Continuous Production of Biodiesel in Supercritical Ethanol: A Comparative Study between Refined and Used Palm Olein Oils as Feedstocks

Biodiesel production from refined palm olein (RPO) and used palm olein (UPO) oils in supercritical ethanol was comparatively investigated in a continuous reactor. After use of RPO for chicken frying, levels of unsaturated fatty acids (UFAs) and free fatty acid (FFA) increased by 10 and 5%, respectively. The optimal conditions for both oils were 300 °C, 30 MPa and 30:1 ethanol:oil molar ratio at 60 min of residence time. Although the UFAs are inactive and sensitive to thermal degradation, the FFA in UPO catalyzed the reactions in supercritical ethanol at the same time. Due to the antagonistic effect of UFAs and FFA, the maximum ester content of UPO biodiesel (73%) was slightly lower than for RPO biodiesel (80%). The other compounds in resultant biodiesel were unreacted glycerides, mainly mono- and di-glycerides. Furthermore, UPO reaction reached equilibrium faster than RPO reaction due to the catalytic effect of FFA.