Phase Equilibrium in Two-Phase, Water-Rich-Liquid, Hydrate Systems:  Experiment and Theory

Two-phase equilibrium between CO2 hydrate (H) and a water-rich liquid (L) are experimentally measured and theoretically described between 273 and 281 K, at pressures below 30 MPa, and at aqueous CO2 concentrations between 0.0163 and 0.0242 mole fraction. These data represent the conditions where hydrates form from a single-phase aqueous solution of fixed composition. Both theoretical and experimental results indicate that the equilibrium pressure is very sensitive to concentration at all temperatures. The concentrations reported represent the solubility of CO2 in a water phase in equilibrium with hydrate at the given temperature and pressure. When a constant aqueous composition LH curve is extrapolated to the three-phase VLH curve, the composition characterizing the LH curve also represents the solubility of CO2 in water at the VLH conditions. Since the solubility of CO2 in water at hydrate-forming conditions is difficult to obtain, this method provides an excellent way of indirectly measuring this three-phase solubility. The effect of salinity on hydrate formation from water-rich-liquid systems was also studied. A modified model was introduced to describe the experimental results and produced good agreement between calculated and experimental pressures. A simplified version of the model can provide quick and reasonable estimations of the equilibrium conditions of hydrates at low concentrations and medium to low pressures. Interestingly, the increase of salt increases the maximum temperature at which hydrates are stable for a constant pressure and constant composition system. This is because the salt increases the chemical potential of the dissolved gases, which more than offsets the reduction in the chemical potential of the liquid water. The model can also be used for prediction of LH equilibrium for other gas hydrates. An example is given for methane hydrate at three different concentrations of methane in water.