Estimation of kinetic parameters and diffusion coefficients for the transesterification of triolein with methanol on a solid ZnAl2O4 catalyst.
Résumé
The transesterification of vegetable oils by methanol implies a set of three parallel–series reversible reactions. Over a heterogeneous catalyst, reactions are limited by molecular diffusion, resulting in the use of a large excess of methanol to achieve a high conversion. This excess is detrimental to process efficiency because it involves large energy requirement for downstream separation. In order to identify the coupled physico-chemical phenomena and to optimize the process, a pilot unit was developed to perform transesterification of triglycerides with methanol over a solid ZnAl2O4 catalyst in a tubular fixed bed reactor under high pressure and temperature. The effects of different parameters such as particle diameter, temperature, residence time and molar feed ratio of methanol to triglyceride on fatty acid methyl ester (FAME) yield and triglyceride (TG) conversion were studied. A pseudo-homogeneous reactor model has been developed to identify rate laws and kinetic parameters by exploiting the experimental data in stationary regime. A set of kinetic constants was also determined for the given catalyst considering two kinetic models: an Eley–Rideal model with the reaction between TG and methanol as the rate determining step and a classical second order model taking into account thermodynamic equilibrium without adsorption. A heterogeneous model has also been established to determine molecular diffusion coefficients for each species in the mixture, through the use of experimental results in the transient regime. These molecular diffusion coefficients are dependent on the mixture viscosity and temperature. As a result, it was determined that diffusion of triglycerides and the first reaction kinetic rates are limiting the global conversion of the system.