Continuous production of biodiesel from high acid value oils in microstructured reactor by acid-catalyzed reactions

Acid-catalyzed esterification and transesterification reactions, suitable for biodiesel production from high acid value oils, face a problem of long reaction time. In this study, we developed a two-step process for fast acid-catalyzed biodiesel production from high acid value oil in a microstructured reactor, which was assembled with an SIMM-V2 micromixer connected with a 0.6mm i.d. stainless steel capillary. Esterification of oleic acid and transesterification of cottonseed oil with methanol were separately carried out to find suitable reaction conditions. The influences of the residence time, the methanol to acid/oil molar ratio, the catalyst concentration, the water/acid concentration and the reaction temperature were examined. The highest oleic acid conversion was obtained when esterification of oleic acid with methanol was carried out at 100°C with the residence time of 5min, while the highest yield of fatty acid methyl ester (FAME) was achieved when transesterification of cottonseed oil with methanol was conducted at 120°C with the residence time of 20min. Thereafter, a two-step process was developed based on the above reaction conditions with the first and second steps separately conducted under the esterification and transesterification reaction conditions. The results indicated that the acid value of the acid oil was reduced from 160 to 1.1mg KOH/g with a methanol to acid molar ratio of 30, the H₂SO₄ concentration of 3wt%, a residence time of 7min at 100°C in the first step. The final FAME yield reached 99.5% with a methanol to triglyceride molar ratio of 20, the H₂SO₄ concentration of 3wt%, a residence time of 5min at 120°C in the second step. Therefore, biodiesel production from high acid value oil can be continuously achieved at high yields by acid-catalyzed transesterification in microstructured reactors with total reaction times of less than 15min.