Patent Publication Number: US-8975457-B2

Title: Methods for producing fuels and solvents substantially free of fatty acids

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. national phase application under 35 USC 371 of international application number PCT/IB 2011/000464, filed Feb. 24, 2011, which claims priority upon U.S. provisional application Ser. No. 61/307,568, filed Feb. 24, 2010. This application is hereby incorporated by reference in its entirety for all of its teachings. 
    
    
     BACKGROUND 
     There are increasing social and economic pressures to develop renewable energy sources as well as renewable and biodegradable industrial and consumer products and materials. The catalytic conversion of natural feedstocks to value-added products has resulted in new approaches and technologies whose application spans across the traditional economic sectors. There is a new focus on biorefining, which can be described as the processing of agricultural and forestry feedstocks capturing increased value by processing them into multiple products including platform chemicals, fuels, and consumer products. The conversion of tallow and other organic oils to biodiesel has been previously studied in depth. Traditionally, this conversion involves the trans-esterification of the triglyceride to produce three methyl-esterified fatty acids and a free glycerol molecule. The chemical, rheological, and combustion properties of the resulting “biodiesel” have also been extensively investigated. Unfortunately, these methyl-ester based fuels have been shown to be far more susceptible to oxidation and have lower heating values than the traditional petroleum based diesel fuels. As a result the traditional biodiesels must be blended with existing diesel stock and may also have to be supplemented with antioxidants to prolong storage life and avoid deposit formation in tanks, fuel systems, and filters. 
     The pyrolysis of fatty acids is one approach to producing the corresponding alkanes and alkenes useful as solvents and fuels. However, fatty of various chain lengths are formed as part of the pyrolysis products as well as unreacted feedstock. The presence of the fatty acids are undesirable for the applications listed above if they are to meet industry and regulatory standards. They must therefore be removed in order to be useful as hydrocarbon solvents and fuels. 
     SUMMARY 
     Described herein are methods for producing fuels and solvents from fatty acid resources. In general, the pyrolysis products of fatty acids are extracted in order to remove residual fatty acids and produce very pure hydrocarbon compositions composed of alkanes and alkenes. The fatty acids removed from the extraction step can be further pyrolyzed to produce additional hydrocarbons or, in the alternative, the fatty acids can be isolated and used in other applications. Also disclosed herein are fuels and solvents produced by the methods described herein. The advantages of the methods and compositions described herein will be set forth-in part in the description which follows, or may be learned by practice of the aspects described below. The advantages described below will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       The accompanying Figures, which are incorporated in and constitute a part of this specification, illustrate several aspects described below. 
         FIG. 1  shows the ASTM D974 acid number of stearic acid pyrolysis product (SAPP) before and after extraction. 
         FIG. 2  shows the GC-MS chromatogram of SAPP before (2A) and after (2B) extraction with 0.1M NaOH in methanol and adding water. 
         FIG. 3  shows the GC-MS chromatogram of back extracted aqueous fraction of SAPP. 
     
    
    
     DETAILED DESCRIPTION 
     Before the present materials, articles, and/or methods are disclosed and described, it is to be understood that the aspects described below are not limited to specific compounds, synthetic methods, or uses as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. 
     In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings: 
     Throughout this specification, unless the context requires otherwise, the word “comprise,” or variations such as “comprises” or “comprising,” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. 
     It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “an oil” includes a single oil or mixtures of two or more oils. 
     “Optional” or “optionally” means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where the event or circumstance occurs and instances where it does not. 
     Described herein are methods for producing fuels and solvents from fatty acid resources. In one aspect, the method comprises:
     a. separating one or more fatty acids from the fatty acid resource;   b. converting the fatty acid to one or more alkanes or alkenes to produce a hydrocarbon composition; and   c. extracting the hydrocarbon composition to substantially remove fatty acids present in the hydrocarbon composition with an extraction solvent.   

     The term “fatty acid resource” as defined herein is any source of free fatty acid or a precursor to a free fatty acid upon subsequent processing. For example, a triglyceride is a precursor to a free fatty acid, where hydrolysis of the glycerol group produces the free fatty acid. Examples of fatty acid resources include, but are not limited to, vegetable oil, animal fats, spent cooking oil, lipids, phospholipids, soapstock, or other sources of triglycerides, diglycerides or monoglycerides. In one aspect, the vegetable oil comprises corn oil, cottonseed oil, canola oil, rapeseed oil, olive oil, palm oil, peanut oil, ground nut oil, safflower oil, sesame oil, soybean oil, sunflower oil, algae oil, almond oil, apricot oil, argan oil, avocado oil, ben oil, cashew oil, castor oil, grape seed oil, hazelnut oil, hemp seed oil, linseed oil, mustard oil neem oil, palm kernel oil, pumpkin seed oil, rice bran oil, walnut oil, a combination thereof. In another aspect, the animal fat comprises blubber, cod liver oil, ghee, lard, tallow, derivatives thereof (e.g., yellow grease, used cooking oil, etc.), or a combination thereof. 
