Correlation and prediction of vapor-liquid equilibrium in electrolytic solutions

Two expressions for the excess Gibbs free energy are presented which correlate and predict vapor-liquid equilibrium and the mean molal activity coefficient of an electrolyte in a ternary mixture containing water and either MeOH or EtOH. The proposed equations take into account coulombic forces between ions and the physical interaction forces between ion-solvent and solvent-solvent molecules in a solution. Model #1 is a combination of an extended Debye and Huckel equation and the local composition of non-random two liquid (NRTL) model. A ternary mixture requires six adjustable binary parameters to predict activity coefficients. These six parameters are evaluated from three separate binary data reductions. Ternary data are predicted with an average error of |ΔY| less than 0.03 up to I = 2. The parameters are considered temperature independent within a 30 to 40°C temperature range for aqueous electrolyte mixtures and within a 15 to 20°C temperature range for nonaqueous electrolyte mixtures. Both isothermal and isobaric ternary experimental data have been tested for ternary correlation. Isothermal ternary data correlation for systems containing water-methanol solvents give results with an average error of |ΔY| less than 0.01 up to I = 6. Whereas isobaric ternary data are correlated with an average error of |ΔY| less than 0.02 up to I = 4. It is possible to extend this model to multi-component mixtures although this has not been investigated in this work. Model #2 is a combination of the Bromley equation, the simplified NRTL equation and an additional ternary salting out expression. The behavior of each electrolyte-solvent binary is described by a one parameter form of the Bromley equation. The temperature dependency of the binary parameters has been established with a two constant equation. Prediction of aqueous electrolyte binary data (γ± and vapor pressure depression) is obtained with an average percent error less than 10.0 at intermediate temperatures. Correlation of ternary VLE and γ± data require four binary parameters and two additional ternary adjustable parameters. This model is limited to binary and ternary data correlation only. The maximum concentration range for the correlation of ternary systems containing water and methanol solvents is about I = 3. The correlation of vapor-liquid equilibrium data results in an average error of |ΔY| less than 0.012, except for the LiCl-H2O-MeOH system at 60°C where the average error in |ΔY| is 0.02