Simultaneous Free Fatty Acid Esterification and Triglyceride Transesterification Using a Solid Acid Catalyst with in Situ Removal of Water and Unreacted Methanol

Abstract

Waste greases used in biodiesel production contain high concentrations of free fatty acids (FFAs). To efficiently utilize these low-cost feedstocks for biodiesel synthesis, a preliminary acid-catalyzed esterification pretreatment is necessary to reduce their FFA contents. In this study, the use of a three-phase reaction has been investigated for the simultaneous esterification of FFAs and transesterification of triglycerides (TGs) using model mixtures of TGs and FFAs [0−25 wt % lauric acid (HLa) in tricaprylin (TCp)] to simulate waste grease and to permit easy distinction between the reactions. Trans/esterification with methanol was carried out using a commercial tungstated zirconia (WZ) solid catalyst in a well-stirred semibatch reactor at 130 °C operating at atmospheric pressure. Methanol was continuously charged to the reactor. Unreacted methanol and water produced during the reaction were continuously removed by vaporization, thereby favoring esterification. Esterification occurred 4 times faster than transesterification. Under our reaction conditions and in the presence of the catalyst, byproduct water was produced not only from HLa esterification but also from methanol dehydration. Because of the presence of this water, TGs were converted into the ester product by two routes: direct TG transesterification and TG hydrolysis followed by esterification. The catalyst, WZ, underwent 37% deactivation after three 2-h reaction cycles but completely recovered its original activity after a simple recalcination in flowing air. The feasibility of a continuous multiphase process for reducing FFAs in low-cost biodiesel feedstocks has been successfully demonstrated.