LIQUID-LIQUID EQUILIBRIUM SURFACES FROM BINARY HEATS OF MIXING
BATTLER, JOHN RAYMOND
Doctor of Philosophy
Experimental heats of mixing were measured for four partially miscible binary systems at six to eight temperatures near their critical solution temperatures (CST) and for the binary pairs of six ternary liquid mixtures at four temperatures between 293.15 and 323.15 K using a Tronac isothermal flow calorimeter. The accuracy is better than 0.75 percent. The data for the partially miscible binary systems indicate the heat of mixing has a critical exponent of zero. The accuracy of the explicit temperature dependence of existing local composition (LC) models was tested by predicting binary liquid-liquid coexistence curves and CSTs using heats of mixing-fit parameters. The explicit temperature dependence of existing models is incorrect and the models are incapable of even qualitatively describing liquid-liquid coexistence curves from heat of mixing-fit parameters. A LC model for the excess enthalpy, h('E), similar to the NRTL model of Renon and Prasunitz, has been integrated between temperature limits to obtain an expression for the excess Gibbs free energy. The decoupling of the preexponential interaction energy from the exponential interaction energy and the model's temperature dependence provide the model with the capability to fit well any commonly found excess enthalpy vs. composition curve. Use of this equation to predict liquid-liquid equilibria (LLE) requires a reference excess free energy term evaluated from vapor-liquid equilibria (VLE) data at conditions removed from those corresponding to phase separation. The model and the method provide a thermodynamically consistent method for extrapolation of VLE data using heats of mixing to obtain reasonably reliable binary LLE predictions. Qualitative and rough quantitative binodal data can be obtained in many cases by using UNIFAC to estimate a reference excess free energy, g('E). This predictive technique has been extended to ternary systems with similarly good results obtained.