Measurements and Predictions of Hydrate-Containing Phase Equilibria for Co2, Methane and Ethane in the Presence of Nacl

Phase equilibria involving gas hydrates are of importance in natural gas recovery and carbon dioxide sequestration. Due to high equilibrium pressures twoand three-phase equilibria for gas hydrates containing systems have been computed using equations of state and van der Waals and Platteeuw model for fugacity of guest components in gas hydrates. So far the inhibition effect has been accounted for by excess Gibbs function models, which are less appropriate than an equation of state for high-pressure applications. In the present study ethane and carbon dioxide solubilities in hydrate-liquid water equilibria containing sodium chloride were measured. Carbon dioxide concentrations in the aqueous phase were determined by expanding dissolved gas from the external sampling loop. Ethane solubilities in the aqueous phase were measured by an indirect method due to very small solubility. The presence of the salt were found to show the inhibition effect that increases equilibrium pressure in three-phase equilibria and the salting-in effect that increases gas solubilities in two-phase equilibria. Phase equilibria of measured and literature data for the systems were predicted using the electrolyte lattice fluid equation of state, in which long-range electrostatic interactions were modeled using the mean spherical approximation. The present model was found to predict various phase equilibria including inhibition and salting-in effects.