A theoretical and experimental study of liquid-liquid equilibrium to refine raw glycerol obtained as a byproduct on the biodiesel production

Abstract

Getting high purity glycerol at a low cost from biodiesel (methylricinoleate) production is of great economic interest. Therefore, exist several strategies for raw glycerol purification. Some involve liquid-liquid extraction steps using ether, toluene, n-butanol, cyclohexanol, aniline, or solvent mixtures to remove organic impurities from glycerol. However, the phase's equilibrium data is not available for the convenient process design in some cases. This work purified raw glycerol obtained from the alkaline methanolysis of castor oil by neutralization, filtration, distillation, wash with acetone, adsorption with activated charcoal, and drying with sodium sulfate. Furthermore, liquid-liquid equilibrium (LLE) of the ternary system glycerol-acetone-biodiesel was monitored by 1H NMR spectroscopy at 281.15, 290.15, and 299.15 K and correlated by the NRTL model. Finally, we evaluated the COSMO-RS implicit solvation model predictive methodology due to the lack of LLE data for glycerol purification systems in the literature. The experimental results indicate that the process produces glycerol at 96.42% purity wt., and the LLE evaluated consists of two pairs of partially miscible liquids, glycerol-biodiesel, and glycerol-acetone. Therefore, the glycerol purification with acetone proposed here allows less process dangerous conditions, a less toxic product for food applications, eliminates 100% of organic impurities with low volumetric ratios, and helps to precipitate the persistent ash content. The theoretical results indicate COSMO-RS model overestimated the solubility glycerol-acetone, predicting only the pair of partially miscible liquids glycerol-biodiesel, this limiting the accuracy of COSMO-RS to predict LLE data for glycerol purification systems with acetone.