Kinetics of sunflower and used vegetable oil methanolysis catalyzed by CaO·ZnO

The kinetics of heterogeneous methanolysis of sunflower oil and used vegetable oil were studied at different temperatures, ranging from 60 to 96 °C using CaO·ZnO as catalyst (2 wt% on the basis of oil) and methanol to oil molar ratio of 10:1. Heterogeneous CaO·ZnO catalyst was synthesized by mechanochemical treatment of CaO and ZnO powder mixture with the addition of water necessary for the formation of corresponding mixed hydroxides, and their calcination at 700 °C in air. It was shown that kinetics of overall process could be described as pseudo-first order reaction. For the sunflower oil methanolysis at 60 and 70 °C, the rate of process at the beginning of biodiesel synthesis was limited by triglycerides mass transfer to the catalyst surface, and after that it is governed by the rate of chemical reaction at catalyst surface. At higher temperatures the influence of mass transfer resistance is almost negligible implying that the rate of chemical reaction determines the overall kinetic of biodiesel synthesis. In the case of used vegetable oil, the influence of mass transfer resistance was not observed either at higher or lower temperature. The kinetic model that describes the whole process well was proposed, and it comprises both the triglycerides mass transfer and chemical reaction controlled regime. The overall volumetric mass transfer coefficient was defined, assuming that it depends on the conversion of triglycerides.