Enzymatic conversion of coconut oil for biodiesel production

The kinetics of the enzymatic conversion of coconut oil to biodiesel (fatty acid alkyl esters) have been investigated using ethanol and 1% (w/v) lipase at 50 °C. Coconut oil is being evaluated for biofuel production and an enzymatic process was selected to minimize side reactions (such as saponification) which can occur with the alkali-catalysed route if free fatty acid (FFA) concentration is significant. Rate data comparison showed that the NaOH-promoted conversion was about 2 orders of magnitude faster than the lipase-catalysed system but resulted in saponification/partial solidification of the reaction mixture. Analysis also revealed that the ratios of the kinetic constants for ester:glycerol (1:1.12) and that for ester:ethanol (1:3.11) during enzymatic transesterification are in agreement with reaction stoichiometry. Additionally, FFA esterification rates were higher (1.5 to 2.5) than triglyceride transesterification rates under similar conditions. Supplementation with ultrasonics at 43 kHz also permitted nearly 20-fold improvement in conversion rate. The study also demonstrated that with Saccharomyces cerevisiae, the growth yield on glycerol of 0.77 g g− 1 is greater than that usually found for growth of yeast on glucose (viz. 0.4–0.45 g g− 1). Thus, opportunities for process enhancement exist through the use of increased enzyme concentrations, ultrasound techniques and growth of yeast on residual by-product glycerol.

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► Species rate constants ratios during transesterification agree with stoichiometry.
► Esterification rates were higher than transesterification for identical conditions.
► 20-fold improvement in conversion rate recorded for the ultrasonic-aided system.
► Yeast growth yield on glycerol was nearly twice that commonly obtained for glucose.