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

Abstract

Methane, nitrogen, and carbon dioxide coexist after the injection of CO 2 /N 2 mixture into CH 4 hydrate. For a better understanding of CH 4 recovery and CO 2 sequestration by gas exchange involving N 2 , the hydrate phase equilibrium of CH 4 /N 2 /CO 2 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 4 and the ratio of N 2 /CO 2 . The cage occupancies of CH 4 , N 2 , and CO 2 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 2 molecules in large cavities.

Experimental Study on the Dissociation Equilibrium of (CH4 + CO2 + N2) Hydrates in the Mixed Sediments

“…By analyzing the measured data, it was found that the hydrate dissociation equilibrium pressure at a given temperature shows a decreasing trend as the CH 4 content increases at a lower CO 2 /N 2 ratio (M 1 –M 3 ), which was opposite to that at a higher CO 2 /N 2 ratio (M 4 –M 6 ). Li et al investigated the dissociation condition of (CH 4 + CO 2 + N 2 ) hydrates in pure water system, and the variation trend was also observed in their results. In addition, as can be seen from Figure , the hydrate phase boundaries of M 4 and M 5 (which has the same CO 2 /N 2 ratio but different CH 4 contents) are very close to each other in the lower-temperature region while they intersected at a higher temperature point (279 K).…”

“…The results demonstrated that (CO 2 + CH 4 + N 2 ) can form coexistence of sI and sII hydrates in the presence of acetone. Li et al tested the hydrate equilibrium condition of (CO 2 + CH 4 + N 2 ) in pure water systems and then used the Chen–Guo model and CSMHYD program to calculate the cage occupancies. They believed that the hydrate dissociation enthalpy increases with the increase of CO 2 molecule occupancy in large cavities.…”

Clathrate hydrates: recent advances on CH4 and CO2 hydrates, and possible new frontiers

“…43 Many studies, including field campaigns, continue to be conducted to determine the optimal operation conditions (T, P, addition of liquid vs. gaseous CO2) to enhance CH4 removal from hydrates. [44][45][46][47][48][49][50] Because it is a very aggressive hydrate former, H2S is proposed as an additive for this application, 48,51 and it could help improving the kinetics of the CO2/CH4 swap process.…”

Molecular Simulations of CO2/CH4, CO2/N2 and N2/CH4 Binary Mixed Hydrates

“…NMR studies frequently have problems detecting signals from CO 2 in small cages, which also suggests that carbon dioxide tends to occupy large cages [16,17]. In ternary CH 4 /CO 2 /N 2 hydrates nitrogen prevails over the two other gases in the competition for the filling small cages, while CO 2 does the same for large cages [18]. Methane occupancy in small cages was shown to be higher than in the large ones [19].…”