A team of researchers in China has developed a two-step enzymatic process for the solvent-free conversion of soybean oil to biodiesel without the production of glycerol. The process uses hydrolysis followed by esterification, with Yarrowia lipolytica lipase.
In an open access paper in the journal Biotechnology for Biofuels, they report that the two-step enzymatic process demonstrated higher efficiency and safety than enzymatic transesterification, "which seems very promising for lipase-catalyzed, large-scale production of biodiesel, especially from high acid value waste oil."
While enzymatic transesterification in solvent media has been investigated intensively, the team notes, glycerol, as a by-product, could block the immobilized enzyme. Furthermore, excess n-hexane, as a solution aid, would reduce the productivity of the enzyme.
The team hydrolyzed soybean oil at 40 °C by 100 U of lipase broth per 1 g of oil with approximately 30% to 60% (vol/vol) water. The free fatty acid (FFA) distilled from this hydrolysis mixture was used for the esterification of FFA to fatty acid ethyl ester by immobilized lipase. A mixture of 2.82 g of FFA and equimolar ethanol (addition in three steps) were shaken at 30 °C with 18 U of lipase per 1 gram of FFA.
The degree of esterification reached 85% after 3 hours. The lipase membranes were taken out, dehydrated and subjected to fresh esterification so that more than 82% of esterification was maintained, even though the esterification was repeated every 3 hours for 25 batches.
