Conversion of acid oil by-produced in vegetable oil refining to biodiesel fuel by immobilized Candida antarctica lipase

Acid oil, which is a by-product in vegetable oil refining, mainly contains free fatty acids (FFAs) and acylglycerols, and is a candidate of materials for production of biodiesel fuel. A mixture (acid oil model) of refined FFAs and vegetable oil was recently reported to be converted to fatty acid methyl esters (FAMEs) at >98% conversion by a two-step reaction system comprising methyl esterification of FFAs and methanolysis of acylglycerols using immobilized Candida antarctica lipase. The two-step system was thus applied to conversion of acid oil by-produced in vegetable oil refining to biodiesel fuel. Under similar conditions that were determined by using acid oil model, however, the lipase was unstable and was not durable for repeated use. The inactivation of the lipase was successfully avoided by addition of excess amounts of methanol (MeOH) in the first-step reaction, and by addition of vegetable oil and glycerol in the second-step reaction. Hence, the first-step reaction was conducted by shaking a mixture of 66 wt% acid oil (77.9 wt% FFAs, 10.8 wt% acylglycerols) and 34 wt% MeOH with 1 wt% immobilized lipase, to convert FFAs to their methyl esters. The second-step reaction was performed by shaking a mixture of 52.3 wt% dehydrated first-step product (79.7 wt% FAMEs, 9.7 wt% acylglycerols), 42.2 wt% rapeseed oil, and 5.5 wt% MeOH using 6 wt% immobilized lipase in the presence of additional 10 wt% glycerol, to convert acylglycerols to FAMEs. The resulting product was composed of 91.1 wt% FAMEs, 0.6 wt% FFAs, 0.8 wt% triacylglycerols, 2.3 wt% diacylglycerols, and 5.2 wt% other compounds. Even though each step of reaction was repeated every 24 h by transferring the immobilized lipase to the fresh substrate mixture, the composition was maintained for >100 cycles.