USE OF CARBON-DIOXIDE TO SHIFT BENZENE/ACETONITRILE AND BENZENE/CYCLOHEXANE AZEOTROPES

A semi-continuous flow apparatus with an on-line gas chromatograph was designed and constructed for conducting experiments to obtain high pressure supercritical fluid-liquid phase equilibrium data for binary and ternary systems. Isothermal P-x-y data were obtained for CO2/benzene, CO2/cyclohexane, CO2/acetonitrile, CO2/benzene/cyclohexane and CO2/benzene/acetonitrile systems up to the mixture critical pressure. Ternary data were obtained with three initial concentrations of liquid mixture to investigate the potential of using supercritical CO2 in breaking or shifting the benzene/acetonitrile and benzene/cyclohexane azeotropes. The analyses of the experimental data indicated that for the CO2/benzene/cyclohexane system at 42.3 degrees C and a pressure of 1.926 MPa, the benzene/cyclohexane azeotrope was shifted from 50.0 mol% cyclohexane to greater than 71.2 mol% cyclohexane (on a CO2-free basis). Similarly, in the case of the CO2/benzene/acetonitrile system at 45.3 degrees C and 2.326 MPa, the benzene/acetonitrile azeotrope shifted from 54.3 mol% benzene to greater than 70.3 mol% benzene. These systems are among the few documented shifts in azeotropic composition attained using supercritical fluids. For the Peng-Robinson equation of state with the one-parameter mixing rule, relative errors of the order of 15% in the prediction of bubble point pressures were encountered at higher pressures, i.e. in the near-critical region. At lower pressures, however, the fit results were in agreement with the experimental bubble point pressures. Excellent representation of binary supercritical fluid-liquid phase behavior for these systems was obtained up to the mixture critical point when pressure- and composition-dependent mixing rules were incorporated. Although the two-parameter mixing rules provided very good descriptions of the two-phase envelope for the ternary mixtures, it was unable to predict any shift in the azeotropic composition (on a CO2-free basis) in the presence of supercritical CO2.