     It is contemplated that the fatty acid resource can be further purified prior to separation step (a). For example, the fatty acid resource can be distilled or extracted to remove any undesirable impurities. In the alternative, the fatty acid resource can be used as-is and proceed to separation step (a). The source of the fatty acid resource will determine if any pre-purification steps are required. 
     Separation step (a) involves removing or isolating one or more fatty acids from the fatty acid resource. A number of different techniques are known in the art for the isolation and purification of fatty acids. For example, U.S. Pat. No. 5,917,501 discloses a process for isolating fatty acids. The process involves hydrolyzing a naturally occurring lipid mixture containing phospholipids, triglycerides, and sterols to form a two-phase product containing a fatty acid phase comprised of free fatty acids and sterols, and an aqueous phase comprised of water, glycerol, and glycerol phosphoric acid esters. The aqueous phase is separated from the fatty acid phase and the crude fatty acid phase is heated to convert the free sterols to fatty acid sterol esters. The free fatty acids are distilled from the fatty acid sterol esters to yield purified fatty acids, which are free of cholesterol and other sterols, and phosphorous compounds. In other aspects, the fatty acid resource is exposed to acid in order to hydrolyze a fatty acid precursor present in the fatty acid resource to produce the corresponding fatty acid. For example, vegetable oils are rich in triglycerides, which upon acid hydrolysis, produce the free fatty acid and glycerol. 
     After the separation step, it is desirable to produce a pure or substantially pure form of the fatty acid. The phrase “substantially pure” as used herein is defined as greater than 90%, greater than 95%, greater than 99%, greater than 99.9%, or 100% by weight fatty acid content. The presence of impurities can adversely affect the final composition of the fuel or solvent. For example, if sulfur, oxygen, or nitrogen compounds are present in the fatty acid prior to step (b), undesirable product characteristics result including high sulfur or nitrogen emissions during combustion or side-reactions may occur during step (b) such as the formation of undesirable aromatic compounds. 
     The nature of the fatty acid will vary depending upon the fatty acid resource. The fatty acid can be a saturated fatty acid, an unsaturated fatty acid, or a combination thereof. Examples of fatty acids include, but are not limited to, butyric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, alpha-linolenic acid, docosahexaenoic acid, eicosapentaenoic acid, linoleic acid, arachidonic acid, oleic acid, erucic acid, a naturally derived fatty acid from a plant or animal source, or a combination thereof. It is contemplated that the fatty acid can be the free acid or the salt/ester thereof. The fatty acid can also be a mixture of fatty acids. 
     The second step involves converting the fatty acid(s) to a hydrocarbon composition composed of one or more alkanes, alkenes, or mixtures thereof. In general, during the conversion step, the fatty acids are decarboxylated and cracked to produce CO 2  and the alkanes or alkenes. The length of the alkane or alkene chain will vary depending upon the fatty acid and reaction parameters, which will be discussed in detail below. In general, the alkanes and alkenes are from C 1  to C 20  hydrocarbons. For example, decarboxylation of stearic acid, which has the formula CH 3 (CH 2 ) 16 COOH, produces CH 3 (CH 2 ) 15 CH 3 , shorter alkanes and alkenes, and CO 2 . 
     In one aspect, the conversion of the fatty acid to the alkane and/or alkene comprises heating the fatty acid to convert the majority of the fatty acid to an alkane, an alkene, or a mixture thereof. As will be discussed in greater detail below, it is not necessary to completely convert all of the fatty acids to alkanes and alkenes. The temperature of the heating step can vary amongst different parameters In one aspect, the temperature of the heating step is from 220° C. to 650° C., 300° C. to 650° C., 350° C. to 650° C., 350° C. to 600° C., or 250° C. to 500° C. Other parameters to consider are the duration of the heating step and the pressure at which the heating step is conducted. The pressure can range from ambient to 2,000 psi, and the duration of the heating step can be from seconds up to 12 hours. In one asepct, the heating step is from two seconds up to 8 hours. In another aspect, the heating step is performed under an inert atmosphere such as, for example, nitrogen or argon. 
