Simultaneous VLLE data correlation for ternary systems: Modification of the NRTL equation for improved calculations

Simultaneous correlation of vapour-liquid-liquid equilibrium (VLLE) data is hardly ever attempted in literature with one common set of parameters for all the equilibrium regions present in the systems. It is common practice to obtain different sets of parameters for VLE, LLE and VLLE regions when experimental equilibrium data for ternary systems are fitted with a given model (e.g. an activity coefficient model for the liquid phase). Besides, when dealing with the correlation of VLE for ternary systems, it is quite frequent to obtain different sets of parameters for each one of the three binary subsystems from those obtained when exclusively binary VLE data are correlated. In the present work, the simultaneous correlation of all the equilibrium regions in the ternary VLL system is attempted. The objective function and the restrictions of the system are clearly presented. In addition, a simple modification of the NRTL equation for the Gibbs energy of mixing (GM) is proposed including a ternary term and a binary correction based on the Wohl equation. This modification has significantly improved the quality of the results obtained in the simultaneous correlation of different equilibrium regions.