CFD study of the supercritical transesterification of triolein: Wall-to-fluid/particle-to fluid transport effects

Abstract

Transesterification of triglycerides is a process by which biodiesel is produced. Biodiesel is a sustainable alternative to petroleum diesel; hence, its efficient production is of significant importance. The transesterification of triolein with methanol under supercritical conditions in the presence of a solid catalyst structured as packed beds is of particular interest due to high yields at moderate temperatures and continuous operability. Fundamental understanding of the heat and mass transfer phenomena in a supercritical transesterification reaction is paramount for a better modelling of the reaction.Computational Fluid Dynamics (CFD) were used to study, by means of a simplified numerical model, heat and mass transport phenomena in a supercritical transesterification reactor. The CFD model was built replicating an experimental pilot plant reactor. Simulations were performed for a 3D geometrical model of a packed bed of cylindrical catalysts. Wall-to-fluid and particle-to-fluid heat and mass transfer were studied for a fluid mixture of methanol (CH3OH) and triolein (C57H104O6) along with co-solvent carbon dioxide (CO2), flown into the packed bed under supercritical conditions (25 MPa, 473.15 K). Detailed flow, temperature and concentration profiles in packed bed were obtained. Free and forced convection effects for different flow rates were analyzed. Large density gradients were observed near the packed bed inlet, generating hydrodynamic instabilities due to natural convection as observed in Metais-Eckert maps. Heat and mass transfer coefficients were estimated from CFD data, and the obtained values of Nuw, Nup, Shp were compared against empirical models for different flow rates for validation of the CFD data. The simulation data agreed well with empirical correlationsfor predicting transport coefficients.