using a computational design process to identify favorable genetic modifications to maximize fatty acid production. Second, fatty acids are converted into jet fuel using a chemical process. The jet fuel can be domestically produced and can be used by the aviation and defense industries, and the gasoline and diesel can be used in conventional gasoline and diesel engines, as well as in flexible fuel engines. 
     Fuel compositions, as well as methods for preparing the compositions, are disclosed. The fuel composition can be used as gasoline, jet fuel, and/or diesel fuel, or used as additives to such fuels. 
     In its broadest aspect, the present invention is directed to an integrated process for producing fuels, including jet fuel, gasoline and diesel. The process involves the thermal decarboxylation of fatty acids to form a thermal decarboxylation product, which can be used as is, or subjected to further steps, such as isomerization, hydrocracking, and hydrotreatment. 
     In some embodiments, the processes described herein are integrated processes. As used herein, the term “integrated process” refers to a process which involves a sequence of steps, some of which may be parallel to other steps in the process, but which are interrelated or somehow dependent upon either earlier or later steps in the total process. 
     There are numerous advantages provided by the processes described herein. The processes convert bacteria-derived fatty acids, which tend to be outside the range of gasoline, diesel and/or jet fuel, into products within these ranges. 
     The following definitions will further define the invention: 
     The term “alkyl”, as used herein, unless otherwise specified, refers to a saturated straight, branched, or cyclic hydrocarbon of C 1-6 , and specifically includes methyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, pentyl, cyclopentyl, isopentyl, neopentyl, hexyl, isohexyl, cyclohexyl, 3-methylpentyl, 2,2-dimethylbutyl, and 2,3-dimethylbutyl. 
     The term “olefin” refers to an unsaturated straight, branched or cyclic hydrocarbon of C 2-10 , and specifically includes ethylene, propylene, butylene, isobutylene, pentene, cyclopentene, isopentene, hexene, cyclohexene, 3-methylpentene, 2,2-dimethylbutene, 2,3-dimethylbutene, 1-heptene, 2-heptene, 3-heptene, 1-octene, 2-octene, 3-octene, 4-octene, 1-nonene, 2-nonene, 3-nonene, 4-nonene, 1-decene, 2-decene, 3-decene, 4-decene, and 5-decene. Ethylene, propylene, butylenes, and isobutylene can be preferred due to their relatively low cost, and C 2-8  olefins, or, preferably, C 2-4  olefins (low molecular weight olefins) can be preferred. Low molecular weight olefins can include other olefins outside the C 2-4  range, but ideally, the majority (51% or more) of the olefins in a low molecular weight olefin feedstock are in this range. In one embodiment, the olefins comprise substituted olefins. 
     I. Fuel Composition 
     The fuel composition prepared using the processes described herein can include alkanes in the gasoline, jet fuel and/or diesel fuel ranges, as desired. While thermal decarboxylation products are present, the composition can optionally include fatty acid alkyl esters, which can provide adequate lubrication when used in an amount of about 2 percent by volume or more. 
     The hydrocarbons produced using the processes described herein typically have average molecular weights in the C 5-20  range. The molecular weight can be controlled by adjusting the molecular weight and proportions of the decarboxylated fatty acids (C 10-20  range), and the low molecular weight olefins, that are subjected to molecular averaging (olefin metathesis) conditions. Fuel compositions with boiling points in the range of between about 68-450 F, more preferably between about 250-370 F, are preferred. The currently most preferred average molecular weight is around C 8-20 , which has a boiling point in the range of roughly 345 F, depending on the degree of branching. Specifications for the most commonly used diesel fuel (No. 2) are disclosed in ASTM D 975 (See, for example, p. 34 of 1998 Chevron Products Company Diesel Fuels Tech Review). The minimum flash point for diesel fuel is 52 C (125 F). Specifications for jet fuel are disclosed in ASTM D 1655, standard Specification for Aviation Turbine Fuels. The minimum flash point for jet fuel is typically 38 C. 
     The process is adaptable to generate higher molecular weight fuels, for example, those in the C 15-20  range, or lower molecular weight fuels, for example, those in the C.sub.5-8 range. Preferably, the majority of the composition includes compounds within about 8, and more preferably, within about 5 carbons of the average. Another important property for the fuel is that it has a relatively high flash point for safety reasons. Preferably, the flash point is above 90 C, more preferably above 110 C, still more preferably greater than 175 C, and most preferably between 175 C and 300 C. 
     The fuel can also be used as a blending component with other fuels. For example, the fuel can be used as a blending component with fuels derived from crude oil or other sources. 
     II. Components used to Prepare the Fuel Composition
         A. Free Fatty Acids       

     All or part of the free fatty acids are obtained from bacterial sources. In addition to the fatty acids derived from bacterial sources, a portion of the fatty acids can be derived from the hydrolysis of triglycerides, including vegetable oils and fats, as well as animal oils and fats. Examples of suitable vegetable oils include, but are not limited to, crude or refined soybean, corn, coconut (including copra), palm, rapeseed, cotton and oils. Examples of suitable animal fats include, but are not limited to, tallow, lard, butter, bacon grease and yellow grease. Naturally-occurring fats and oils are the preferred source of triglycerides because of their abundance and renewability. Oils with a higher boiling point are preferred over oils with a lower boiling point. Animal carcasses can be used, though this may not be preferred due to the presence of various by-products (i.e., compounds other than decarboxylated fatty acids) in the thermal decarboxylation product stream (although the by-products can be removed, if desired). In one embodiment, all or part of the fatty acids can come from the hydrolysis of vegetable oil. 
     Fatty acid biosynthesis (FAB) is necessary for the production of bacterial cell walls, and therefore is essential for the survival of bacteria (Magnuson et al., 1993, Microbiol. Rev. 57:522-542). The fatty acid synthase system in  E. coli  is the archetypal type II fatty acid synthase system. Multiple enzymes are involved in fatty acid biosynthesis, and genes encoding the enzymes fabH, fabD, fabG, acpP, and fabF are clustered together on the  E. coli  chromosome. Clusters of FAB genes have also been found in Bacillus subtilis, Haemophilus influenza Rd, Vibrio harveyi, and Rhodobacier capsulatus. Examples of FAB genes in B. subtilis include fabD, yjaX and yhjB (encoding synthase III), fabG, ywpB, yjbW, yjaY, ylpC, fabG, and acpA. The ylpC, fabG, and acpA genes are contained within a single operon that is controlled by the PylpC promoter. 
     Compounds that promote the production of fatty acids can be introduced to bacteria to encourage the production of fatty acids. 
     Genetically modified  E. coli  microorganisms, or other bacteria, can be produced such that they overproduce fatty acids, which are used in the instant process to make fuels. Enzymes can be expressed which can enhance fatty acid production. 
