Transesterification reaction was performed in the presence of soybean oil, methanol, and Demineralized water plant sedimentation catalyst at 60°C for 8 h in this study. Central composite design method was used to study the effect of catalyst concentration and methanol to oil molar ratio on purity and yield of produced biodiesel. The results showed catalyst concentration of 9.08 wt% and methanol to oil molar ratio of 22.49 as the optimum condition in which the values of purity and yield of the produced biodiesel in the second-order models were 99.89% and 81.83%, respectively. Experiments are in good agreement with the mentioned values as corresponded values are 99.95% and 86.68%, respectively. The catalyst was reused five times and XRF analyses showed a reduction in content of available CaO in catalyst after further usages, because of CaO extraction by methanol. Also for kinetics investigation, effects of temperature and time of the reaction on the produced biodiesel conversion at optimal condition were investigated. Rate constant at different temperatures and then activation energy were determined. Results showed high accuracy of first order kinetic model. Mean relative error of experimental data for the presented model was 2.42%.
