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| - Phase Equilibrium in Two-Phase, Water-Rich-Liquid, Hydrate Systems: Experiment and Theory
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| - Two-phase equilibrium between CO2 hydrate (H) and a water-rich liquid (L) are experimentally measuredand theoretically described between 273 and 281 K, at pressures below 30 MPa, and at aqueous CO2concentrations between 0.0163 and 0.0242 mole fraction. These data represent the conditions where hydratesform from a single-phase aqueous solution of fixed composition. Both theoretical and experimental resultsindicate that the equilibrium pressure is very sensitive to concentration at all temperatures. The concentrationsreported represent the solubility of CO2 in a water phase in equilibrium with hydrate at the given temperatureand 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 VLHconditions. Since the solubility of CO2 in water at hydrate-forming conditions is difficult to obtain, this methodprovides an excellent way of indirectly measuring this three-phase solubility. The effect of salinity on hydrateformation from water-rich-liquid systems was also studied. A modified model was introduced to describe theexperimental results and produced good agreement between calculated and experimental pressures. A simplifiedversion of the model can provide quick and reasonable estimations of the equilibrium conditions of hydratesat low concentrations and medium to low pressures. Interestingly, the increase of salt increases the maximumtemperature at which hydrates are stable for a constant pressure and constant composition system. This isbecause the salt increases the chemical potential of the dissolved gases, which more than offsets the reductionin the chemical potential of the liquid water. The model can also be used for prediction of LH equilibriumfor other gas hydrates. An example is given for methane hydrate at three different concentrations of methanein water.
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