Toward a Possibility To Exchange CO sub(2) and CH sub(4) in sI Clathrate Hydrates

We study the thermodynamic conditions for exchange of CH sub(4) with CO sub(2) in sI clathrate hydrates using Grand Canonical Monte Carlo simulations. From the variations in the Helmholtz energy, we suggest a thermodynamic path for exchange of CH sub(4) by CO sub(2). The results can be understood from single-component occupancy isotherms. Simulations of pure component systems show that all methane sites are essentially equivalent, whereas carbon dioxide distinguishes between two types of sites, large or small. Mixture of CO sub(2) and CH sub(4) in the clathrate can be regarded as ideal, as long as only the large sites are occupied. A strong preference in selectivity is demonstrated for methane when the smaller sites become filled. The Helmholtz energies of the hydrate with a CO sub(2)-CH sub(4) gas mixture for temperatures between 278 and 328 K and pressures between 10 super(4) and 10 super(9) Pa indicate that there exists a region of stability of a mixed hydrate referred to single-component hydrates.