Optimization of Mechanical Agitation and Evaluation of the Mass-Transfer Resistance in the Oil Transesterification Reaction for Biodiesel Production

The KOH-catalyzed transesterification of sunflower oil with methanol was studied in batch conditions in a 22 L stirred reactor in order to (i) develop criteria for the energetic optimization of mechanical agitation in the biodiesel synthesis reaction; (ii) obtain preliminary information on the decantation of the reaction products; (iii) evaluate the influence of the mass-transfer resistance under different mixing conditions. Different combinations of radial, axial, and pitched-blade impellers were tested. For each combination an optimal rotational speed was identified, characterized on the one hand by a drastic reduction of the specific mixing energy, and on the other by a 20−30% increase of reaction time. An evaluation of the reaction and mass-transfer characteristic times showed that the optimized tests are characterized by a not negligible mass-transfer resistance. The complete halt of the agitation after a 1−2 min mixing time led to a further decrease of mixing energy, without any increase of reaction time. Preliminary decantation tests showed that the individuation of an optimized agitation speed for the reaction step, combined with the agitation halt after a short mixing time, has positive effects on the liquid−liquid separation step. The results were compared with those of a twin study conducted with static mixing.