Using a polar version of PC-SAFT to understand the impact of added CO2 on the phase behavior of biodiesel systems

Abstract Two obstacles to efficient transesterification of triglycerides to form biodiesel are the initial immiscibility of the reactants and the potential depletion of the short chain alcohol into the glycerol-rich phase formed as a result of the reaction. Consequently, co-solvents have been used to promote mixing and lower the energetic requirements of the process. Amongst the multiple proposed co-solvents in the literature carbon dioxide exhibits the highest vapor pressure, which provides multiple benefits in the downstream separation process of the biodiesel products and excess reactants. In previous work the presence of an optimal content of CO2 for each set of P-T conditions was demonstrated for a system containing CO2, methanol and triglycerides. In this work, that approach has been extended to the quaternary system of CO2, methanol, glycerol and biodiesel, with an aim towards understanding how added CO2 can reduce the loss of the reactant methanol to the glycerol-rich phase that forms during the transesterification reaction. Variation of the phase separation over a range of pressures (10–40 MPa) temperatures (40–200 °C) and different methanol to glycerol ratios (2–30:1) and the influence on the optimal conditions are reported using a polar version of PC-SAFT that can easily be extended to multiple substances and process conditions. In general, it has been found that adding CO2 promotes retention of methanol in the oil-rich phase until a very light third phase is formed; the appearance of this third phase is predicted by the PC-SAFT model.