Hydrate Phase Equilibrium of CH4+N2+CO2 Gas Mixtures and Cage Occupancy Behaviors

Methane, nitrogen, and carbon dioxide coexist after the injection of CO2/N2 mixture into CH4 hydrate. For a better understanding of CH4 recovery and CO2 sequestration by gas exchange involving N2, the hydrate phase equilibrium of CH4/N2/CO2 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 CH4 and the ratio of N2/CO2. The cage occupancies of CH4, N2, and CO2 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 CO2 molecules in large cavities.