     Although most bacteria produce fatty acids as cell envelope precursors, the biosynthesis of fatty acids is tightly regulated at multiple levels. By introducing four distinct genetic changes into the  E. coli  genome, Lu et al. (Metabolic Engineering, Volume 10, Issue 6, November 2008, Pages 333-339, Engineering Metabolic Pathways for Biofuels Production) have engineered an efficient producer of fatty acids. Under fed-batch, defined media fermentation conditions, 2.5 g/L fatty acids were produced by this metabolically engineered  E. coli  strain, with a specific productivity of 0.024 g/h/g dry cell mass and a peak conversion efficiency of 4.8% of the carbon source into fatty acid products. At least 50% of the fatty acids produced were present in the free acid form. The contents of the Lu et al. paper are hereby incorporated by reference in their entirety. 
     Streen et al., Nature, Vol 463, 28 Jan. 2010, discloses the production of fatty acids from the industrial microorganism  E. coli. E. coli  is approximately 9.7% lipid, produces fatty acid metabolites at the commercial productivity of 0.2 g l21 h21 per gram of cell mass just to grow, can achieve product-dependent mass yields of 30-35% 4, and is exceptionally amenable to genetic manipulation. Combining this natural fatty acid synthetic ability with new biochemical reactions realized through synthetic biology has provided a means to divert fatty acid metabolism directly towards fuel and chemical products of interest The product of microbial fatty acid biosynthesis is fatty acyl-ACP (acyl carrier protein), which can then be directed to cellular components such as structural or storage lipids 5,6. The accumulation of fatty acyl-ACP feedback inhibits fatty acid biosynthesis. The expression of a cytoplasmic thioesterase was previously shown to result in hydrolysis of these acyl-ACPs, deregulation of fatty acid biosynthesis, and overproduction and secretion of significant levels of free fatty acids. 
     By cytosolic expression of a native  E. coli  thioesterase (&#39;tesA—a ‘leaderless’ version of TesA that is targeted to the cytosol), normally localized to the periplasm, Streen demonstrated free fatty acid production of 0.32 g l21, similar to previous findings (FIG. 2)6,8. &#39;TesA exhibits a preference for C14 fatty acyl-ACPs, although a range of free fatty acids (C8-C18) is observed when &#39;TesA is produced The length of the fatty acid chain produced can be controlled by expressing alternative thioesterases from plants9. To improve free fatty acid production further, we eliminated the first two competing enzymes associated with b-oxidation, FadD and FadE, resulting in an extra three- to fourfold increase in titre to, 1.2 g l21. &#39;TesA-DfadE affected a 6% yield of fatty acids from 2% glucose in shake flasks, 14% of the theoretical limit. The contents of the Streen et al. paper are hereby incorporated by reference in their entirety. 
     These are just a few references related to the bacterial production of fatty acids. Efforts to date have involved esterifying the fatty acids to produce biodiesel. The use of these fatty acids to produce jet fuel, gasoline, or diesel (not biodiesel) range hydrocarbons is first described herein. Using these teachings, and other knowledge already present in the art, one can use bacteria to produce fatty acids as a feedstock for the process described herein. 
     Efforts are underway, for example, by the company LS9, to modify bacteria so as to produce fatty acids, and then, within the bacteria, to cleave the acid into a hydrocarbon. However, it is unclear whether bacteria can tolerate a high concentration of linear hydrocarbons. It is clear, and it is an object of the present invention, that the bacteria can produce fatty acids, and that these fatty acids can be isolated separately from the bacteria, and then subjected to thermal decarboxylation conditions to produce hydrocarbons, which are then subjected to further process steps, as described herein, to produce jet fuel, gasoline, and/or diesel range hydrocarbons, with jet fuel being particularly preferred due to its relatively high market price.
         B. Additional Components       

     The fuel compositions can include various additives, such as lubricants, emulsifiers, wetting agents, densifiers, fluid-loss additives, corrosion inhibitors, oxidation inhibitors, friction modifiers, demulsifiers, anti-wear agents, anti-foaming agents, detergents, rust inhibitors and the like. Other hydrocarbons, such as those described in U.S. Pat. No. 5,096,883 and/or U.S. Pat. No. 5,189,012, can be blended with the fuel, provided that the final blend has the necessary octane/cetane values, pour, cloud and freeze points, kinematic viscosity, flash point, and toxicity properties. The total amount of additives is preferably between 50-100 ppm by weight for 4-stroke engine fuel, and for 2-stroke engine fuel, additional lubricant oil may be added. 
     Diesel fuel additives are used for a wide variety of purposes; however, they can be grouped into four major categories: engine performance, fuel stability, fuel handling, and contaminant control additives. 
     Engine performance additives can be added to improve engine performance. Cetane number improvers (diesel ignition improvers) can be added to reduce combustion noise and smoke. 2-Ethylhexyl nitrate (EHN) is the most widely used cetane number improver. It is sometimes also called octyl nitrate. EHN typically is used in the concentration range of 0.05% mass to 0.4% mass and may yield a 3 to 8 cetane number benefit. Other alkyl nitrates, ether nitrates some nitroso compounds, and di-tertiary butyl peroxide can also be used. 
     Fuel and/or crankcase lubricant can form deposits in the nozzle area of injectors--the area exposed to high cylinder temperatures. Injector cleanliness additives can be added to minimize these problems. Ashless polymeric detergent additives can be added to clean up fuel injector deposits and/or keep injectors clean. These additives include a polar group that bonds to deposits and deposit precursors and a non-polar group that dissolves in the fuel. Detergent additives are typically used in the concentration range of 50 ppm to 300 ppm. Examples of detergents and metal rust inhibitors include the metal salts of sulfonic acids, alkylphenols, sulfurized alkylphenols, alkyl salicylates, naphthenates and other oil soluble mono and dicarboxylic acids such as tetrapropyl succinic anhydride. Neutral or highly basic metal salts such as highly basic alkaline earth metal sulfonates (especially calcium and magnesium salts) are frequently used as such detergents. Also useful is nonylphenol sulfide. Similar materials can be prepared by reacting an alkylphenol with commercial sulfur dichlorides. Suitable alkylphenol sulfides can also be prepared by reacting alkylphenols with elemental sulfur. Also suitable as detergents are neutral and basic salts of phenols, generally known as phenates, wherein the phenol is generally an alkyl substituted phenolic group, where the substituent is an aliphatic hydrocarbon group having about 4 to 400 carbon atoms. 
     Lubricity additives can also be added. Lubricity additives are typically fatty acids and/or fatty esters. Examples of suitable lubricants include polyol esters of C.sub.12-28 acids. The fatty acids are typically used in the concentration range of 10 ppm to 50 ppm, and the esters are typically used in the range of 50 ppm to 250 ppm. 
     Some organometallic compounds, for example, barium organometallics, act as combustion catalysts, and can be used as smoke suppressants. Adding these compounds to fuel can reduce the black smoke emissions that result from incomplete combustion. Smoke suppressants based on other metals, e.g., iron, cerium, or platinum, can also be used. 
     Anti-foaming additives such as organosilicone compounds can be used, typically at concentrations of 10 ppm or less. Examples of anti-foaming agents include polysiloxanes such as silicone oil and polydimethyl siloxane; acrylate polymers are also suitable. 
     Low molecular weight alcohols or glycols can be added to diesel fuel to prevent ice formation. Additional additives can lower a diesel fuel&#39;s pour point (gel point) or cloud point, or improve its cold flow properties. 
