High shear-assisted extraction of lipid from wet microalgal biomass

Abstract

The importance of microalgae as the potential source of biofuel and value-added products (omega-3 lipids, pigments) have led to the development of the microalgae-based biorefinery in recent years. For industrialization of microalgae-based biofuel production, development of energy and cost-efficient lipid extraction process is necessary. The conventional wet lipid extraction process composed of cell disruption and lipid recovery step required high energy consumption and solvent usage, and long process time, which reducing the overall economic feasibility. To overcome these problems, high shear-assisted lipid extraction method was proposed using a high shear mixer (HSM) for industrial microalgal strains, wet Aurantiochytrium and Nannochloropsis sp. These strains can achieve high biomass and lipid productivity with producing high amounts of omega-3 lipids (DHA and EPA). HSM can induce strong turbulence flow and high shear stress that cause strong dispersion and cavitation effects. The concentrated wet biomass was mixed with various organic solvent directly by using a HSM to enhance accessibility of solvent to lipids. The results showed that high shear mixing achieved high lipid extraction yield compared to conventional stirring without severe cell disruption. Furthermore, high shear mixer shows same lipid extraction yield for dry and wet biomass, which indicating that high shear mixing can overcome water barrier preventing mass transfer of solvent. Additional process optimization for Aurantiochytrium sp. was performed in large scale and achieved high lipid extraction yield (90 %) with low energy consumption (4.83 MJ/kg dry cell) and solvent usage (5.9 ml/g dry cell), which is lower more than 10 times than conventional methods developed for Thraustochytrids. In case of Nannochloropsis sp., it contained thick cell wall composed of cellulose and algaenan (hydrocarbon with various functional group) so that it was not enough to extract lipids with only hexane. Thus, we added alcohol and acid to solvent mixture and achieved high lipid extraction yield with short process time. In addition, acid played important role of convert polar lipids to free fatty acid, which increased lipid quality and prevents emulsion formation. Optimization of solvent composition and operating condition was resulted in high lipid extraction yield (83 % of total lipid and 94 % of EPA) with 30 % lower energy consumption (1.38 MJ/kg dry cell) and 7-times shorter process time (5 min), compared to the conventional methods for photoautotrophic microalgae. In conclusion, high shear-assisted lipid extraction process was successfully demonstrated to increase economic feasibility for the microalgae-based biorefinery. The high shear-assisted lipid extraction process was able to achieve high lipid yield with low energy consumption, solvent usage and short process time compared to conventional methods. For future work, development of a continuous system for microalgae lipid extraction process using in-line HSM and another downstream process that using biphasic system is recommended.