Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 28, 2006
Publication Date: February 1, 2007
Citation: Haas, M.J., Scott, K.M. 2007. Moisture removal substantially improves the efficiency of in situ biodiesel production from soybeans. Journal of the American Oil Chemists' Society. 84(2):197-204. Interpretive Summary: Biodiesel, a renewable diesel engine fuel that consists of the simple alkyl esters of fatty acids and can be synthesized from domestically-produced fats and oils, is making the transition from a research item to a commercial fuel. Economics is a major barrier to this transformation, in that biodiesel produced from refined oils is generally not cost competitive with petroleum fuels. We have previously developed and described a novel method for biodiesel production, termed 'in situ transesterification.' This method eliminates the need for the prior isolation and purification of the fat or oil to be used for biodiesel production. Instead, the synthesis uses raw agricultural oil-bearing material as feedstock. This eliminates the cost and technology of oil isolation, and could reduce overall production costs. However, as first developed, in situ transesterification required the use of prohibitively large amounts of methanol co-reactant and alkali catalyst. Here we describe that by drying the oilseed feedstock a very desirable reduction of 55 to 60%, respectively, in the methanol and alkali requirements is achieved. This should greatly improve the economics of biodiesel synthesis by this process. Reaction conditions consistent with maximum predicted yields of biodiesel were identified, which again should allow the efficient conduct of this method. Finally, sufficient biodiesel was produced to allow its analysis by the accepted quality specifications for biodiesel, establishing that the in situ transesterification process can produce an acceptable biodiesel.
Technical Abstract: In an effort to reduce the production cost of biodiesel, we have previously described an approach termed 'in situ transesterification' wherein the transesterification of a vegetable oil occurs directly in its raw agricultural material (Haas, M.J., et al., J. Am. Oil Chem. Soc. 81:83-89, 2004). In that method, substantial quantities of reagents were required in order to achieve high efficiency transesterification: the molar ratio of methanol to triacylglycerol in the feedstock was 543:1. Here we report that by drying the flaked soy substrate, which had an initial moisture level of 9.4%, a marked reduction in the reagents requirement was achieved. Using statistical experimental design methods, reaction conditions were optimized for the production of fatty acid methyl esters (FAME) in reactions conducted at room temperature. Thus, flakes (5.0 g) with 2.6% moisture were efficiently transesterified in reaction of 16 h duration containing 18 mL of 0.10 N NaOH in methanol. The observed transesterification rate was 97% of theoretical maximum. For completely dry flakes, optimal transesterification was predicted to occur with a 10 h reaction containing, per 5 g of flakes, 12 ml of 0.10 N NaOH in methanol. This is a molar ratio of methanol/triacylglycerol of 227, and represents a 60% reduction in methanol use compared to the transesterification of full moisture flakes. A 56% reduction in NaOH use relative to the situation with full moisture flakes was also achieved. Under these conditions, the observed transesterification efficiency was 100% of theoretical maximum. The transesterification of 20.036 kg of flakes with a moisture content of 0.8% was conducted, in 9 batches, using the optimal reaction conditions for dry flakes. An average transesterification efficiency of 94.4% was attained. The amount of unreacted TAG remaining in the flakes post-reaction was 1.0% of input. The amount of FAME left on the flakes was 3.9% of maximum theoretical yield. Following washing of the crude FAME, 81.2% theoretical maximum FAME yield was recovered. The FAME product met all currently accepted biodiesel specifications, except that for acid number, which it exceeded by 12%. A wash with dilute NaOH brought the material into compliance with this specification. The tocopherol levels of soy FAME produced by in situ transesterification exceed by 68% those of a representative commercial biodiesel. Soy flakes retained 97.0% of their protein following in situ transesterification.