     Drag reducing additives can also be added to increase the volume of the product that can be delivered. Drag reducing additives are typically used in concentrations below 15 ppm. 
     Antioxidants can be added to the distillate fuel to neutralize or minimize degradation chemistry, typically in the concentration range of 10 ppm to 80 ppm. Examples of antioxidants include those described in U.S. Pat. No. 5,200,101. 
     Acid-base reactions are another mode of fuel instability. Stabilizers such as strongly basic amines can be added, typically in the concentration range of 50 ppm to 150 ppm, to counteract these effects. 
     Metal deactivators can be used to tie up (chelate) various metal impurities, neutralizing their catalytic effects on fuel performance. They are typically used in the concentration range of 1 ppm to 15 ppm. 
     Multi-component fuel stabilizer packages may contain a dispersant. Dispersants are typically used in the concentration range of 15 ppm to 100 ppm. 
     Biocides can be used when contamination by microorganisms reaches problem levels, typically used in the concentration range of 200 ppm to 600 ppm. 
     Demulsifiers are surfactants that break up emulsions and allow fuel and water phases to separate, and are typically are used in the range of 5 ppm to 30 ppm. 
     Dispersants are well known in the lubricating oil field. 
     Corrosion and oxidation inhibitors are compounds that attach to metal surfaces and form a barrier that prevents attack by corrosive agents, and are typically are used in the range of 5 ppm to 15 ppm. 
     Friction modifiers, such as fatty acid esters and amides, glycerol esters of dimerized fatty acids, and succinate esters or metal salts thereof, can be used. 
     Pour point depressants such as C 8-18  dialkyl fumarate vinyl acetate copolymers, polymethacrylates and wax naphthalene, can be used. 
     Examples of anti-wear agents include zinc dialkyldithiophosphate, zinc diary diphosphate, and sulfurized isobutylene. Additional additives are described in U.S. Pat. No. 5,898,023 to Francisco, et al. 
     III. Alternative Fuel Composition 
     The fuels prepared as described herein can be used directly, or combined with conventional fuels to form an alternative fuel composition. When formulated as gasoline, diesel or jet fuels, the compositions can be combined with gasoline, diesel and/or jet fuels, as appropriate, or used as is, to run gasoline, diesel and/or jet engines. The blended ratios with petroleum-based fuels are typically such that the resulting blended fuel composition ideally contain between about 25 to about 98 percent of the conventional fuel and between about 2 to about 75 percent of the compositions described herein. The components can be mixed in any suitable manner. 
     IV. Methods for Preparing the Fuel Composition 
     
         
         
           
             A. Hydrolysis 
           
         
       
    
     If one starts with fatty acids derived from bacterial sources, there is no need to perform a hydrolysis step. However, if all or a portion of the fatty acids is derived from triglycerides, the first step in the process involves either hydrolysis or saponification of the triglyceride to form free fatty acids and glycerol. Conditions for hydrolyzing/saponifying triglycerides are well known to those of skill in the art. Although triglycerides can be hydrolyzed during the thermal decarboxylation step, they can also be hydrolyzed beforehand. Any acid catalyst that is suitable for performing triglyceride hydrolysis can be used, in any effective amount and any effective concentration. Examples of suitable acids include, but are not limited to, hydrochloric acid, hydrobromic acid, sulfuric acid, nitric acid, and solid catalysts such as Dowex 50™. 
     The presence of glycerol and water in the subsequent thermal decarboxylation step is not deleterious, although to increase throughput, it may be desirable to remove the glycerol/water fraction before thermal decarboxylation. 
     In one aspect, the hydrolysis occurs in a batch-type process, where water, triglycerides and an acid catalyst are heated until the hydrolysis is complete. The resulting aqueous phase includes glycerol, water, and, if the acid catalyst is water soluble, an acid. In another aspect, the triglycerides are thermally hydrolyzed by heating them with water at a temperature at or near, and, ideally, above, the boiling point of water. High pressure steam (steam hotter than the boiling point of water) can quickly hydrolyze triglycerides to glycerol and free fatty acids.
         B. Thermal Decarboxylation/Deoxygenation of Free Fatty Acids       

     Free fatty acids are converted to alkanes via thermal decarboxylation and/or thermal decarbonylation. In one aspect, the thermal decarboxylation of fatty acids is performed in the same step in which a triglyceride is hydrolyzed to form glycerol/water and free fatty acids. 
     In a batch or continuous process, free fatty acids, optionally in water or an organic solvent, and optionally in the presence of a catalyst, are subjected to thermal decarboxylation conditions. Typically, this involves heating the free fatty acids to a temperature of between 400-600 C, though this temperature can be lowered by appropriate selection of catalyst. 
     By-products, including primarily carbon dioxide and hydrogen, can be separately collected. The hydrogen and carbon dioxide can be collected, with the hydrogen ideally being separated from the carbon dioxide. The hydrogen can be used for hydrocracking, for hydrofinishing, and/or other processing steps. 
     In another aspect, the thermal decarboxylation of the fatty acids is performed using a static mixer or other suitable means for mixing high pressure steam and triglycerides. The high pressure steam and triglycerides initially form the glycerol/water stream, and the fatty acids then form the decarboxylation products, including carbon dioxide and hydrogen. Using a static mixer or similar apparatus, the hydrogen and carbon dioxide gases thus formed can flow in the direction of the other products. Since the other products tend to liquefy at higher temperatures than the hydrogen and carbon dioxide, use of a static mixer can both facilitate collection of the gases and minimize pressure and foaming in the reactor, as might otherwise occur in a batch process in which water and fatty acids are heated above the boiling points of either. The decarboxylation reaction can occur in a time frame suitable for using a static mixer or other suitable mixing apparatus in a continuous process, and the shorter contact times can minimize product degradation, as has been reported in cases where whole animal carcasses have been subjected to these types of elevated temperatures and pressures. 
     Following the conversion of the lipidic biomass to free fatty acids, the FFAs are converted to straight-chain paraffins (i.e., n-alkanes) via a reduction process. This step can be carried out in the gas phase (e.g., using a fixed bed catalyst) or in the liquid phase (e.g., using a stirred autoclave reactor with a catalyst slurry/dispersion). 
     Although reduction processes have been previously performed, the present invention recognizes that catalytic reduction processes are needed to provide reliable, consistent decarboxylation of the FFAs to provide a constant stream of n-alkanes, which are then hydrocracked and, optionally, isomerized and/or hydrotreated. Accordingly, the FFAs are contacted with an appropriate catalyst. In one embodiment, the FFAs can be passed through a fixed-bed catalyst, such as palladium on carbon (Pd/C). In another embodiment, the FFAs can be combined with a slurry of Pd/C in a stirred autoclave using solvent. 
     Deoxygenation is generally understood as relating to a chemical reaction resulting in the removal of oxygen. In the present invention, deoxygenation of FFAs is a reversible reaction. 
