Kinetic Selectivity of SF₆ during Formation and Dissociation of SF₆ + N₂ Hydrates and Its Significance in Hydrate-Based Greenhouse Gas Separation

Abstract

The time-dependent selectivity of SF6, the most potent global warming gas, in the hydrate-based gas separation process was investigated through both experimental and computational approaches. Guest-enclathrating and guest-releasing behaviors in SF6 + N2 hydrate were observed via gas chromatography, in situ Raman spectroscopy, and molecular dynamics (MD) simulations. The increasing pattern of the normalized area ratio of the Raman peak for enclathrated SF6 molecules was similar to that for enclathrated N2 during hydrate formation, and the composition of SF6 in the hydrate phase was almost constant throughout hydrate formation. MD simulations also showed that the captured SF6/N2 ratio in the hydrate structure was nearly constant over time. These results evidenced no remarkable difference in kinetic selectivity between SF6 and N2 during hydrate formation. The in situ Raman spectra and MD simulations examined during hydrate dissociation also demonstrated that SF6 was not kinetically selective in the guest-releasing process. The overall experimental and computational results indicated that none of the guest molecules in the SF6 + N2 hydrate were kinetically selective during formation and dissociation despite the superior thermodynamic selectivity of SF6. The findings of this work provide the features of guest-filling and guest-liberating behaviors during the formation and dissociation of SF6 + N2 hydrate. They will contribute to the determination of the optimal operation time for hydrate formation and thus to the development of the hydrate-based SF6 separation process.