Modelling the kinetics of lipid extraction from wet microalgal concentrate: A novel perspective on a classical process

• We examined organic solvent extraction of lipid from wet microalgal concentrate.
• Spectrophotometric method was developed to determine instantaneous lipid yield.
• Model described lipid extraction as a first-order equilibrium-driven process.
• Increase in the water content of the solvent did not reduce extraction efficiency.
• Microalgal lipid was non-polar with a Hildebrand value between 24 and 28 MPa1/2.

This study reports a detailed examination of the sustainability of using organic solvents to extract lipid from wet microalgal concentrate (Tetraselmis suecica) to produce biodiesel. In the preliminary section of the study, a simple surrogate method that facilitated the rapid determination of lipid yield during organic solvent extraction was developed. Based on monitoring the change in the area of the spectrophotometric absorbance of the microalgal lipid solution, the method appeared to accurately predict the lipid yield measured by a traditional gravimetric technique (r2 = 0.97). In the main section of the study, the kinetics of lipid extraction from wet microalgal concentrate was examined by developing a model that described microalgal lipid extraction as a first-order equilibrium-driven process. A monophasic methanol-based mixture or hexane/isopropanol-based mixture was selected as an extraction system. The model was successfully fitted to the evolution of lipid concentration in the extraction solvent obtained from the experiments (0.68 ⩽ r2 ⩽ 1.00). The lipid extraction efficiency was shown to increase with an increase in the speed of agitation, the extraction temperature, and the degree of cellular disruption. When water content of the extraction mixture was less than 20 vol%, increase in the water content of the extraction mixture did not significantly reduce the lipid extraction efficiency as long as the mixture remained monophasic. This finding was critical for the economics of organic solvent extraction as it removed the prerequisite for dried biomass normally achieved via an energy-intensive drying process. The Hildebrand solubility parameter of the microalgal lipid extracted in the study was estimated to be between 24 and 28 MPa1/2. Any monophasic combination of organic solvents with Hildebrand value lying within this range was able to effectively extract lipid from the microalgal biomass with marginal variation in extraction efficiency.