Experimental measurement and thermodynamic modeling of dissociation conditions of hydrogen sulfide hydrate in the presence of electrolyte solutions

Abstract Understanding of hydrogen sulfide (H2S) hydrates phase equilibrium is important for sour gas production and transportation. In this study, hydrate dissociation conditions of H2S were measured in aqueous NaCl, KCl, MgCl2, NH4Cl, Na2SO4, and K2SO4 solutions at temperatures ranging from 273.4 to 298.8 K, pressures ranging from 1.13 to 21.90 bar, and weight percent of electrolyte ranging from 3.4 to 20.0 wt%. The experimental measurements were conducted in microcapillary silica tubes using an isothermal pressure search method. The dissociation enthalpies of H2S hydrate in electrolyte solutions were calculated by fitting the Clausius-Clapeyron equation to LHV (Liquid-Hydrate-Vapor) equilibrium curves. The result shows that the dissociation enthalpies of H2S hydrate in electrolyte solutions range from 57.7 to 71.5 kJ/mol. The inhibitory effect of six electrolytes on H2S hydrate formation is in the order of MgCl2 > NH4Cl > NaCl > KCl > Na2SO4 = K2SO4. Additionally, a thermodynamic model was developed to predict dissociation conditions of H2S hydrate in electrolyte solutions. The van der Waals-Platteeuw (vdW-P) model was used to describe the hydrate phase, and the liquid phase was modeled with the thermodynamic equation improved by Holder et al. The Duan-Moller-Weare-92 equation of state (DMW-92 EoS) and the Pitzer model were used to calculate fugacity and water activity, respectively. The average absolute deviations of pressure (AADP) indicate that the developed model can accurately predict the dissociation conditions of H2S hydrate for aqueous NaCl, KCl, Na2SO4, and K2SO4 solutions. Moreover, the valence band of hydroxyl group was used to characterize electrolytes effects on gas hydrate equilibrium conditions.