Biodiesel synthesis under supercritical conditions provides both energy and economic benefits over the conventional base catalyzed biodiesel production process. Development of this new process and technology requires a deep understanding of the mechanisms and kinetics of supercritical transesterification reactions. In this study, supercritical transesterification of triolein, a model triglyceride compound, with methanol was investigated at 355-400 °C and 15 MPa. The methanol-to-triolein molar ratio was fixed at 9, while the residence time varied from 0.5 to 10 min. The conversion of triolein and the composition of the reaction intermediates were determined for different reaction conditions. Free glycerol and bound glycerol fractions in the methyl ester phase were determined by the ASTM method. A three-step kinetic model is proposed to predict concentrations of triolein, diolein and monoolein during reaction. Reactions from diolein to monoolein and from monoolein to glycerol are assumed to be reversible reactions. Reaction rate constants have been determined. Compared with the simple one-step model, the three-step model better represents intermediate reaction product evolutions which are related to bound glycerol fractions in the biodiesel. It was found that bound and free glycerol fractions were in the ranges of 0.36 - 0.45 wt % and 0.18 - 0.54 wt %, respectively.
