Study on the Phase Behaviors, Viscosities, and Thermodynamic Properties of CO2/[C4mim][PF6]/Methanol System at Elevated Pressures

Abstract

An apparatus to determine the vapor–liquid equilibria of CO₂/ionic liquid (IL)/organic solvent multisystems and the viscosity of the liquid phase at elevated pressures has been constructed. The solubility of CO₂ in 3-butyl-1-methyl-imidazolium hexafluorophosphate ([C₄mim][PF₆]) and the viscosity of CO₂-saturated [C₄mim][PF₆] have been studied at 313.15, 323.15, and 333.15 K and at pressures up to 12.5 MPa. The phase behavior of CO₂/[C₄mim][PF₆]/methanol ternary mixture and the viscosity of the liquid phases at equilibrium condition have also been determined at 313.15 K and at 7.15 and 10.00 MPa. The partition coefficients of the components in the ternary system are calculated. Peng–Robinson equation of state and some thermodynamic functions are combined to calculate the fugacity coefficients of the components in the system. It demonstrates that the viscosity of the IL-rich phase decreases significantly with increasing pressure of CO₂, and the effect of temperature on the viscosity of CO₂/IL mixture is not noticeable at high pressure, although the viscosity of the CO₂-free IL decreases dramatically with increasing temperature. Compressed CO₂ may become an attractive reagent for reducing the viscosity of ILs in many applications. The mole fraction of methanol in the CO₂-rich phase is much lower than that in the IL-rich phase; this indicates that the interaction between the IL and methanol is stronger than that between CO₂ and methanol. The fugacity coefficient of CO₂ in IL-rich phase is larger than unity, while that of methanol is much small than unity, which further suggests that methanol–IL interaction is much stronger than CO₂–IL interaction. However, the CO₂–IL interaction is stronger than the CO₂–methanol interaction.