Enzymatic biodiesel production of microalgae lipids under supercritical carbon dioxide: Process optimization and integration

Enzymatic biodiesel production in supercritical CO₂ (SC-CO₂) has recently received an increasing attention, as an alternative to the conventional chemical processes. In this study, enzymatic production of biodiesel from microalgal lipids was investigated in batch and integrated extraction-reaction systems. In the batch system, the effect of enzyme loading (15-50 wt%), temperature (35-55°C) and methanol to lipid molar ratios (3-15:1) were studied, and response surface methodology was employed to optimize selected factors effect. The optimum transesterification yield of 80% was obtained at 47°C, 200 bar, 35% enzyme loading, and 9:1 molar ratio after 4 h reaction in the batch system. The experimental results were also used to determine the kinetics parameters of the Ping-Pong Bi Bi model, with methanol inhibition, suggested to describe the reaction. In the continuous integrated extraction-reaction system, the effect of methanol to lipids molar ratio was investigated, and enzyme operational stability and reusability were tested. Bed regeneration by tert-butanol washing was also assessed. The optimum methanol to lipid ratio was found to be 10:1. At this ratio, the enzyme was able to attain 78% of its original activity when reused for 6 continuous cycles, and the bed was successfully reused by washing with tert-butanol.