     While decarboxylation and decarbonylation will both proceed over a Pd/C catalyst, decarboxylation is the primary reaction pathway, and the rate of decarboxylation is generally at least an order of magnitude faster than that of decarbonylation. When the n-alkane reaction product from the deoxygenation reaction is used as the reaction solvent (which is more fully described below) and the deoxygenation reaction is performed under hydrogen, the decarbonylation pathway is more significant, since it is not slowed due to microscopic reversibility. It is notable, however, that stearic acid decarboxylation is much slower in heptadecane solvent with a 10% H 2  atm. The reaction is driven toward the reaction product by constant 10% H 2  sparge, which purges the formed CO 2  from the reactor. The decarboxylation rate is slowed in heptadecane due to equilibrium limitations. The decarbonylation pathway is unaffected by the change in solvent since both CO and heptadecane are kept at low concentrations keeping the reverse decarbonylation reactions to a minimum. 
     This reaction pathway can generically be referred to as a reduction reaction or a deoxygenation reaction. Both terms are meant to encompass both the decarboxylation reaction and the decarbonylation reaction. Since decarboxylation is the primary reaction pathway, particularly when using preferred catalysts, the discussion relating to conversion of FFAs to n-alkanes may be particularly described in terms of the decarboxylation reaction. However, the invention is not to be considered as being limited to decarboxylation. Rather, a decarbonylation mechanism is fully encompassed by the invention, particularly in embodiments where n-alkane product is recycled as the reaction solvent. 
     Although decarboxylation can be achieved through application of high heat in the presence of a high boiling solvent, such thermal decarboxylation is ineffective for complete and consistent reaction of FFAs into their corresponding n-alkanes. In comparison, however, catalytic decarboxylation according to the present invention provides for very good selectivity and a conversion rate approaching 100%. In specific embodiments, the catalytic decarboxylation has a conversion rate to the corresponding n-alkane of at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 92%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, or at least about 99%. 
     The addition of catalyst apparently helps to drive the reaction to completion. Accordingly, in one embodiment, decarboxylation is performed using a solvent such as dodecane under either catalytic or non-catalytic conditions, at temperatures of around 300 C at 1.5 MPa pressure for varying residence times, with a catalyst such as 5% Pd/C. 
     Decarboxylation of carboxylic acids was first reported by Maier, et al. (Chemische Berichte 115: 225-229, 1982) using gas-solid (heterogeneous) catalysis over supported palladium and nickel catalysts in the presence of hydrogen. For straight-chain carboxylic acids, palladium was preferred over nickel. The longest straight-chain acid investigated by Maier et al. was octanoic acid (C.sub.8). According to the present invention, however, it is possible to successfully decarboxylate a longer chain carboxylic acid (e.g., C18 compounds, such as stearic acid, or even higher carbon compounds) in the gas phase. Such gas phase decarboxylation generally comprises vaporization of the lipidic feedstock. For example, in one embodiment when using a feedstock comprising stearic acid, it is necessary to heat to a temperature of at least about 361 C (the boiling point of stearic acid). 
     Gas phase catalytic deoxygenation can be carried out by injecting the FFAs from the hydrolysis step into a suitable reactor vessel in fluid communication with a catalyst chamber and heating to a temperature suitable to vaporize the FFAs. The vaporized FFAs move through the catalyst chamber where conversion to the corresponding n-alkane on the order of 100% is achieved. The product can then proceed through a cooling zone for condensation of the n-alkanes. In certain embodiments, it can be useful to purge the system with H 2  to remove oxygen prior to heating to the FFA vaporization temperature. 
     Liquid phase deoxygenation is also effective according to the present invention. For example, stearic acid or other fatty acids in a dodecane or other alkane solvent can be heated to about  300  C while contacted with a palladium catalyst, such as a 5% Pd/C catalyst. Heptadecane is formed as the major reaction product. Thus, when such a typical solvent is used, it is necessary to isolate the reaction product from the solvent prior to introducing the reaction product into the hydrocracker. 
     Supercritical water can optionally be used as the solvent. 
     Snare, et al. (Industrial Engineering Chemistry Research 45(16): 5708-5715, 2006) investigated the deoxygenation of stearic acid as an alternative process for biodiesel production from FFAs using a liquid-phase batch process with dodecane as the solvent (requiring a solvent-to-FFA mass ratio of 19:1). As pointed out above, a separation process was required to recover the products and remove the solvent. According to the present invention, however, it is possible to carry out liquid phase decarboxylation of stearic acid in heptadecane, which is the decarboxylation product of stearic acid. Thus, in certain embodiments, the present invention provides for liquid phase catalytic decarboxylation of a long chain FFA into its corresponding n-alkane while recycling a portion of the reaction product as the solvent. Employing the product of the reaction as the solvent greatly increases the thermodynamic efficiency because the need to heat a separate solvent stream is eliminated. This is further advantageous because it eliminates the need for an additional separation process because the product and the solvent are the same. Thus, the continuous nature of the inventive process is conserved by recycling a portion of the decarboxylation reaction stream as the decarboxylation solvent in a liquid phase reaction. 
     As previously noted, deoxygenation is a reversible process, and there can thus be equilibrium limitations on the decarboxylation and/or decarbonylation reactions taking place. For example, when using recycled n-alkane reaction product as reaction solvent, deoxygenation can be slowed in both the decarboxylation and decarbonylation pathways. Accordingly, in certain embodiments, it is beneficial to including a purging step to facilitate reaction. For example, removal of CO 2  (a decarboxylation product) can be useful to drive equilibrium toward reactants. 
     Since decarboxylation is the dominant deoxygenation pathway over a Pd/C catalyst, hydrogen generally is not required for the reaction. Nevertheless, in specific embodiments, it can be particularly useful to introduce hydrogen into the reaction. 
     The decarboxylation kinetics of stearic acid and oleic acids in H 2  are closely similar, with complete FFA conversion occurring in approximately 30 minutes and providing essentially 100% yield of n-heptadecane. 
     Other known processes that purport to form fuels rely heavily on the use of H 2  as a reactant, particularly in hydrotreating processes, to achieve oxygen removal. The present invention, however, is not so limited. Rather, as pointed out above, deoxygenation according to the present process is catalytically achieved, and amount of H 2  used is generally a function of the lipidic biomass feedstock. For example, when using highly saturated materials, H 2  can be relegated to a basically non-reactive status, being used mainly as a purge material, such as described above in relation to gas-phase catalytic decarboxylation. When using a less saturated (i.e., more olefinic) feedstock, additional H 2  can be used to encourage production of n-alkanes. 
     While Pd/C can be used as an efficient FFA decarboxylation catalyst, the use of other catalysts is not excluded. Rather, any catalyst effective in facilitating FFA decarboxylation can be used as a catalyst, or, alternatively, no catalyst need be used. In particular, any noble metal may be used, particularly platinum and palladium. Moreover, bimetallic catalysts may also be used according to the invention and may have the formula M N -X, wherein M N  is a noble metal and X is a complementary metal, which can include other noble metals or transition metals. Moreover, supports other than carbon can be used according to the invention. Non-limiting examples of supports useful according to the invention in addition to carbon include silicates, as well as any other support-type material which, preferably, is non-acidic and substantially or completely inert (i.e., have little or no inherent catalytic function). Non-limiting examples of further catalysts that could be used according to the invention include Ni, Ni/Mo, Ru, Pd, Pt, Ir, Os, and Rh metal catalysts. 
