Hydrate Phase Equilibrium of CH<sub>4</sub>+N<sub>2</sub>+CO<sub>2</sub> Gas Mixtures and Cage Occupancy Behaviors

Methane, nitrogen, and carbon dioxide coexist after the injection of CO<sub>2</sub>/N<sub>2</sub> mixture into CH<sub>4</sub> hydrate. For a better understanding of CH<sub>4</sub> recovery and CO<sub>2</sub> sequestration by gas exchange involving N<sub>2</sub>, the hydrate phase equilibrium of CH<sub>4</sub>/N<sub>2</sub>/CO<sub>2</sub> ternary gas mixtures in pure water system was determined in a sapphire cell using the pressure-search method under isothermal conditions. The hydrate equilibrium data were analyzed with respect to the concentration of CH<sub>4</sub> and the ratio of N<sub>2</sub>/CO<sub>2</sub>. The cage occupancies of CH<sub>4</sub>, N<sub>2</sub>, and CO<sub>2</sub> were calculated by Chen–Guo hydrate model and CSMHYD program. The result revealed that the cage-specific guest distributions and preferential partitioning of guest molecules in cages were involved in the formation of mixed gas hydrates. The enthalpy of the ternary hydrates was calculated using the Clausius–Clapeyron equation. It was deduced that the enthalpy of the mixed hydrates was changed with the occupancy of CO<sub>2</sub> molecules in large cavities.