Supercritical CO2 extraction of neutral lipids from dry and wet Chlorella vulgaris NIES 227 microalgae for biodiesel production

This study investigates supercritical CO2 extraction of neutral lipids from Chlorella vulgaris NIES 227 microalgae, for biodiesel applications. While microalgae feedstocks are typically desiccated before extraction, drying is energy intensive and supercritical extraction from wet microalgae would be beneficial in reducing overall energy costs. The effects on the supercritical extraction kinetics, yields and extract composition of the water content of the initial biomass (either 0 or 27 wt%), and of the operating pressure (200 and 250 bar) and temperature (40 and 60 degrees C) were investigated. The highest total extraction yield (16.16 wt% extract/dry biomass) was obtained with freeze-dried microalgae at 40 degrees C and 200 bar. However, the extracts obtained at 40 degrees C and 250 bar were the richest in fatty acids (0.96 g FA/extract), and the extracts obtained at 60 degrees C and 250 bar had the highest cetane number (69) and oxidative stability (18 h). Water in the initial biomass had a negative impact on total extraction yields (3.89 wt% extract/dry biomass at 200 bar and 40 degrees C) and led to increased pigment extraction, but results suggest the presence of water may promote the extraction of fatty acids, in particular monounsaturated ones, highly desirable for biodiesel. In comparison, Soxhlet extraction with hexane was found to have higher lipid yields (31.20 wt% extract/dry biomass) than any of the tested supercritical conditions, but more pigments were also extracted alongside, having a negative impact on oxidative stability. All the extracts obtained with supercritical CO2 and Soxhlet hexane met biofuel European standards for density, cetane numbers and iodine values.