Reaction kinetics with catalyst deactivation in simultaneous esterification and transesterification of acid oils to biodiesel (FAME) over a mesoporous sulphonated carbon catalyst

Abstract

In this work, a careful analysis of the reaction kinetics upon simultaneous esterification and transesterification of acidic oils over a mesoporous sulphonated carbon catalyst is discussed. A batch reactor system was used and the synthesized carbon catalyst were characterized by N2-physisorption, transmission electron microscopy, elemental analysis and NH3-TPD. A second order pseudo-homogeneous kinetic model was proposed which explained the experimental results obtained for three different feedstock oils with ⩾98% accuracy. The rate constants (k), activation energies (Ea) and equilibrium constants (Keq) of the individual reactions were determined by regression analysis which confirmed that the reaction steps were kinetically controlled and not limited by inter-particle diffusion or external mass transfer limitations (Ea>25kJmol−1). Furthermore, the composition feedstock was found to have a distinct effect on the solubility of methanol and oil phase which influenced k, Keq and Ea values, eventually determining the final biodiesel (FAME) yield. To account for the loss of activity upon catalyst reuse, a deactivation model was also proposed which explained our results with ∼94% accuracy. In fact, the loss of activity was accounted for by incorporating a concentration-independent ‘deactivation constant’ kd in the reaction rate equations. Moreover, under optimized reaction conditions (120°C and 20:1 methanol-to-FFA molar ratio), FAME yields up to 79–91wt% could be obtained in one step process from oils containing 21–41wt% FFA.