Biodiesel production from high acid value waste cooking oil using supercritical methanol: Esterification kinetics of free fatty acids

In this study, low quality waste cooking oil (WCO) with high total acid value has been used for biodiesel production. The main factors affecting the reaction have been analysed using Response Surface Methodology (RSM). A quadratic model representing the interrelationships between reaction variables and free fatty acids (FFA) conversion has been developed. Analysis of variance (ANOVA) has been used to evaluate the significance of the predicted model. Numerical optimisation predicted the optimum conditions for maximum conversion of FFA at methanol to oil (M:O) molar ratio, temperature, pressure and time of 35:1, 260 °C, 110 bar and 16 min, respectively for 98 % conversion. The predicted optimum conditions have been validated experimentally resulting in 97.7 % conversion of FFA with 0.3 % relative error. Kinetic and thermodynamic data of the esterification reaction has been studied resulting in pseudo first-order reaction with reaction rate constant of 0.00103 s^-1, activation energy of 34.5 kJ/mol and Arrhenius constant of 1.26 s^-1. Finally, a kinetic reactor has been simulated resulting in 97 % conversion of FFA with 0.716 % relative error from the experimental results.