     By using a catalyst, the temperature requirements are generally lower than when no catalyst is used, although the decarboxylation is still performed when no catalysts are present. In the absence of a catalyst, temperatures in excess of 400 C are typically used to achieve appreciable decarboxylation. Even greater temperatures (e.g., in excess of 500 C) can be required to achieve useful levels of decarboxylation. The use of catalysts, therefore, can be preferred. Particularly, it is possible to proceed with significantly lower reaction temperatures while still achieving excellent decarboxylation. In certain embodiments, to carry out catalytic decarboxylation in a liquid phase reaction, the FFAs are heated to a temperature of up to about 325 C. In other embodiments, the FFAs are heated, in the presence of a suitable catalyst, to a temperature in the range of about 200 C to about 320 C, about 250 C to about 320 C., about 270 C to about 320 C, or about 290 C to about 310 C. Reaction pressure can be in the range of about 400 kPa to about 800 kPa, preferably about 500 kPa to about 700 kPa. 
     Catalytic decarboxylation occurs at around 300 C in the liquid-phase under conditions, where this temperature may be too low to achieve thermal decarboxylation in the absence of a catalyst. Moreover, reaction selectivity for n-alkanes. For example, in certain embodiments, decarboxylation occurs in a manner such that greater than 90% of the hydrocarbon reaction products are n-alkanes. In further embodiments, decarboxylation occurs in a manner such that greater than 92%, greater than 95%, greater than 97%, or greater than 98% of the hydrocarbon reaction products are n-alkanes. 
     The largest single energy cost in the process of the invention is the cost of heating the solvent to reaction temperature. Accordingly, in preferred embodiments, the inventive process can be optimized to minimize or eliminate the use of an added solvent in the reaction process. In one particular embodiment, the reaction can proceed in liquid n-alkane (without additional solvent) that is recycled from the reaction process. In such an embodiment, the catalyst can be used in a slurry/dispersion with the FFAs. Moreover, since the decarboxylation is proceeding catalytically and is not dependent upon temperature alone, less heat is required to maintain the lower process heat used in the catalytic decarboxylation process. 
     The benefits of thermal decarboxylation are particularly seen in the liquid phase reaction using recycled n-alkane as the reaction solvent. As pointed out above, traditional thermal decarboxylation is typically carried out in the liquid phase using a hydrocarbon solvent, such as dodecane. By using a portion of the n-alkane that is produced as the solvent for the decarboxylation reaction, one need not separately isolate the product from the solvent. In certain embodiments, it is possible to use a catalyst slurry/dispersion with a solvent that is recycled n-alkane decarboxylation reaction product. 
     In further embodiments, the reaction can be carried out in a continuous stirred autoclave with recycling of reaction components. Further, gas phase fixed bed reactors, as well as liquid phase slurry reactors could be use. Of course, these are merely representative types of reactors and are not intended to limit the scope of the invention. One example of a method for heterogeneous catalytic deoxygenation is disclosed by Snare et al., I. &amp; E. Chem. Res. 45(16) 5708-5715 (2006), which is incorporated herein by reference in its entirety. 
     U.S. Patent Publication No. 20080071125 describes the use of supercritical water to affect decarboxylation. Using a Diels Alder reaction, the fatty acids (if they include double bonds) can be cyclized before they are decarboxylated, if desired. The contents of this patent application are hereby incorporated by reference in their entirety. 
     U.S. Pat. No. 8,389,782 discloses thermal decarboxylation reactions, and these can also be used. 
     The &#39;872 patent discloses that catalysts can be used, including metal titanates, also referred to herein interchangeably as titanates, which can be expressed as MTiO 3  wherein M is a metal having a valence of 2+. The metal M may also be capable of multiple valences. In one embodiment, the catalyst consists essentially of at least a metal titanate of the formula MTiO 3 . Pure metal titanates have a perovskite crystalline structure. The catalyst can contain at least 80% by weight titanate. In another embodiment, the catalyst contains at least 1% by weight titanate; in another embodiment at least 5% by weight titanate; in another embodiment at least 10% by weight titanate based on the total weight of the catalyst, including any other desirable active components as well as optional support material. The actual amount of titanate needed will vary depending on whether or not a support is used, and how the catalyst is dispersed on the support. Examples of suitable metal titanates for use in the catalyst include, but are not limited to, magnesium titanate, copper titanate, nickel titanate, iron(II) titanium oxide, cobalt titanium oxide, manganese(II) titanium oxide, lead(II) titanate, calcium titanate, barium titanate, zinc titanate, and mixtures thereof. In one embodiment, the catalyst has a BET surface area greater than 20 m 2 /g; in another embodiment the BET surface area is greater than 200 m 2 /g; in yet another embodiment the BET surface area is greater than 400 m 2 /g. 
     In one embodiment, the catalyst is a supported catalyst. Suitable support materials include silica, alumina, silica-alumina, carbon, molecular sieves and mixtures thereof. In one embodiment, the catalyst is deposited on a carbon support having a BET surface area of between 500 m 2 /g and 1500 m 2 /g. In another embodiment, the catalyst is deposited on a support selected from silica, alumina, silica-alumina and mixtures thereof, and the support has a BET surface area of between 150 m 2 /g and 600 m 2 /g. In one embodiment, the support can be a monolithic support. Alternatively, the catalyst can be unsupported. 
     The feed is contacted with the catalyst at a temperature of less than 500 C, in one embodiment from 200 C to 500 C, and in one embodiment from 200 C to 400 C. In one embodiment, the pressure within the reactor is between 100 kPa and 1000 kPa (all pressures indicated herein are absolute). The pressure can be below 100 kPa, although depending on the pressure in the surrounding equipment, it may be necessary to pump the stream exiting the reactor to a higher pressure. In one embodiment, the LHSV is between 0.1 and 10 in another embodiment, the LHSV is between 0.2 and 5.0 h −1 ; in another embodiment, between 0.4 and 2.0 h −1 . LHSV refers to the volumetric liquid feed rate per total volume of catalyst and is expressed in the inverse of hours (h −1 ). 
     In one embodiment, the reaction is conducted in the absence of added hydrogen. 
     In the working examples, the &#39;782 patent used ZnTiO 3  (product number 634409, obtained from Sigma-Aldrich Corp., St. Louis, Mo.) at a temperature of around 350 C in a batch reactor, with N 2  as a purge gas to remove CO 2  formed during the reaction. 
     Decarboxylation in Supercritical Water 
     The authors of the &#39;125 application used catalytic hydrothermolysis to both hydrolyze triglycerides and thermally decarboxylate the fatty acids. Since fatty acids are used as feedstocks, the hydrolysis step is not performed, but otherwise, the conditions used for catalytic hydrothermolysis are one way to carry out the desired thermal decarboxylation. 
