Diffusion of gas mixtures in the sI hydrate structure

Abstract

Replacing methane with carbon dioxide in gas hydrates has been suggested as a way of harvesting methane, while at the same time storing carbon dioxide. Experimental evidence suggests that this process is facilitated if gas mixtures are used instead of pure carbon dioxide. We studied the free energy barriers for diffusion of methane, carbon dioxide, nitrogen, and hydrogen in the sI hydrate structure using molecular simulation techniques. Cage hops between neighboring cages were considered with and without a water vacancy and with a potential inclusion of an additional gas molecule in either the initial or final cage. Our results give little evidence for enhanced methane and carbon dioxide diffusion if nitrogen is present as well. However, the inclusion of hydrogen seems to have a substantial effect as it diffuses rapidly and can easily enter occupied cages, which reduces the barriers of diffusion for the gas molecules that co-occupy a cage with hydrogen.

Insights into multistep depressurization of CH4/CO2 mixed hydrates in unconsolidated sediments

“…2. For CH 4 /CO 2 hydrates formed as a result of hydrate swapping, CO 2 gas diffusion dominated mass transfer through outer CO 2 hydrate films and inner CH 4 hydrates [49,50]. Laboratory-scale experiments on CO 2 injection into CH 4 hydrates would trigger inlet blockage in that CO 2 hydrate formation and thus cause low CH 4 /CO 2 hydrate saturation and heterogeneous hydrate distribution [44].…”

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

“…According to simulation data, small cages are preferably occupied by CH 4 ; large cages do not show any preference for either of the gases, except for high pressures (100 bar or more), for which preferential occupancy by CH 4 molecules was observed. Finally, the energy barriers of gas diffusion through rings of water between the cages of sI hydrate were calculated [27]. Simulation data suggest that nitrogen has more possibilities than CO 2 to diffuse into the large cages, which are already occupied by methane, though this effect is relatively weak.…”

Effect of Injecting N2/H2 on Methane Hydrate Decomposition

“…Sun et al proposed a combined exploitation technology by integrating CO 2 /H 2 injection and CH 4 production with in situ steam reformation of CH 4 , in which the involved separation of CO2/H2 is much easier than that of CH 4 /N 2 [7,8]. Waage et al studied the diffusion of gas mixtures in the sI hydrate using molecules simulation techniques [9]. They suggested that using a mixture of CO 2 /H 2 as injection gases could boost the bulk diffusivities of both CH 4 and CO 2 in sI hydrate.…”