Exergy-based screening of biocompatible solvents for in situ lipid extraction from Chlorella vulgaris

The biocompatibility and high extraction efficiency are two characteristics of an optimal solvent for in situ lipid extraction from microalgae. These parameters were evaluated for seven potential solvents at extraction times of 5, 10, 15, and 20 min from Chlorella vulgaris ISC33. The solvent biocompatibility was assessed by determination of cell viability and biological activity after different extraction times. It was found to be associated with water solubility of the solvent and the exposure time. The microalgae treatment with n-decane increased growth up to 90 %, while treatment with polar solvents inhibited biological activity. Short-term exposure of C. vulgaris ISC33 to hydrophobic solvents increased algal growth due to improved membrane permeability, while long contact times impaired growth due to molecular toxicity. The extraction efficiency of the solvents was evaluated by exergy analysis to find the optimal solvent and also to pinpoint the sites of primary exergy loss. The solvent evaporation during the lipid extraction was determined as the main site of exergy loss in the process. The biocompatible solvents require less power for mixing with the microalgal suspension due to lower viscosity and density than polar solvents. The exergy required for solvent/lipid separation is also lower for apolar solvents. Considering exergy, biocompatibility, and economic objectives, the lipid extraction from a 5 g dry weight L-aEuroi1 of C. vulgaris ISC33 by 15-min exposure to n-decane yields the best performance.