Investigation of mass transfer limitations in simultaneous esterification and transesterification of triglycerides using a heterogeneous catalyst

Abstract

The process for the production of biodiesel can be intensified significantly by employing heterogeneous catalysts that can tolerate free fatty acid in the feedstock. The synthesis of biodiesel was experimentally investigated in a fixed bed reactor using refined sunflower oil (triglyceride) and raw Jatropha oil (triglyceride with free fatty acid). A zinc-based catalyst was used in the form of cylindrical pellets of 6 mm in diameter and 8–10 mm in length. For the raw Jatropha oil, conversions greater than 90% were realized for both triglycerides and free fatty acids (FFAs), typically at 200 °C and a molar ratio of 6 : 1 (methanol : oil) for a residence time of 40 g_cat ml⁻¹ min. The experimental results for the refined oil in the fixed bed reactor (FBR) are explained using an appropriate model that incorporates both simplified kinetics and external mass transfer resistances at liquid–liquid and solid–liquid interfaces. The comparison of FBR results with those of the independent experiments in a spinning basket reactor, in the absence of external mass transfer limitations, shows that the intrinsic reaction is very fast. Hence, in the fixed bed reactor, mass transfer limitations especially at the liquid–liquid interface play an important role under the conditions employed. The rates in the case of refined sunflower oil are significantly lower compared to those of Jatropha oil, due to the presence of FFA, mono- and diglycerides in the latter.