Transesterification Kinetics of Soybean Oil for Production of Biodiesel in Supercritical Methanol

A kinetic study on soybean oil transesterification without a catalyst in subcritical and supercritical methanol was made at pressures between 8.7 and 36 MPa. It was found that the conversion of soybean oil into the corresponding methyl esters was enhanced considerably in the supercritical methanol. The apparent activation energies of the transesterification are different with the subcritical and the supercritical states of methanol, which are 11.2 and 56.0 kJ/mol (molar ratio of methanol to oil: 42, pressure: 28 MPa), respectively. The reaction pressure considerably influenced the yield of fatty acid methyl esters (FAME) in the pressure range from ambient pressure up to 25 MPa (280 °C, 42:1). The reaction activation volume of transesterification in supercritical methanol is approximately −206 cm³/mol. The PΔV ^≠ term accounts for nearly 10% of the apparent activation energy, and can not be ignored (280 °C, 42:1).