Modeling of Gas Solubility in Aqueous Electrolyte Solutions with the eSAFT-VR Mie Equation of State

In this work, the eSAFT-VR Mie equation of state (EoS) is applied to water-methane-salt and water-carbon dioxide-salt mixtures. Initially, the EoS parameters for nonelectrolyte systems are fitted, and temperature-dependent water-ion interaction parameters are introduced to improve the behavior of the model at high temperatures. Furthermore, a database of experimental methane and carbon dioxide solubility in single-salt aqueous solutions was compiled. Four different parameterization schemes regarding gas-ion dispersion energy have been pursued in this work, namely, (a) setting the gas-ion dispersion energy parameter to zero, (b) using the SAFT-VR Mie combining rules without any binary interaction parameters, (c) using the predictive Hudson-McCoubrey combining rules, and last but not least (d) fitting to experimental solubility of gases in aqueous solutions of single salts. The fitting approach yields an average deviation from experimental data of about 10%, compared to more than 25% for the other approaches. However, despite the great overall deviations of the predictive methods, there are cases where they yield excellent results, such as methane solubility in aqueous NaCl solutions. The salting-out behavior predicted with the fitted parameters is excellent at room temperature but deviates from experimental data at higher temperatures. Finally, the model is applied to gas solubility in mixed-salt solutions, without further adjustable parameters, and the predictions are in excellent agreement with experiments. Overall, eSAFT-VR Mie shows great potential for gas solubility calculations under salinity.