Effect of additives to supercritical methyl acetate on biodiesel production

•Additives for biodiesel production by supercritical methyl acetate were studied.•CH3COOH(aq) (26 wt.% conc.) was a potential additive.•10 wt.% additive addition yielded 109.7 wt.% products under 300 °C/20 MPa/45 min.

Supercritical methyl acetate has been proven to convert triglycerides into fatty acid methyl esters (FAMEs) and triacetin. Due to the low reactivity of supercritical methyl acetate on triglycerides, reaction condition of 350 °C/20 MPa/45 min is needed to obtain the highest yield of 96.7 wt.% FAME and 8.8 wt.% triacetin (total 105.5 wt.%). Yet, under such condition, thermal decomposition of products is inevitable, thus, further yield increments are hard to attain. This study was, therefore, done to know the possible use of additives in increasing the reaction rate of the supercritical process. Oleic acid, acetic acid, methanol and water were evaluated as additives. It was discovered that acetic acid and water additions improved the performance of supercritical methyl acetate method. Surprisingly, when both additives were used simultaneously as aqueous acetic acid, the higher yield was obtainable at a lower reaction temperature. The addition of 10 wt.% aqueous acetic acid (25 wt.% concentration) at 300 °C/20 MPa/45 min yielded 96.8 wt.% FAME and 12.9 wt.% triacetin (total 109.7 wt.%), thus, it is considered as a potential additive in biodiesel production by supercritical methyl acetate method.