     There is a growing interest in hot-compressed water as alternatives to organic solvents and as a medium for unique and/or green chemistry. Of particular interest is processes in water near its critical point (T c =374 C, P c  =221 bar, and rho c =0.314 g/ml). One of the attractive features of hot-compressed water is the adjustability of its properties by varying process temperature and pressure. Specific to its solvent properties, the dielectric constant of water can be adjusted from 80 at room temperature to 5 at its critical point. Therefore, water can solubilize most nonpolar organic compounds including most hydrocarbons and aromatics starting at 200-250 C and extending to the critical point. The reversal of the solvent characteristics of hot-compressed water also results in precipitation of salts that are normally soluble in room temperature water. Most inorganic salts become sparingly soluble in supercritical water. This is the basis for unique separation of ionic species in supercritical water. The precipitated salts can serve as heterogeneous catalysts for reactions in supercritical water. 
     Hot-compressed water has been exploited in a number of novel processes including oxidation, partial oxidation, hydrolysis, and cracking/thermal degradation of small molecular compounds and polymeric materials. The last processing area of this list is the most pertinent to the process of the present invention. Studies have already been conducted on the thermal degradation of polyethylene in subcritical and supercritical water since the late 1990s. It has also been shown that supercritical water suppressed coke formation and increased oil yield in cracking polyethylene as compared to thermal cracking. As a comparison, conventional cracking inevitably produces large fractions of coke and light hydrocarbons. For example, catalytic cracking of palm oil without water can achieve 99 wt % conversion, but only 48 wt % gasoline yield at 450 C using zeolite catalysts with the balance (52%) being coke and light hydrocarbons. The unique and established contributions by hot-compressed water to various oxidation and hydrolysis processes are translated to modified soybean oil in the CH process of the present invention. 
     The CH process as used in the present invention can trigger the following simultaneous or sequential reactions: cracking, hydrolysis, decarboxylation, dehydration, isomerization, recombination, and/or aromatization. 
     Cracking is the key reaction pathway to manipulate the carbon chain length distribution and structural variation of the resulting hydrocarbon mixture. In such a water/oil homogeneous state and uniform and rapid heating environment provided by hot-compressed water, the cracking reactions of the current invention produce a more desirable spectrum of hydrocarbon products than any conventional thermal cracking process. Unsaturated fatty acids and derivatives are more susceptible for cracking at lower temperatures than the saturated fatty acids. Specifically, cracking of unsaturated fatty acids and pre-conditioned derivatives are likely to occur at the carbon adjacent to either side of a double bond or a junction point of three carbon-carbon bonds resulting from cross-linking. The apparent activation energy for hydrothermal of cracking heavy saturated hydrocarbons has a reported value of 76 Id/mol. Cracking reactions and products are summarized in the following table. 
     Catalytic Decarboxylation/Dehydration oxygen, as carbon dioxide, from fatty acids. Reactions of long chain saturated fatty acids in hot-compressed water primarily produce alkanes and alkenes with one less carbon than that of the starting compound. Specifically, decarboxylation of stearic acid in supercritical water follows monomolecular and bimolecular reaction pathways. The former produces C 17  alkane, and the latter renders C 35  ketone (i.e., combining two fatty acid molecules) and C 16  alkene. The formation of C 35  ketone may increase C 10  and C 11  fractions in the cracked hydrocarbon mixture. The presence of a water solvent field greatly facilitates the decarboxylation reaction. For soybean saturated fatty acids (14 wt %), mostly palmitic acid (11 wt %), decarboxylation is likely to occur before cracking. While decarboxylation removes carbon dioxide from the fatty acid, the proton stays with the carbon backbone to form the alkane. Therefore, for each mole of soybean oil, 1.5 moles of H 2  are extracted from water and added to the resulting hydrocarbon. 
     Processing parameters govern the effectiveness and efficiency of the CH process and its product quality and distribution include temperature, water to oil ratio, catalysts, and rate of heating and depressurization. Specifically, temperature effects the rates of parallel reactions, hence influencing carbon chain length distribution and characteristics. The CH process is conducted at temperatures ranging from 240 to 450 C under corresponding pressures either above or below the saturation or critical pressure. The deviation of pressure from saturation or critical pressure may be determined by process operability, product quality and economics. In addition, isomerization and aromatization may take place under CH conditions at the higher temperatures, in the range of 400 to 500 C. 
     The water to fatty acid ratio is another key factor to control the rates of cracking and decarboxylation, hence impacting on product distributions. It also has process economic implications, since more water would require more thermal energy input. The water to oil mass ratio is controlled in the range from 10:1 to 1:100, preferably from 1:1 to 1:10. 
     Most catalysts used in conventional organic phase oil conversion processes are likely to be deactivated by water, particularly high-temperature water. Two types of materials have been used as catalysts in high-temperature water applications: metal oxides and inorganic salts. Catalysts suitable for use in the CH process include salts, oxides, hydroxides, clays, minerals, and acids. Preferably, catalysts are selected from metal oxides, preferably transition metal oxides, such as ZrO 2 , TiO 2  and Al 2 O 3 ; high melting point salts which are insoluble in supercritical water, such as Na 2 CO 3 , Cu 2 Cl 2  and Cu 2 Br 2 ; low melting point salts, such as ZnCl 2 ; hydroxides, such as alkali and alkali earth metal hydroxides; clays such as bentonite and montmorillonite; minerals such as silicates, carbonates, molybdates, or borates; or mineral or boric acids. 
     Finally, the rate of heating and depressurization of the CH process effluent can be used to manipulate product yield and quality. Critically, the rate of heating the fatty acids by contact with the water should be rapid, preferably no less than 10 C per second. Similarly, the pressure of the oil/water mixture should be reduced before releasing the same through a nozzle, or otherwise allowing sudden expansion, to ensure continued separation of the oil and water. 
     Dehydrogenation 
     If the decarboxylated fatty acids do not include carbon-carbon double bonds, and it is desirable that they do so, they can be dehydrogenated before the olefin metathesis reaction. Similarly, before or following the olefin metathesis, any alkanes present in the low molecular weight olefin fraction can be dehydrogenated. The dehydrogenation catalyst must have dehydrogenation activity to convert at least a portion of the paraffins to olefins, which are believed to be the actual species that undergo olefin metathesis. 
     Platinum and palladium or the compounds thereof are preferred for inclusion in the dehydrogenation/hydrogenation component, with platinum or a compound thereof being especially preferred. As noted previously, when referring to a particular metal in this disclosure as being useful, the metal can be present as elemental metal or as a compound of the metal. As discussed above, reference to a particular metal in this disclosure is not intended to limit the invention to any particular form of the metal unless the specific name of the compound is given, as in the examples in which specific compounds are named as being used in the preparations. 
     The dehydrogenation step can be conducted by passing the decarboxylated fatty acids over a dehydrogenation catalyst under dehydrogenating reaction conditions. If it is desirable to reduce or eliminate the amount of diolefins produced or other undesired by-products, the reaction conversion to internal olefins should preferably not exceed 50%, and more preferably not exceed 30%, but proceed by at least 15-20%. 
