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 pr essure supercritical fluid-liquid phase equilibrium data for binary an d ternary systems. Isothermal P-x-y data were obtained for CO2/benzene , CO2/cyclohexane, CO2/acetonitrile, CO2/benzene/cyclohexane and CO2/b enzene/acetonitrile systems up to the mixture critical pressure. Terna ry data were obtained with three initial concentrations of liquid mixt ure to investigate the potential of using supercritical CO2 in breakin g or shifting the benzene/acetonitrile and benzene/cyclohexane azeotro pes. 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 M Pa, the benzene/cyclohexane azeotrope was shifted from 50.0 mol% cyclo hexane to greater than 71.2 mol% cyclohexane (on a CO2-free basis). Si milarly, in the case of the CO2/benzene/acetonitrile system at 45.3 de grees C and 2.326 MPa, the benzene/acetonitrile azeotrope shifted from 54.3 mol% benzene to greater than 70.3 mol% benzene. These systems ar e among the few documented shifts in azeotropic composition attained u sing supercritical fluids. For the Peng-Robinson equation of state wit h the one-parameter mixing rule, relative errors of the order of 15% i n the prediction of bubble point pressures were encountered at higher pressures, i.e. in the near-critical region. At lower pressures, howev er, the fit results were in agreement with the experimental bubble poi nt pressures. Excellent representation of binary supercritical fluid-l iquid phase behavior for these systems was obtained up to the mixture critical point when pressure- and composition-dependent mixing rules w ere incorporated. Although the two-parameter mixing rules provided ver y 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.