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

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 (E_a) and equilibrium constants (K_eq) 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 (E_a > 25 kJ mol⁻¹). Furthermore, the composition feedstock was found to have a distinct effect on the solubility of methanol and oil phase which influenced k, K_eq and E_a 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’ k_d in the reaction rate equations. Moreover, under optimized reaction conditions (120 °C and 20:1 methanol-to-FFA molar ratio), FAME yields up to 79–91 wt% could be obtained in one step process from oils containing 21–41 wt% FFA.