Lipase-Catalyzed Production of Biodiesel by Hydrolysis of Waste Cooking Oil Followed by Esterification of Free Fatty Acids

Biodiesel is conventionally produced by alkaline-catalyzed transesterification, which requires high-purity oils. However, low-quality oils can be used as feedstocks for the production of biodiesel by enzyme-catalyzed reactions. The use of enzymes has several advantages, such as the absence of saponification side reactions, production of high-purity glycerol co-product, and low-cost downstream processing. In this work, biodiesel was produced from lipase-catalyzed hydrolysis of waste cooking oil (WCO) followed by esterification of the hydrolyzed WCO (HWCO). The hydrolysis of acylglycerols was carried out at 30 A degrees C in salt-free water (WCO/water ratio of 1:4, v/v) and the esterification of HWCO was carried out at 40 A degrees C with ethanol in a solvent-free medium (HWCO/ethanol molar ratio of 1:7). The hydrolysis and esterification steps were carried out using immobilized Thermomyces lanuginosus lipase (TLL/WCO ratio of 1:5.6, w/w) and immobilized Candida antarctica lipase B (10 wt%, CALB/HWCO) as biocatalysts, respectively. The hydrolysis of acylglycerols was almost complete after 12 h (ca. 94 %), and in the esterification step, the conversion was around 90 % after 6 h. The purified biodiesel had 91.8 wt% of fatty acid ethyl esters, 0.53 wt% of acylglycerols, 0.003 wt% of free glycerol, viscosity of 4.59 cP, and acid value of 10.88 mg KOH/g. Reuse hydrolysis and esterification assays showed that the immobilized enzymes could be recycled five times in 10-h batches, under the conditions described above. TLL was greatly inactivated under the assay conditions, whereas CALB remained fully active. The results showed that WCO is a promising feedstock for use in the production of biodiesel.