Kinetic Study on Nannochloropsis Oculata's Lipid Extraction Using Supercritical CO₂ and n-Hexane for Biodiesel...

Biodiesel as a renewable fuel has attracted increasing attention in recent years. Microalgae biomass is becoming an attractive raw material for producing biodiesel using supercritical CO₂ (SC-CO₂) as a safe and environmentally friendly technique with high efficiency for lipid extraction. In this study, the lipid of Nannochloropsis oculata was extracted under different conditions of SC-CO₂ to assess the kinetics of supercritical fluid extraction. The effective parameters on lipid extraction, including temperature, pressure, and the existence of n-hexane as a co-solvent, were investigated. The results show that an increase in temperature at low or high pressures causes the kinetic constant of lipid extraction to decrease or increase, respectively. Also, an increase in pressure causes the kinetic constant of lipid extraction to increase at low or high temperatures. The most yield and the most kinetic constant value during extraction with pure CO₂ are about 0.262 [g extracted lipid/g microalgal biomass] and 0.062 min^-1, respectively, at the highest pressure and temperature (i.e., 550 bar and 75 °C). Using SC-CO₂ laced with n-hexane increases both the final yield and the rate of lipid extraction. Also, it improves the quality of the biodiesel fuel through the extraction of unsaturated fatty acids with a concentration of almost two times more than saturated fatty acids. Additionally, results reveal that the effect of adding n-hexane to CO₂ in lipid extraction would be more efficient by increasing the temperature and lowering the pressure.