     By varying reaction conditions during the conversion of the fatty acid to the alkane/alkene, one of ordinary skill in the art can produce short or long chain alkanes/alkenes for fuels and solvents. For example, prolonged heating at elevated temperatures can produce short chain alkanes/alkenes that can be useful as fuels. Alternatively, long chain alkanes/alkenes can be produced by one of ordinary skill in the art by reducing the heating time and temperature. If short chain alkanes or alkenes are produced, reaction conditions can be controlled such that these products are gasses (e.g., methane, propane, butane, etc.) that can be readily removed from the reactor. 
     In another aspect, the use of a decarboxylation catalyst can be used to facilitate the conversion of the fatty acid to the alkane or alkene. Depending upon the selection of the decarboxylation catalyst, the catalyst can reduce the heating temperature and time. This is desirable in certain instances, particularly if degradation of the alkane/alkene or side reactions (e.g., aromatization) are to be avoided. Examples of to decarboxylation catalysts include, but are not limited to, activated alumina catalysts. 
     Steps (a) and/or (b) can be performed in batch, semi-batch, or continuous modes of operation. For example, with respect to step (b), a continuous reactor system with unreacted acid recycle could be employed to enhance the yield of desirable alkane/alkene by limiting the duration and exposure of the alkane/alkene in the high temperature reactor. Carbon dioxide and small hydrocarbon products could be recovered, with the gas phase hydrocarbons used as fuel for the reactor or other applications. When a continuous reactor system is used, process conditions can be optimized to minimize reaction temperatures and times in order to maximize product yields and composition. As the reaction can be adjusted to select for a preferred carbon chain length (long, short or medium), the technology has the capability of enriching for a particular product group. From these groups, individual chemicals could be recovered, purified, and sold as pure platform chemicals. 
     After step (b), the pyrolysis product (i.e., the hydrocarbon composition) is extracted with an extraction solvent to substantially remove fatty acids present in the hydrocarbon composition. As discussed above, the hydrocarbon composition contains alkanes and/or alkenes as well as residual fatty acids. The term “substantially remove” with respect to the fatty acids is defined as completely removing the fatty acids from the hydrocarbon composition or, in the alternative, removing the fatty acids so that they are not detectable using analytical techniques known in the art. For example, the acid number of the hydrocarbon composition after extraction can be at or near zero as measured by ASTM D974 (see for example  FIG. 1 ). 
     The hydrocarbon composition can be extracted neat or dissolved in an organic solvent prior to extraction. In the case when the hydrocarbon composition is dissolved in a solvent, the solvent selected should substantially if not completely dissolve the hydrocarbon composition. The selection of the solvent can vary depending upon the types of alkanes, alkenes, and fatty acids present in the hydrocarbon composition. For example, non-polar solvents such toluene, petroleum ether, hexane, heptane, octane, pentane, diethyl ether, or any combination thereof can be used herein. 
     In one aspect, the extraction solvent comprises an aqueous base. In general, the amount of base used is in molar excess to the amount of fatty acid present in the hydrocarbon composition. Techniques for determining the amount of fatty acid present in the hydrocarbon composition and, thus, the amount of base to use are known in the art. The selection of the base can vary as well. A variety of inorganic and organic bases can be used. In one aspect, the base comprises a hydroxide, a carbonate, a sulfate, a phosphate, an acetate, an amide, or any combination thereof. In another aspect, the base comprises an alkali metal hydroxide, alkaline earth metal hydroxide, or a combination thereof. 
     In certain aspects, it is desirable to change or modify the conditions in order to enhance phase separation between the different phases. For example, the extraction solvent can further comprise one or more organic co-solvents. The co-solvent selected is generally soluble in water. Examples of co-solvents useful herein include, but are not limited to ethers, ketones, alcohols, or any combination thereof. Acyclic ethers (e.g., dimethyl ether, methyl t-butyl ether) and cyclic ethers (e.g., THF) can be used herein. Ketones useful herein include symmetrical and asymmetrical aryl and alkyl compounds. Alcohols such as, for example, a C 1  to C 5  branched or straight chain alcohol can be used herein as well (e.g., methanol, ethanol, etc). It is also possible to use mixtures of two or more different co-solvents. The order in which the extraction solvent is introduced to the hydrocarbon composition can vary. For example, a solution of base and co-solvent can be added first to the hydrocarbon composition followed by the addition of water to generate the aqueous base. In this aspect, the extraction solvent is produced in situ. Alternatively, a mixture of water, co-solvent, and base can be produced prior to extraction. 
