Mass transfer modeling of Scenedesmus sp. lipids extracted by supercritical CO₂

Mathematical modeling of supercritical extraction of lipids from Scenedesmus sp. microalgae is presented in this study. Broken and intact cells (BIC) model, proposed by Sovova in 1994, was used to predict the extraction curves. The effects of extraction pressure, temperature and fluid flowrate on predicted model parameters and extraction rates were assessed. The results indicates that BIC model represents well the experimental data, and the mass transfer coefficients in the fluid and solid phases changed in ranges of 6.7-30.9×10^-6ms^-1 and 2.08-13.2×10^-10ms^-1, respectively. The fluid phase coefficient increased with increasing the flowrate and temperature, but was not affected by the pressure, whereas the solid phase showed a significant decrease with the increase in pressure. According to the external mass transfer coefficient, mass transfer correlations were developed to relate Sherwood number to Reynolds, Schmidt, and Grashof numbers. It was found that when Schmidt exponent was fixed at 1/3 (model 1), the model did not fit the experimental data well, whereas when the power was adjusted and considered as a fitting parameter the coefficients of determination increased significantly exceeding 90%. The natural convection effect was found to be insignificant compared to the forced convection. The validity of the developed correlations with coefficients determined from experimental data collected from a small scale extraction cell was confirmed by predicting the overall extraction curve (OEC's) of a larger scale extraction cell.