Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 24, 2003
Publication Date: January 1, 2004
Citation: Haas, M.J., Scott, K.M., Marmer, W.N., Foglia, T.A. 2004. In situ alkaline transesterification: an effective method for the production of fatty acid esters from vegetable oils. Journal of the American Oil Chemists' Society. 81(1):83-89. Interpretive Summary: 'Biodiesel' is a new diesel engine fuel presently entering fuel markets around the world. Interest in biodiesel is strong because it burns cleaner than petroleum diesel, reducing air pollution, and because it is made from domestically produced fats and oils. Thus it offers both a means to reduce dependence on foreign imports and a means to increase farm income. However, biodiesel is not economically competitive with petro-diesel, a fact that retards its more widespread adoption. The research described here was designed to investigate a new paradigm for biodiesel production. Current technology employs first an extraction of the oil source (e.g., soybeans) with an organic solvent to recover the oil it contains. This oil is then converted to fatty acid esters (the chemical description of biodiesel) by incubation with alcohol and alkali. In this paper we investigated whether the direct incubation of the oil source with the alcohol and alkali could produce and liberate fatty acid esters from the oilseed. Success in this approach could simplify the technology of biodiesel production and serve also to reduce the cost of vegetable oil. Such changes could reduce the cost of biodiesel, fostering the more widespread adoption of this desirable new fuel. It was determined that the simple incubation of flaked soybeans with methyl alcohol and alkali does cause ester synthesis, at both elevated temperatures and room temperature. The best conditions for the conduct of the reaction were determined, and it was found that under these conditions 77% of the oil in soy was converted to methyl esters. Glycerol is a coproduct of the synthesis of these esters. It was determined that 94% of the produced glycerol goes to the ester fraction, while 6% stays on the treated flakes. These results offer a first demonstration of the feasibility of the proposed new method for biodiesel production. Subsequent work will be required to test its suitability with other alcohols and to examine the nutritional suitability of the spent meal, since the sale of extracted meal for use as an animal feed is a key component of the profitability of an oilseed operation.
Technical Abstract: The production of simple alkyl fatty acid esters by 'in situ' transesterification of the acylglycerols present in flaked soybeans by incubation with alkaline alcohol solutions was examined. Commercially produced soy flakes had a hexane-extractable lipid content of 23.9%. Initial experiments showed that this lipid was readily transesterified to produce fatty acid esters during agitation at 60°C in sealed containers of alcoholic NaOH. Free fatty acids were also produced. Methyl, ethyl and isopropyl alcohols readily participated in the reaction, suggesting that the phenomenon is a general one. Statistical experimental design methods and response surface regression analysis were used to further study the reaction, using methanol as the alcohol. At 60°C, highest yields of methyl ester with minimum contamination by free fatty acids (FFA) and acylglycerols (AG) were predicted at a molar ratio of methanol/acylglycerol/NaOH of 226/1/1.6 with an approximately 8 h incubation. This corresponds to the reaction of 5 g of soy flakes with 12.5 mL of 0.18N NaOH in methanol. Increased methanol volumes with reduced alkali concentrations also gave high ester yields with low FFA and AG contamination. The reaction also proceeded well at 23°C (room temperature), giving higher predicted ester yields than at 60°C. At room temperature maximum esterification was predicted at a molar ratio of 543/1/1.1 for methanol/acylglycerol/NaOH, again in 8 h. Substantial amounts of ester were formed within the first hours of reaction at both temperatures examined. The efficiency of removal of lipid from 100 g of soy flakes subjected to in situ transesterification at room temperature under optimal conditions was 90%. The efficiency of production and recovery of FAME from this solubilized material was 86%, corresponding to a transesterification efficiency of 77% for the overall process. The FAME fraction contained only 0.6% (mass basis) FFA and no acylglycerols. Of the glycerol produced by transesterification of lipid, 94% was located in the alcoholic ester phase and 6% was on the post-transesterification flakes.