Prediction of Thermodynamic Consistency of Vapour-liquid Equilibrium of a Two-Phase System in the Presence of the Salting-in and Salting-out Effects

Salt effect on vapour-liquid equilibrium (VLE) has been a subject of intense investigation from experimental and modelling perspectives since the salting-in and salting-out effects might eliminate the possibility of azeotrope formation at saturation. Nevertheless, thermodynamic consistency evaluation of data generated from this type of studies, accounting for non-idealities of one or two of the phases involved, is mandatory. In this communication, a primary evaluation of thermodynamic consistency under a non-ideal VLE situation of data generated for an ethanol-water system with and without the addition of inorganic potassium chloride (KCl) salt is presented. The equilibrium data (T-x-y diagram) for ethanol-water system is generated under isobaric conditions, with and without 2 and 4 g/L concentration levels of KCl. The isobaric VLE data of ethanol-water system saturated with KCl concentrations are correlated by means of a modified Raoult's law to predict the activity coefficient of the volatile components. Activity coefficients are evaluated using a state equation and fugacities as well. Both methodologies give identical activity coefficients, indicating that Raoult's law is a simple approach to calculate these parameters. Thermodynamic consistency of VLE data with and without the addition of KCl is validated from a simple but reliable strategy that makes use of the Gibbs-Duhem plot and Murthy's approach which avoids the use of enthalpy of mixing.