     The dehydrogenation is typically conducted at temperatures between about 500 F and 1000 F (260 C and 538 C), preferably between about 600 F and 800 F (316 C and 427 C). The pressures are preferably between about 0.1 and 10 atms, more preferably between about 0.5 and 4 atms absolute pressure (about 0.5 to 4 bars). The LHSV (liquid hourly space velocity) is preferably between about 1 and 50 hr −1 , preferably between about 20 and 40 hr −1 . The products generally and preferably include internal olefins. 
     The dehydrogenation is also typically conducted in the presence of a gaseous diluent, typically and preferably hydrogen. Although hydrogen is the preferred diluent, other art-recognized diluents may also be used, either individually or in admixture with hydrogen or each other, such as steam, methane, ethane, carbon dioxide, and the like. Hydrogen is preferred because it serves the dual-function of not only lowering the partial pressure of the dehydrogenatable hydrocarbon, but also of suppressing the formation of hydrogen-deficient, carbonaceous deposits on the catalytic composite. Hydrogen is typically used in amounts sufficient to insure a hydrogen to hydrocarbon feed mole ratio of about from 2:1 to 40:1, preferably in the range of about from 5:1 to 20:1. 
     Suitable dehydrogenation catalysts which can be used include Group VIII noble metals, e.g., iron, cobalt, nickel, palladium, platinum, rhodium, ruthenium, osmium, and iridium, preferably on an oxide support. 
     Less desirably, combinations of Group VIII non-noble and Group VIB metals or their oxides, e.g., chromium oxide, may also be used. Suitable catalyst supports include, for example, silica, silicalite, zeolites, molecular sieves, activated carbon alumina, silica-alumina, silica-magnesia, silica-thoria, silicaberylia, silica-titania, silica-aluminum-thora, silica-alumina-zirconia kaolin clays, montmorillonite clays and the like. In general, platinum on alumina or silicalite afford very good results in this reaction. Typically, the catalyst contains about from 0.01 to 5 wt. %, preferably 0.1 to 1 wt. % of the dehydrogenation metal (e.g., platinum). Combination metal catalysts, such as those described in U.S. Pat. Nos. 4,013,733; 4,101,593 and 4,148,833, can be used. 
     Since dehydrogenation produces a net gain in hydrogen, the hydrogen may be taken off for other plant uses or as is typically the case, where the dehydrogenation is conducted in the presence of hydrogen, a portion of the recovered hydrogen can be recycled back to the dehydrogenation reactor. Further information regarding dehydrogenation and dehydrogenation catalysts can, for example, be found in U.S. Pat. Nos. 4,046,715; 4,101,593; and 4,124,649. A variety of commercial processes also incorporate dehydrogenation processes, in their overall process scheme, which dehydrogenation processes may also be used in the present process to dehydrogen the paraffinic hydrocarbons. Examples of such processes include the dehydrogenation process portion of the Pacol process for manufacturing linear alkylbenzenes, described in Vora et al., Chemistry and Industry, 187-191 (1990); Schulz R. C. et al., Second World Conference on Detergents, Montreaux, Switzerland (October 1986); and Vora et al., Second World Surfactants Congress, Paris France (May 1988). 
     If desired, diolefins produced during the dehydrogenation step may be removed by known adsorption processes or selective hydrogenation processes which selectively hydrogenate diolefins to monoolefins without significantly hydrogenating monoolefins. One such selective hydrogenation process known as the DeFine process is described in the Vora et al. Chemistry and Industry publication cited above.
         A. Isomerization Chemistry       

     Optionally, various fractions resulting from the thermal decarboxylation of free fatty acids (i.e., a fraction already in the desired molecular weight range for preparing the desired distillate fuel product), the fractions being molecularly averaged, and/or the products of the molecular averaging chemistry, are isomerized. The isomerization products have more branched paraffins, thus improving their pour, cloud and freeze points. Isomerization processes are generally carried out at a temperature between 200 F and 700 F, preferably 300 F to 550 F, with a liquid hourly space velocity between 0.1 and 2, preferably between 0.25 and 0.50. The hydrogen content is adjusted such that the hydrogen to hydrocarbon mole ratio is between 1:1 and 5:1. Catalysts useful for isomerization are generally bifunctional catalysts comprising a hydrogenation component (preferably selected from the Group VIII metals of the Periodic Table of the Elements, and more preferably selected from the group consisting of nickel, platinum, palladium and mixtures thereof) and an acid component. Examples of an acid component useful in the preferred isomerization catalyst include a crystalline zeolite, a halogenated alumina component, or a silica-alumina component. Such paraffin isomerization catalysts are well known in the art. 
     Optionally, but preferably, the resulting product is hydrogenated. The hydrogen can come from a separate hydrogen plant, can be derived from syngas, made directly from methane or other light hydrocarbons, or come directly from the thermal decarboxylation step. 
     After hydrogenation, which typically is a mild hydrofinishing step, the resulting distillate fuel product is highly paraffinic. Hydrofinishing is done after isomerization. Hydrofinishing is well known in the art and can be conducted at temperatures between about 190 C to about 340 C, pressures between about 400 psig to about 3000 psig, space velocities (LHSV) between about 0.1 to about 20, and hydrogen recycle rates between about 400 and 1500 SCF/bbl. 
     The hydrofinishing step is beneficial in preparing an acceptably stable fuel. Fuels that do not receive the hydrofinishing step may be unstable in air and light due to olefin polymerization. To counter this, they may require higher than typical levels of stability additives and antioxidants.
         B. Thermal Cracking       

     The thermal decarboxylation products are subjected to hydrocracking steps, to reduce their molecular weight, and, ideally, to reduce the viscosity of the products to be the same as, or lower than, diesel fuel. The viscosity can be lowered by thermally cracking, hydrocracking, or pyrolyzing the composition, preferably in the presence of a Lewis acid catalyst. 
     Methods for thermally cracking or hydrocracking hydrocarbons are known to those of skill in the art. Representative Lewis acid catalysts and reactions conditions are described, for example, in Fluid Catalytic Cracking II, Concepts in Catalyst Design, ACS Symposium Series 452, Mario Occelli, editor, American Chemical Society, Washington, D.C., 1991. The pyrolysis of vegetable oils is described in Alencar, et al., Pyrolysis of Tropical Vegetable Oils, J. Ag. Food Chem., 31:1268-1270 (1983). The hydrocracking of vegetable oils is described in U.S. Pat. No. 4,992,605 to Craig, et al. 
     In one embodiment, the fuel additive composition is heated to a temperature of between approximately 100 and 500 F, preferably to between approximately 100 and 200 F, and more preferably to between approximately 150 and 180 F, and then passed through a Lewis acid catalyst. Any Lewis acid catalyst that is effective for thermally cracking hydrocarbons can be used. Suitable catalysts for use in the present invention include, but are not limited to, zeolites, clay montmorrilite, aluminum chloride, aluminum bromide, ferrous chloride and ferrous bromide. Preferably, the catalyst is a fixed-bed catalyst. 
     A preferred catalyst is prepared by coating a ceramic monolithic support with lithium metal. Supports of this type are manufactured, for example, by Dow-Coming. Lithium is coated on the support by first etching the support with zinc chloride, then brushing lithium onto the support, and then baking the support. 
     The retention time through the Lewis acid catalyst can be as little as one second, although longer retention times do not adversely affect the product. 
