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A two-stage biodiesel production directly from wet microalgae ( Chlorella vulgaris ) assisted by radio frequency (RF) heating was optimized in this study with the response surface methodology (RSM) applied to both two steps. In order to apply RSM to the two stages rather than to a one-step reaction, a three-variable, five-level central composite design (CCD) was employed to evaluate the effects of three key parameters, i.e. HCl to MeOH ratio (v/v), MeOH volume, and RF heating time. Two regression equations (one regression equation for each stage) were obtained, and two sets of optimized production conditions with a slight difference were also determined. The best predicted FAME yield of 93.1% was obtained at a HCl to MeOH ratio of 4.27 (v/v), a MeOH volume of 28.5 mL, and a RF heating time of 19.2 min. Although the second stage of conversion increased the FAME yield from 82.9% to 93.1%, this process would cause additional consumption of time, solvent, and energy. Verification experiments were carried out at the optimized point and resulted in a yield of 92.7 ± 0.1%, which validated the reliability of the prediction model. All processing steps, including cell destruction, esterification, and transesterification, were carried out at temperatures below 100°C and atmospheric pressure. Therefore, no pressure-proof nor high-temperature apparatus was required. The procedure shows great potential for industrial application because of its high FAME yield, simple operation, low chemical consumption, and short processing time. The principles of the procedure can also be applied to other microalgae with high lipid contents.

Processing wet microalgae for direct biodiesel production: optimization of the two-stage process assisted by radio frequency heating