A Helmholtz free energy equation of state for the vapor-liquid equilibrium and PVTx properties of the H2S[sbnd]H2O mixture and its application to the H2S[sbnd]H2O[sbnd]NaCl system

Abstract

An equation of state (EOS) explicit in Helmholtz free energy has been developed to calculate the vapor-liquid equilibrium (VLE) and pressure-volume-temperature-composition (PVTx) properties of the H2SH2O fluid mixture. This EOS, where five mixing parameters are used, is based on the highly accurate EOSs of pure H2S and H2O fluids, and contains a simple departure function. Compared to reliable experimental data available, the average absolute deviations of H2S solubility in liquid phase, water content in vapor phase, and liquid density of the H2SH2O system are 3.88%, 5.03% and 0.20%, respectively. The EOS of the H2SH2O fluid mixture, together with the Pitzer activity coefficient of H2S in aqueous NaCl solution from previous study, can be used to predict the H2S solubility in aqueous NaCl solution with an average absolute deviation of 7.52%. The water content of vapor phase in the H2SH2ONaCl system can be reproduced with the fluid EOS of H2SH2O system by a fugacity-activity method within experimental uncertainties. The fluid EOS of H2SH2O system, combined with the Helmholtz free energy EOS of H2ONaCl fluid mixture, can predict the PVTx properties of the H2SH2ONaCl mixture without using additional mixing parameters. The developed EOS can be safely used under the conditions of CO2 capture and sequestration (273–473 K, 0–400 bar and 0–6 mol kg−1), beyond which the EOS also has some extrapolated ability. The computer codes are in the supplemental data and can be downloaded from Applied Geochemistry or obtained from the corresponding author.