Process Optimization Photo-Esterification of Free Fatty Acids in Waste Cooking Oils under UV Irradiation via the RSM Method

In this research study, UV irradiation was employed to decrease the free fatty acids (FFAs) of waste cooking oil (WCO). In the first step, under the UV irradiation, the photocatalytic reaction of Cr-TiO2 with methanol and free fatty acids in WCO was studied. Optimization was performed with Design Expert Software and using the Box-Behnken method. We investigated the effects of the main parameters, including the molar ratio of methanol to WCO, irradiation time, and type of photocatalyst. By using UV radiation, 0.9661% of the response changes were justified with a Quadratic model. The methanol-to-WCO ratio parameter showed a positive effect with a greater slope. The transesterification reaction of triglycerides with methanol was performed in the second step using the Verma method. Solid catalysts were found to be highly active for the photo-esterification of FFAs with methanol in the UV irradiation. While performing the esterification step in the presence of the UV irradiation in the presence of Cr-TiO2 photocatalysts, the efficiency was 76.3%-89.5%. The order of photocatalytic esterification reaction by Cr-TiO2 was approximately equal to one. The kinetics of the transesterification stage was investigated. The first-order kinetic model showed a correlation coefficient of R2 = 0.997. The amount of activation energy was 7.103 kJ/mol, indicating the warming nature of the reaction. Concerning the production of H+, CH3O. and R-COOH on the photocatalyst surface, a possible reaction mechanism for the esterification process is proposed. For the mechanism of the transesterification reaction, -OH or -OR were considered active species.