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Abstract
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 cm^sup 3^/mol. The PΔV^sup ≠^ term accounts for nearly 10% of the apparent activation energy, and can not be ignored (280 °C, 42:1).
Keywords Apparent activation energy * Activation volume * Biodiesel * Kinetics * Pressure * Supercritical methanol * Soybean oil
(ProQuest-CSA LLC: ... denotes formulae omitted.)
Introduction
The transesterification reaction of vegetable oil with supercritical methanol without a catalyst provides a new way of producing biodiesel fuel and was recently reported by Saka [1, 2], Madras [3], Demirbas [4, 5] and He [6]. Compared to conventional transesterification processes catalyzed by acid and alkali catalysts, there are several outstanding advantages of the supercritical method. It was reported that the transesterification reaction in supercritical methanol was completed in several minutes, while the conventional catalytic transesterification takes several hours. It was believed that the two phase oil and methanol mixture could be converted into a single phase under supercritical methanol conditions due to the great decrease in methanol's dielectric constant [1]. As a result, the good contact between the oil and alcohol greatly increases the rate of reaction. In addition, the supercritical alcohol is expected to act as an acid catalyst to accelerate thereaction [7]. The high reaction temperature and reaction pressure of the supercritical condition also help to accelerate the transesterification reaction. Because the transesterification reaction under supercritical conditions does not require catalysts, the purification of the products after transesterification in supercritical methanol is much simpler and more environmentally-friendly compared to conventional catalytic processes in which all the catalyst and saponification products have...