Robust kinetic modeling of heterogeneously catalyzed free fatty acids esterification in monophasic liquid/solid packed bed reactor: rival model discrimination

Abstract Biodiesel production could be economically sustainable if raw oils, i.e., waste cooking oil or oils with a high acid content, are used for its production. However, the high content of free fatty acid (FFA) of these oils makes necessary a pretreatment before the transesterification step. Esterification using methanol and a heterogeneous catalyst should be very convenient because this reaction permits to decrease FFA content by producing simultaneously methyl esters, i.e., biodiesel. This technology was applied in a continuous packed bed reactor for the deacidification of sunflower oil, using Amberlyst 46 as heterogeneous acid catalyst at pressure of 6 bar and temperature in the range of 54–95 °C. An amount of methanol five times greater than the stoichiometric was added to sunflower oil, in order to shift the reaction equilibrium but avoiding the formation of a double liquid phase. Two different kinetic models were considered: a pseudo-homogeneous and an adsorption based, which accounts for the different affinities toward the polymeric matrix of all the species involved in the reaction and the solvent (triglycerides). For both the models, either the ideal or non-ideal behavior of the mixture was considered, and the results were compared. The kinetic parameters of the esterification of free fatty acid were regressed using a robust mathematical model and using equilibrated resins, i.e., all the experiments started only after having let the catalyst adsorb reactants and products at the operative conditions.