Tri-phase behavior of ionic liquid–water–CO2 system at elevated pressures

Abstract

The phase behavior of CO₂–water–1-n-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF₄]) system at different conditions has been studied by the static method in the temperature range of 5 to 25 °C and pressures up to 16 MPa. At a fixed temperature or pressure there can exist four phases. However, this work focuses on the compositions of the phases in three-phase region (ionic liquid-rich phase, water-rich phase, CO₂-rich phase). The phase diagrams at 20.0 °C and 5.0 MPa have been discussed in detail. At 20.0 °C the concentration difference of the ionic liquid (IL) in IL-rich phase and water-rich phase becomes larger and larger with increasing pressure. At 5.0 MPa, IL concentration in the IL-rich phase decreases with increasing temperature, while that in the water-rich phase increases as temperature rises, and the difference of IL concentration in the IL-rich phase and water-rich phase is sensitive to temperature. On the basis of phase equilibrium data the distribution coefficients of [bmim][BF₄] between IL-rich phase and water-rich phase are calculated, and separation of IL and water is discussed.