Modelling of liquid-vapour equilibria in the H2O-CO2-NaCl and H2O-H2S-NaCl systems to 270°C

Abstract

An unsymmetric thermodynamic model for the liquid-vapour equilibria in the H2O-CO2-NaCl and H2O-H2S-NaCl systems for temperatures below the critical point of water (250°C for H2S, 270°C for CO2) is presented. The vapour phase is described by a cubic Equation of state. The water and gas components in the liquid aqueous phase are respectively described by Raoult's law and Henry's law combined with a Redlich-Kister's model of regular solutions for the activity coefficients of these two components. After an analysis of the experimental data base, more than 80% for CO2 and 92% for H2S of predicted pressures of aqueous isopleths deviate less than 5%, which is comparable to experimental uncertainty. Although the model is not fitted on the composition of the vapour phase, the predicted values are correct above 100°C. The salt effect is modelled by a combination of the model of Pitzer for the water activity and an extension of Setchenow's law. The hypotheses behind this model makes it applicable at pressures below 300 bar. Copyright © 2005, Institut français du pétrole.