CFD MODELING AND SIMULATION OF TRANSESTERIFICATION REACTIONS IN STIRRED TANKS

Resumo

Methanol and triglycerides are poorly miscible. The low miscibility causes transesterification between those compounds to proceed slowly. Mechanical agitation can increase the contact surface between alcohol and oil phases, so it provides higher reaction rates and higher yields. Agitation is therefore widely used in biodiesel production processes in industry. By studying reactions inside stirred tanks, it is possible to establish optimal conditions for the design and operation of these reactors. Computational modeling and simulation of stirred tanks and chemical reactions can be employed with this purpose, and are much less expensive than experimental methods of investigation of reactions in flow fields. In this study, computational fluid dynamics (CFD) simulations of transesterification reactions inside mechanically stirred tanks were carried out. Reaction rates were evaluated by differential equations that describe the kinetics of transesterification process, and were used as source terms of generation and consumption of compounds in the reaction mixture. The concentration profiles obtained for the mixture components over time were in accordance with the experimental data. The mass fractions patterns showed efficient mixing in a short stirring time. Many turbulence models were analyzed, and some influence of the adopted turbulence model in final results was observed.  The height of the impeller blades relatively to the tank walls also influenced the mixing efficiency and thus the transesterification yield. The stirring speed, on the other hand, had no significant effect on the reaction yield.