Impact of transesterification mechanisms on the kinetic modeling of biodiesel production by immobilized lipase

Three kinetic models of the transesterification of palm oil fatty acids to ethanol using an immobilized lipase were developed. The models differ from one another with respect to the rate-limiting step and the point at which the ethanol molecule becomes involved in the reaction. The kinetic parameters were estimated by fitting experimental data of the transesterification of palm oil with various ethanol concentrations. The models are able to account for the effects of substrates and products involved in the transesterification throughout the entire reaction. There was a good agreement between experimental results and those predicted by the proposed model equations in which ethanol was assumed to be involved directly in an alcoholysis reaction with palm oil. Furthermore, the calculated results show that the rate constants for alcoholysis of palm oil with ethanol are much higher than those for the hydrolysis reaction. From the proposed model equations, the effects of ethanol concentration on the initial production rates and yields of fatty acid ethyl ester and free fatty acids were simulated. The simulation results show that increasing the initial ethanol concentration produces an increase in the initial production rate and yield of fatty acid ethyl ester and lowers the final concentration of free fatty acid whereas lower ethanol concentration led to a higher final concentration of free fatty acid. (C) 2008 Elsevier B.V. All rights reserved.