The continuous production of biodiesel (fatty acid methyl esters) by the transesterification reaction of coconut oil and palm kernel oil was studied in supercritical methanol without using any catalyst. Experiments were carried out in a tubular flow reactor, and reactions were studied at 270, 300, and 350 °C at a pressure of 10 and 19 MPa with various molar ratios of methanol-to-oils from 6 to 42. It was found that the best condition to produce methyl esters from coconut oil and palm kernel oil was at a reaction temperature of 350 °C, molar ratio of methanol-to-vegetable oil of 42, and space time 400 s. The % methyl ester conversions were 95 and 96 wt % for coconut oil and palm kernel oil, respectively. The regression models by the least-squares method were adequate to predict % methyl ester conversion with temperature, molar ratio of methanol-to-oil, and space time as the main effects. The produced methyl ester fuel properties met the specification of the ASTM biodiesel standards.

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Previous studies on the transesterification of vegetable oil in supercritical methanol in a batch reactor resulted in a non-saponified product with high methyl esters content and high glycerol purity. For the continuous reactor, the high viscosity of vegetable oil might result in problems in the flow system. This study selected THF and hexane as co-solvents to reduce the viscosity of the vegetable oil. The effect of co-solvents was investigated in both 250 mL and 5.5 mL batch reactors by 2-replicate 2 3 factorial design at temperatures from 290-350 uC, a molar ratio of methanol to vegetable oil from 12-42 and a molar ratio of co-solvent to vegetable oil from 0-5. The reaction time was fixed at 10 min. The products from the employed and unemployed co-solvent process were analyzed by GC-MS to confirm that the reaction among the vegetable oil, methanol and cosolvent was non-existent. However, some thermal cracking was observed in a 250 mL reactor at 350 uC and 30 min reaction time. The amount of co-solvents had no significant effect on methyl esters content and also did not allow the reaction to be completed under milder conditions. Thus, it was concluded that both THF and hexane were appropriate co-solvents to reduce the viscosity of vegetable oil for the continuous production of biodiesel in supercritical methanol.