Measurement and prediction of phase equilibria for water+CO2 in hydrate forming conditions

CO2 solubility in H–LW equilibria for which no reliable solubility data are available in the literature were measured together with H–LW–VCO2 and H–LW–LCO2 equilibria for water–CO2–hydrate. The pressure range was from 2 to 20 MPa and the temperature range was from 273 to 285 K. Equations of state have been applied to three-phase equilibria and to gas-rich phase in two-phase equilibria by other investigators. However, they have not been applied to H–LW equilibria so far. In this study applicability of the lattice fluid equation of state was investigated for the unified description of various phase equilibria. With Langmuir constants in the Van der Waals and Platteeuw model for hydrates and hydrogen-bonding free energy of water fitted to data, the method was found to consistently describe various two- and three-phase equilibria of LW–VCO2, LW–LCO2, LW–VCO2–LCO2, H–LW–VCO2/LCO2, H–I–VCO2, H–VCO2–LCO2, H–LW, and H–VCO2/LCO2. With a single binary interaction parameter, good agreements between observed and calculated results were obtained except for liquid composition in CO2-rich phases for H–LCO2 and LW–VCO2–LCO2 equilibria in the vicinity of CO2 vapor pressure.