Empirical model of the Gibbs free energy for saline solutions of arbitrary concentration: Application for H2O-NaCl solutions at 423.15K-573.15K and pressures from saturation up to 5kbar

TL;DR

This paper presents an empirical, analytical model for the Gibbs free energy of saline solutions, accurately describing H2O-NaCl solutions across wide concentration, temperature, and pressure ranges, useful for thermodynamic property predictions.

Contribution

The authors develop a simple four-parameter analytical model for Gex that accurately fits experimental data for H2O-NaCl solutions from dilute to saturated concentrations, extending to high pressures up to 5 kbar.

Findings

Accurately reproduces experimental osmotic coefficients across concentrations.

Extends model applicability to high pressures up to 5 kbar.

Provides thermodynamic properties for H2O-NaCl solutions over wide conditions.

Abstract

An empirical model of the concentration dependence of the excess Gibbs free energy Gex for saline solutions is proposed. Our simple analytical form of Gex allows obtaining equations of state of saline solutions equally precise in the whole range of the salt concentrations, from dilute solutions up to the limit of solubility. Our equation for Gex includes one term responsible for concentration dependence of Gex at low salt concentrations and two terms of Margules type dependent on powers of mole fractions of the components. These terms contain four parameters dependent on temperature and pressure. As an example of application of the proposed model we took the system H2O-NaCl. For fixed T and P, our four-parameter form of Gex allows precisely reproduce experimental data for NaCl water solutions, including both regions of low and high concentrations. An introduction of temperature dependence of the parameters allowed us to build a precise analytic fit of experimental data on osmotic coefficients of NaCl solutions for saturation pressures of NaCl at temperatures 423.15K-573.15K. On the basis of PVTx data by Driesner (2007) the model was extended towards pressures up to 5kbar. Our form of Gex allows obtaining the molar volumes, activities of components and other thermodynamic properties of NaCl-H2O solutions in the range of temperatures 423.15K-573.15K and pressures from saturation up to 5kbar in the whole range of concentrations possible for H2O-NaCl solutions (from zero up to approximately 10 molal at 573.15K).