Transesterification of soybean oil with methanol and acetic acid at lower reaction severity under subcritical conditions

•(trans)Esterification of oils under subcritical conditions.•Acetic acid as catalyst and co-solvent in biodiesel production.•Influence of reactor hydrodynamic (loading and stirring) on FAME yield.•High methyl ester yield can be obtained at less severe reaction conditions.

Soybean oil (56–80 g) was reacted with methanol (40–106 mL) to produce fatty acid methyl ester in the presence of 1–6% acetic acid under subcritical condition at 250 °C. Stirring and loading of the reaction system affected the yield and severity of the process. The presence of acetic acid improved the yield of FAME from 32.1% to 89.5% at a methanol to oil molar ratio of 20 mL/g. Acetic acid was found to act strongly as an acid catalyst and to some extent improved the solubility between oil and methanol. Reaction pressure higher than the supercritical pressure of methanol (7.85 MPa) was not required to achieve high FAME yield (89.5–94.8%) in short time (30–60 min).