Separation efficiency and equilibrium recovery ratio of SF6 in hydrate-based greenhouse gas separation

Abstract

The feasibility of hydrate-based sulfur hexafluoride (SF6) separation was investigated by primarily focusing on the thermodynamic, kinetic, and structural characteristics of SF6 + N2 hydrates, the separation efficiency, and the equilibrium recovery ratio. Three-phase (hydrate (H)–water (LW)–vapor (V)) equilibria of SF6 + N2 hydrates were measured to examine the effect of guest occupation on their thermodynamic stability. A pressure–composition diagram, which was obtained at 275.15 K, was constructed to elucidate the separation efficiency. The final SF6 compositions in the vapor phase during hydrate formation in isochoric and isobaric conditions showed agreement with the corresponding equilibrium compositions. SF6 + N2 hydrates were identified as sII via powder X-ray diffraction (PXRD). The Rietveld refinement of the PXRD patterns offered quantitative cage occupancy of SF6 and N2 in the SF6 + N2 hydrates. The dissociation enthalpy (ΔHd) of SF6 + N2 hydrates was measured using a high-pressure micro-differential scanning calorimeter (HP μ-DSC). The overall experimental results clearly demonstrated that SF6 was selectively captured in the hydrate phase. The hydrate-based method required a lower initial SF6 concentration and pressure to attain a specified recovery ratio of SF6 compared with the liquefaction method; however, it offered lower SF6 purity. Therefore, the hydrate-liquefaction combined method is suggested to supplement the drawbacks of each method and conserve power consumption for pressurization.