Optimization and kinetic modeling of esterification of the oil obtained from waste plum stones as a pretreatment step in biodiesel production

This study reports on the use of oil obtained from waste plum stones as a low-cost feedstock for biodiesel production. Because of high free fatty acid (FFA) level (15.8%), the oil was processed through the two-step process including esterification of FFA and methanolysis of the esterified oil catalyzed by H2SO4 and CaO, respectively. Esterification was optimized by response surface methodology combined with a central composite design. The second-order polynomial equation predicted the lowest acid value of 0.53 mg KOH/g under the following optimal reaction conditions: the methanol:oil molar ratio of 8.5:1, the catalyst amount of 2% and the reaction temperature of 45 °C. The predicted acid value agreed with the experimental acid value (0.47 mg KOH/g). The kinetics of FFA esterification was described by the irreversible pseudo first-order reaction rate law. The apparent kinetic constant was correlated with the initial methanol and catalyst concentrations and reaction temperature. The activation energy of the esterification reaction slightly decreased from 13.23 to 11.55 kJ/mol with increasing the catalyst concentration from 0.049 to 0.172 mol/dm3. In the second step, the esterified oil reacted with methanol (methanol:oil molar ratio of 9:1) in the presence of CaO (5% to the oil mass) at 60 °C. The properties of the obtained biodiesel were within the EN 14214 standard limits. Hence, waste plum stones might be valuable raw material for obtaining fatty oil for the use as alternative feedstock in biodiesel production.