Synthesis of biodiesel from wastes of food industry via direct transesterification with methanol/carbon dioxide mixtures

Abstract

Direct transesterification with supercritical methanol of spent coffee ground oil in the temperature range of 523 to 603 K and in the pressure range of 15 to 30 MPa has been studied, and fatty acid methyl esters yield up to 85% were obtained. The percentage of oil contained in spent coffee grounds has been determined by soxhlet extraction with ethanol, resulting a value of 14% in oil. Carbon dioxide as co-solvent was added to methanol with the aim of decreasing the operating temperature and pressure. In order to determine the optimal conditions of the various factors that influence the transesterification with CO2 in a minimum number of experiments, the design of experiments (DOE) was applied. It was demonstrated that,at a reaction temperature of 573 K, pressure of 10 MPa and a molar ratio CO2/MeOH of 0.1 (optimal operation conditions), a 93% yield of methyl esters was obtained. The percentage in C16 (methyl palmitate) and C18 (methyl stearate, methyl oleate and methyl linoleate) has been determined in all samples. The C16 and C18 yields obtained in the experiment at 573 K, 10 MPa and a molar ratio CO2/MeOH of 10% were of 41.5% and 52%, respectively.