Moisture removal substantially improves the efficiency of in situ biodiesel production from soybeans

In an effort to reduce the production cost of biodiesel, we previously described an approach termed "in situ transesterification" wherein the transesterification of a vegetable oil occurred directly in its raw agricultural material. In that method, substantial quantities of reagents were required to achieve high-efficiency transesterification. Here we report that by drying the substrate a marked reduction in the reagent requirements was achieved. Reaction conditions for maximum fatty acid methyl ester (FAME) production were identified using statistical experimental design methods. In 16-h reactions with 5 g of flakes (2.6% moisture) and 18 mL of 0.10 N NaOH in methanol, 97% theoretical maximum transesterification was achieved. For dry flakes, optimal transesterification was achieved in 10 h in reactions containing 5 g of flakes and 12 mL of 0.10 N NaOH in methanol. This represented a 60% reduction in methanol and a 56% reduction in NaOH use compared with the transesterification of full-moisture flakes. Under these conditions the degree of transesterification was 100% of the theoretical maximum. The transesterification of 20 kg of flakes with a moisture content of 0.8% was conducted under optimal reaction conditions. Both triacylglycerols (TAG) and phosphoacylglycerols (PL) were transesterified to high degrees, with an overall efficiency of 97.3% of the maximum theoretical efficiency. PL were not detected in the flakes following transesterification, and the amount of remaining unreacted TAG was only 1.0% of that input. Following washes with water, 0.5 M NaCl, and dilute NaOH, the recovered FAME met the ASTM specifications for biodiesel. The tocopherol levels of the FAME exceeded by 76% those of a representative commercial biodiesel. Soy flakes retained 97% of their protein following in situ transesterification.