A study of tri-phasic behavior of ionic liquid–methanol–CO2 systems at elevated pressures

The phase behavior of CO2–methanol–1-n-butyl-3-methyl-imidazolium hexafluorophosphate ([bmim][PF6]) system under different conditions has been studied by a static method. The results demonstrate that at 40.0 °C there can exist three phases (IL-rich phase, the methanol-rich phase, CO2-rich phase) in the pressure range from 6.95 to 8.21 MPa when the concentration of the IL is low. In the high IL concentration region, however, there is only one liquid phase in the entire pressure range. In the three-phase region, the difference of the concentrations of the IL in the two liquid phases becomes larger and larger with increasing pressure. The results also show that the three-phase region exists only in the temperature range from 35 to 44.5 °C at 7.6 MPa. In the three-phase region IL concentration in the IL-rich phase decreases with increasing temperature, while that in the methanol-rich phase increases as temperature rises, and the difference of IL concentration in the two liquid phases is sensitive to temperature. On the basis of phase equilibrium data the distribution coefficients of [bmim][PF6] in the two liquid phases are calculated and discussed.