Response surface study and kinetic modelling of biodiesel synthesis catalyzed by zinc stearate

Abstract

This contribution reports experimental and theoretical studies on the transesterification of soybean oil with methanol catalyzed by zinc stearate. This reaction produces fatty acid methyl esters (FAME) and glycerol, and di- and monoglycerides as intermediate products. Response Surface Methodology was used to study the relationship between process variables and triglyceride conversion, FAME yield and initial rate. An increase in the catalyst concentration and in the methanol/oil molar ratio increased triglyceride conversion and FAME yield and decreased the initial turnover frequency values. The latter was associated with the formation of an emulsion in the reaction medium. A kinetic study of the reaction was performed. Two models were proposed. Model 1 assumed a complete mixing of the dissolved catalyst with the reactants and a second-order mechanism for the forward and reverse reactions, where the reaction system could be described as pseudo-homogeneous and the catalyst was dissolved in the reaction medium. Model 2 supposed that the dissolved catalyst formed part of a macro emulsion, with mass transfer resistance in the boundary layer around the droplets. The kinetic constants were determined, and Model 2 showed a better fit to the experimental data. The model and the kinetic parameters allow to generate reaction operation strategies.