In this study, a thermo-responsive switchable solvent (TSS), with a tunable hydrophobicity by simply changing the temperature (between 25 to 45oC) was used for simultaneous lipids extraction from wet microalgae and biodiesel production. By manipulating the hydrophobicity of the solvent, the cell wall disruption, lipid extraction and transesterification, and product separation steps were all carried out in a single pot, while eliminating the need for the energy intensive and time-consuming drying step. To overcome the problems currently encountered by using conventional alkaline catalysts in the transesterification of lipids, immobilized enzyme has been used. The proposed TSS consisted of an ionic liquid (N,N diethyl-Nmethylammonium methane sulfonate), a polymer poly (propylene) glycol (PPG) and water. The effectiveness of the proposed process was compared to that using conventional organic solvent, n-hexane, and other CO₂ triggered amine based switchable solvents, namely 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU)-1-hexanol and DBU-Mono-ethanolamine (MEA). At the same conditions and solvent switching program, using immobilized lipase as a biocatalyst, the biodiesel yields were 45.5 ± 0.38 %, 37.8 ± 1.03 % and 5.9 ± 1.50 %, using TSS, DBU-hexanol, and DBU-MEA respectively. Using n-hexane resulted in insignificant yield of 3.1 ± 0.43 %. Furthermore, a reusability of the TSS-immobilized lipase system was investigated, and it was shown that the reusability biodiesel yield dropped from 50 ± 1.46 % in the first cycle to 20.4 ± 0.60 % in the fourth.
A parametric study was performed, using response surface methodology (RSM) to evaluate the effects of cell disruption and extraction/reaction durations in the range of 0-3 h, and methanol amount used in the range of 0.02 – 0.2 mL on the biodiesel production yield from 1 g of wet biomass. The results were used to develop a statistical model to predict the biodiesel yield under different conditions and to optimize the process. The optimum conditions were estimated to 0.5 hr, 3 hr and 0.15 mL for the cell disruption time, extraction-reaction time and methanol amount respectively, at which the yield was predicted to be 78.65 %. The experiment was repeated at the optimum conditions, and the actual yield was found to be 75.11 ± 1.03 %.
The successful use of TSS for simultaneous extraction-reaction and product separation from wet biomass has a significant effect on the simplification of microalgae to biodiesel process. By simply changing the temperature, the hydrophobicity of TSS can be manipulated, rendering the overall process easier, as compared to the CO₂ triggered Switchable Solvents. A process similar to the one presented in this work has never been reported before in literature