     After passing through the Lewis acid catalyst, the derivative stream is then preferably heated to a temperature of between approximately 200 and 600 F, preferably between approximately 200 and 230 F, to thermally crack the product. The resulting product is suitable for blending with distillate fuel, such as gasoline, diesel, or jet fuel, to form an alternative fuel composition.
         C. Hydrotreating and/or Hydrocracking Chemistry       

     Fractions used in the process described herein may include heteroatoms such as sulfur or nitrogen, diolefins and alkynes that may adversely affect the catalysts used in the various reactions. If sulfur impurities are present in the starting materials, they can be removed using means well known to those of skill in the art, for example, extractive Merox, hydrotreating, adsorption, etc. Nitrogen-containing impurities can also be removed using means well known to those of skill in the art. Hydrotreating and hydrocracking are preferred means for removing these and other impurities from the heavy wax feed component. Removal of these components from the light naphtha and gas streams must use techniques that minimize the saturation of the olefins in these streams. Extractive Merox is suitable for removing sulfur compounds and acids from the light streams. The other compounds can be removed, for example, by adsorption, dehydration of alcohols, and selective hydrogenation. Selective hydrogenation of diolefins, for example, is well known in the art. One example of a selective hydrogenation of diolefins in the presence of olefins is UOP&#39;s DeFine process. 
     Hydrogenation catalysts can be used to hydrotreat the products resulting from the hydrocracking and/or isomerization reactions. 
     As used herein, the terms “hydrotreating” and “hydrocracking” are given their conventional meaning and describe processes that are well known to those skilled in the art. Hydrotreating refers to a catalytic process, usually carried out in the presence of free hydrogen, in which the primary purpose is the desulfurization and/or denitrification of the feedstock. Generally, in hydrotreating operations, cracking of the hydrocarbon molecules, i.e., breaking the larger hydrocarbon molecules into smaller hydrocarbon molecules, is minimized and the unsaturated hydrocarbons are either fully or partially hydrogenated. 
     Hydrocracking refers to a catalytic process, usually carried out in the presence of free hydrogen, in which the cracking of the larger hydrocarbon molecules is a primary purpose of the operation. Desulfurization and/or denitrification of the feed stock usually will also occur. 
     Catalysts used in carrying out hydrotreating and hydrocracking operations are well known in the art. See, for example, U.S. Pat. Nos. 4,347,121 and 4,810,357 for general descriptions of hydrotreating, hydrocracking, and typical catalysts used in each process. 
     Suitable catalysts include noble metals from Group VIIIA, such as platinum or palladium on an alumina or siliceous matrix, and unsulfided Group VIIIA and Group VIB, such as nickel-molybdenum or nickel-tin on an alumina or siliceous matrix. U.S. Pat. No. 3,852,207 describes suitable noble metal catalysts and mild hydrotreating conditions. Other suitable catalysts are described, for example, in U.S. Pat. Nos. 4,157,294 and 3,904,513. The non-noble metal (such as nickel-molybdenum) hydrogenation metal are usually present in the final catalyst composition as oxides, or more preferably or possibly, as sulfides when such compounds are readily formed from the particular metal involved. Preferred non-noble metal catalyst compositions contain in excess of about 5 weight percent, preferably about 5 to about 40 weight percent molybdenum and/or tungsten, and at least about 0.5, and generally about 1 to about 15 weight percent of nickel and/or cobalt determined as the corresponding oxides. The noble metal (such as platinum) catalyst contains in excess of 0.01 percent metal, preferably between 0.1 and 1.0 percent metal. Combinations of noble metals may also be used, such as mixtures of platinum and palladium. 
     The hydrogenation components can be incorporated into the overall catalyst composition by any one of numerous procedures. The hydrogenation components can be added to matrix component by co-mulling, impregnation, or ion exchange and the Group VI components, i.e., molybdenum and tungsten can be combined with the refractory oxide by impregnation, co-mulling or co-precipitation. Although these components can be combined with the catalyst matrix as the sulfides, that may not be preferred, as the sulfur compounds may interfere with some molecular averaging or Fischer-Tropsch catalysts. 
     The matrix component can be of many types including some that have acidic catalytic activity. Ones that have activity include amorphous silica-alumina or may be a zeolitic or non-zeolitic crystalline molecular sieve. Examples of suitable matrix molecular sieves include zeolite Y, zeolite X and the so-called ultra-stable zeolite Y and high structural silica:alumina ratio zeolite Y such as that described in U.S. Pat. Nos. 4,401,556, 4,820,402 and 5,059,567. Small crystal size zeolite Y, such as that described in U.S. Pat. No. 5,073,530, can also be used. Non-zeolitic molecular sieves which can be used include, for example, silicoaluminophosphates (SAPO), ferroaluminophosphate, titanium aluminophosphate, and the various ELAPO molecular sieves described in U.S. Pat. No. 4,913,799 and the references cited therein. Details regarding the preparation of various non-zeolite molecular sieves can be found in U.S. Pat. No. 5,114,563 (SAPO); U.S. Pat. No. 4,913,799 and the various references cited in U.S. Pat. No. 4,913,799. Mesoporous molecular sieves can also be used, for example, the M415 family of materials (J. Am. Chem. Soc. 1992, 114, 10834-10843), MCM-41 (U.S. Pat. Nos. 5,246,689, 5,198,203 and 5,334,368), and MCM-48 (Kresge et al., Nature 359 (1992) 710). 
     Suitable matrix materials may also include synthetic or natural substances as well as inorganic materials such as clay, silica and/or metal oxides such as silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-berylia, silica-titania as well as ternary compositions, such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia, and silica-magnesia zirconia. The latter may be either naturally occurring or in the form of gelatinous precipitates or gels including mixtures of silica and metal oxides. Naturally occurring clays which can be composited with the catalyst include those of the montmorillonite and kaolin families. These clays can be used in the raw state as originally mined or initially subjected to calumniation, acid treatment or chemical modification. 
     Furthermore, more than one catalyst type may be used in the reactor. The different catalyst types can be separated into layers or mixed. Typical hydrotreating conditions vary over a wide range. In general, the overall LHSV is about 0.25 to 2.0, preferably about 0.5 to 1.0. The hydrogen partial pressure is greater than 200 psia, preferably ranging from about 500 psia to about 2000 psia. Hydrogen recirculation rates are typically greater than 50 SCF/Bbl, and are preferably between 1000 and 5000 SCF/Bbl. Temperatures range from about 300 F to about 750 F, preferably ranging from 450 F to 600 F.
         D. Filtration of the Fuel Composition       

     In one embodiment, the fuel composition is filtered, preferably through a filter with a pore size of between approximately 5 and 50 microns, more preferably, between approximately 10 and 20 microns, to remove solid impurities. This can be especially important when animal fats are used, since rendering processes can inadvertently place small pieces of bone and other particulate matter in the animal fat that needs to be removed. 
     Modifications and variations of the present invention will be obvious to those skilled in the art from the foregoing detailed description of the invention. The disclosures of each of the patents and papers discussed above are incorporated herein by reference in their entirety.