Modeling gas hydrate-containing phase equilibria for carbon dioxide-rich mixtures using an equation of state

Thermodynamic modeling of phase behaviors for CO2-rich mixtures in gas hydrate forming conditions are required for the process design in the field of carbon dioxide sequestration and enhanced oil recovery. With recent experimental data published for solubility of water in CO2–rich mixtures that are significantly different from those previously published, improved modeling studies become necessary for phase equilibria containing gas hydrates. In the present study, an equation of state based on hydrogen-bonding nonrandom lattice fluid theory was applied for both vapor and liquid phases. The model for hydrogen-bonding contribution is simplified and a weak hydrogen bonding between water and carbon dioxide was included for improved calculation of mutual solubility. Hydrate phase was modeled by van der Waals and Platteeuw method but without guest specific parameters other than Kihara potential parameters. The method was applied to single and binary CO2-rich guest mixtures containing methane, ethane, propane, isobutene, nitrogen, hydrogen sulfide and methanol for temperatures above 180 K and pressures below 100 MPa. Results of two- and three-phase equilibrium calculations containing gas hydrates were found to be comparable with those of CSMGem (Sloan and Koh, Clathrate and Hydrates of Natural Gases, 3rd ed., CRC Press, Boca Raton, FL, 2008) in general and better for water contents in liquid carbon dioxide in equilibrium with gas hydrates.