Hydrate Stability in the H₂S–H₂O systemVisual Observations and Measurements in a High-Pressure Optical Cell and Thermodynamic Models

A visual method was developed to investigate the equilibrium phase relations of hydrate in the hydrogen sulfide (H₂S)–H₂O system using a high-pressure optical cell coupled with a heating–cooling stage under a microscope. The equilibrium pressure (P)–temperature (T) phase boundaries for the assemblages (1) hydrate (H)–liquid water (Lw)–vapor H₂S [H₂S (g)] and (2) H–Lw–liquid H₂S [H₂S (l)] were determined over a wide P–T range (0.0986 to 66.128 MPa and 272.4 to 307.7 K, respectively). In this study, a smaller visual cell was used, which has four advantages: (1) the phase transition process in a high-pressure optical cell under a microscope at various well–controlled P–T conditions can be fully observed; (2) the experimental run time is considerably reduced owing to the small experimental device, and <1 h is required to collect each equilibrium datum; (3) the experiment was sufficiently safe because the high-pressure optical cell contains a small amount of H₂S; and (4) the high-pressure optical cell is suitable for studying sample fluids at temperatures up to 873.15 K and pressures up to 100 MPa; the measurement range of P and T being far beyond that of previous studies. The van der Waals–Platteeuw model was used to calculate the equilibrium P–T lines for these two three-phase assemblages. To achieve more accurate results, the Kihara potential parameters were optimized using the improved values of H₂S fugacity and its solubility in pure water, calculated from high-quality thermodynamic models, and the experimental data measured in this and previous studies. The average absolute deviations of the new model from the experimental data of this study are 1.1% and 5.8% for the H–Lw–H₂S (g) and H–Lw–H₂S (l) phase boundaries, respectively.