Phase equilibria of ternary systems of carbon dioxide + ethanol, dimethyl sulfoxide, or N,N-dimethyl formamide + water at elevated pressures including near critical regions

Abstract

A visual and volume-variable high-pressure phase equilibrium analyzer was used for measuring the isothermal vapor-liquid equilibrium (or vapor-liquid-liquid) phase boundaries of CO2 with the mixed solvents of ethanol/water = 9/1, dimethyl sulfoxide (DMSO)/water = 9/1, and N,N-dimethyl formamide (DMF)/water = 7.5/2.5 in molar ratio at temperatures from 298.2 K to 348.2 K over wide composition range, including near critical regions. The isothermal phase equilibrium data obtained from this work are compared with those of water-free binary systems to investigate the effect of phase boundary shifts caused by introducing the phase modifier, water, into the organic solvents. The comparison shows that the bubble, dew, and critical pressures substantially increase in the presence of water for each system, especially for DMSO-containing system. The Peng-Robinson equation of state with Mathias-Copemen alpha function and the Wong-Sandler mixing rule is employed to the phase equilibrium calculation. While the UNIFAC model estimates the excess Helmholtz free energy at infinite pressure, the binary interaction parameters for calculating the second virial coefficient are determined by the phase equilibrium data of constituent binaries. The obtained parameters are directly adopted to the phase boundary predictions for the investigated ternary systems.