Modelling phase behavior of triglycerides, diglycerides and monoglycerides related to biodiesel transesterification in mixtures of alcohols and CO2 using a polar version of PC-SAFT

Abstract Soh et al. [1] showed that one can achieve high conversion (98%) of triglycerides to fatty acid methyl esters at 95 °C when carbon dioxide (CO2) is used as a co-solvent to enhance mixing of the immiscible alcohol and triglyceride. Soh's results demonstrated the benefits of using CO2 as a co-solvent yet did not provide information on the evolution of the phase behavior during the reaction. The PC-SAFT equation of state has been employed to understand and predict the phase behavior of the various biodiesel components versus temperature and pressure in the presence of CO2. In previous work [2] the interaction between alkyl-esters and low molecular weight alcohols, glycerol, and CO2 were investigated. Polar PC-SAFT and the Group Contribution PPC-SAFT EoS were used to model phase equilibria amongst the various multicomponent systems. The same approach is extended here, specifically applied to mixtures containing tri-, di- and monoglycerides plus low molecular weight alcohols and CO2. Data for different types of glycerides are scarce and in general the phase boundary curves in CO2-based mixtures are only weakly dependent on CO2 concentration, creating curves with very high slopes in P-T space that accumulate larger prediction errors compared to the previously investigated alkyl-ester systems. Nonetheless, it was found that one could generate a general correlation for binary interaction coefficient that depends only on carbon chain length and degree of unsaturation for description of triglyceride-CO2 mixtures. Binary interaction parameters as low as 12% were obtained for all glyceride-methanol systems and binary interaction coefficient below 3.5% are sufficient to correctly capture CO2 – glyceride vapor-liquid and liquid-liquid phase equilibria.