     In certain aspects, in order to enhance phase separation, one or more salts can be added to the extraction solvent. The addition of the salt can be used with or without the co-solvent depending on the extraction solvent and the hydrocarbon composition that is being extracted. The amount of salt likewise can vary as needed. In one aspect, the salt comprises an alkali metal halide (e.g., NaCl) or an alkaline earth metal halide. In other aspects, during and/or after extraction step (c), heat can be applied for a sufficient time and temperature in order to increase phase separation. In certain aspects, the heating step is performed less than 100° C. to avoid the water present in the extraction solvent from boiling. However, if a pressurized system is used the extraction can be performed up to 180° C. 
     Similar to steps (a) and (b), the extraction step (c) can be performed in a batch, semi-batch, or continuous process using techniques known in the art. The methods described herein provide numerous advantages over current techniques for producing bio-fuel. In the absence of the extraction step, it is necessary to pyrolyze the fatty acids for longer periods of time and higher temperatures to ensure most if not all of the fatty acids are converted to alkanes and/or alkenes. This is not the case with the methods described herein. Because the extraction step is highly efficient at removing fatty acids from the hydrocarbon composition, it is not necessary to completely convert the fatty acids to alkanes and/or alkenes during pyrolysis. Thus, lower pyrolysis temperatures and shorter pyrolysis times are required. This is particularly advantageous in larger scale, continuous applications. In addition to efficiently removing fatty acids, the fatty acids that are removed can be recycled into the process described herein to further convert the fatty acids to additional alkanes and/or alkenes. Alternatively, the isolated fatty acids can be used in other applications. For example, the polar aqueous phase from the extraction step (c) can be treated with an acid to re-generate the free fatty acid and subsequently extracted to isolated the free fatty acid (see the Examples). Thus, the methods described herein provide an efficient way to convert fatty acids to useful hydrocarbons with minimal waste of resources and energy. 
     EXAMPLES 
     The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the materials, articles, and methods described and claimed herein are made and evaluated, and are intended to be purely exemplary and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.) but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in ° C. or is at ambient temperature, and pressure is at or near atmospheric. There are numerous variations and combinations of reaction conditions, e.g., component concentrations, desired solvents, solvent mixtures, temperatures, pressures and other reaction ranges and conditions that can be used to optimize the product purity and yield obtained from the described process. Only reasonable and routine experimentation will be required to optimize such process conditions. 
     Example 1 
     A 0.1M NaOH solution was prepared by dissolving solid NaOH (reagent grade, 97% EMD chemicals Darmstadt) in methanol (HPLC grade, Fisher scientific, New Jersey). A 0.1M NaCl solution was prepared by dissolving crystallized NaCl (food grade 99.6%, Millinckrodt, USP) in distilled water. Stearic acid pyrolysis product was dissolved in toluene (HPLC grade, fisher scientific, New Jersey) to be used in the extraction. The pyrolysis product of stearic acid was produced by heating 1 gram of stearic acid at 410° C. for 1 hour in a batch reactor. The stearic acid pyrolysis product (5 mL) was dissolved in toluene and pipetted into two 50 mL plastic centrifuge tube. To each tube, 20 mL of the 0.1M NaOH in methanol was added. The contents of the tubes were then shaken to mix them thoroughly. Next, 15 mL of distilled water was added to one tube and 15 mL 0.1M NaCl solution was added to the other. The contents of the tubes were shaken then allowed to stand for about 4 hour for the phases to separate. To ensure proper phase separation, the tubes were centrifuged at 3,000 rpm for 10 minutes. The toluene layer of each tube was then pipetted into glass vials and capped for further experiments whereas the polar/aqueous layer was left in the centrifuged tubes. 
     The polar layer from each tube was acidified using HCl and then back extracted with toluene using a separating funnel. The toluene layer was stored in a glass vial for later analysis. Similar extractions were carried out using oleic acid and linoleic acid pyrolysis products. 
     The extent of extraction was determined using ASTM D974 acid number determination and gas chromatography coupled to mass spectrometer. The results of the acid number ( FIG. 1 ) shows that using NaOH in methanol and adding either water or aqueous NaCl resulted in the complete extraction of the fatty acids from stearic acid pyrolysis products. To confirm the results, GC-MS was used and the results ( FIG. 2 ) show the same outcome (i.e., the complete extraction of the fatty acids). 
     To further confirm that the fatty acids were extracted, the back extracted toluene layer was run on the GC-MS and the results are presented in  FIG. 3 . The results of the back extraction further confirm that the fatty acids were indeed extracted. Similar results were obtained for the oleic acid and linoleic acid pyrolysis products. 
     Throughout this application, various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the compounds, compositions and methods described herein. 
     Various modifications and variations can be made to the materials, methods, and articles described herein. Other aspects of the materials, methods, and articles described herein will be apparent from consideration of the specification and practice of the materials, methods, and articles disclosed herein. It is intended that the specification and examples be considered as exemplary.