Kinetics of the Transesterification Reaction with a Fourth Order Shunt Mechanism for Biodiesel Production with Alcohols

A new kinetic model for the non-catalytic transesterification of triglycerides in supercritical alcohols has been proposed. Based on the conditions commonly employed in non-catalytic transesterification for biodiesel production, i.e., high temperatures (>250°C) and high alcohol to triglyceride molar ratios (~40), a fourth order shunt mechanism has been shown to dominate the reaction. In the more traditional alkali-catalyzed transesterification processes for biodiesel production, 3 consecutive and reversible reactions account for biodiesel yield. Triglycerides are first converted to diglycerides, which are then converted to monoglycerides, and finally monoglycerides to glycerol; an alkyl ester molecule is produced at each step of the reaction. The proposed model for transesterification in supercritical alcohols involves a single fourth-order reaction in which 3 alcohol molecules directly react with a single triglyceride molecule. The activation energy of the transesterification reaction was found to depend on the reactants, i.e., the alcohol and triglyceride source, as well as the exact reaction conditions, i.e., temperature and molar ratio. For rapeseed oil, the activation energy of transesterification increased with the carbon number of the alcohol.