Biodiesel from soybean oil in supercritical methanol with co-solvent

Biodiesel synthesis from soybean oil using methanol was investigated at supercritical and subcritical conditions of methanol in a high pressure vessel of 250 cm3 volume. Under the supercritical conditions, the maximum methyl ester yield exceeded 98% when the molar ratio of methanol to oil was 42:1 and the reaction temperature ranged from 260 °C to 350 °C. In order to decrease the operational temperature and pressures and increase the conversion efficiency of methanol, a novel idea was presented in which a co-solvent (hexane, carbon dioxide, KOH) was added to the reactants. The results indicated that the yield of methyl ester was improved when hexane or CO2 was added. With CO2 or hexane as co-solvent in the reaction system at 300 °C, there was a significant increase of the methyl esters yield. With the optimal reaction temperature of 160 °C and methanol to oil ratio of 24, a 98% yield of methyl esters was observed in 20 min at the subcritical condition with 0.1 wt% potassium hydroxide (KOH).