Influence of structural transformation on guest exchange behavior in the sII hydrate – (CO2 + N2) replacement for energy recovery and CO2 sequestration.

[Display omitted] • The guest exchange in the sII (CH 4 + C 3 H 8) hydrate – (CO 2 + N 2) replacement was investigated. • CO 2 + N 2 mixtures with varying compositions and partial pressures of CO 2 were injected. • The extent of replacement and structural transition increased as the CO 2 composition and partial pressure increased. • The newly formed sI hydrates mainly consisted of CO 2 and N 2. • The guest exchange behavior in the sII small cages significantly affected the CO 2 /N 2 ratio in the replaced hydrates. In this study, the guest exchange behavior in the sII (CH 4 + C 3 H 8) hydrate – (CO 2 + N 2) replacement at various CO 2 concentrations was experimentally investigated to elucidate the influence of the structural transformation of the initial sII hydrate on the replacement efficiency at different feed gas compositions and injecting pressures. The extent of replacement, structural identification, and cage occupancy of guest molecules in the replaced hydrates were determined by a combination of gas chromatography (GC), powder X-ray diffraction (PXRD), and 13C NMR spectroscopy. The experimental results demonstrated that injecting feed gas with higher CO 2 concentrations (% CO2) at a higher partial pressure of CO 2 (P CO2) resulted in a higher weight fraction of sI hydrates (a greater degree of structural transformation from sII to sI) after replacement and consequently, higher replacement efficiency. The newly formed sI hydrates after replacement were primarily composed of CO 2 and N 2. The guest-inclusion behavior in the small (512) cages of the sII hydrates after replacement, as revealed by Rietveld refinement of the PXRD patterns, had a dominant influence on the total CO 2 /N 2 ratios in the replaced hydrates. The findings of this study offer valuable insights into the guest exchange mechanism occurring in sII hydrates during flue gas injection and can aid in estimating the optimal compositions of flue gas for energy recovery and CO 2 sequestration through guest replacement in natural gas hydrates. [ABSTRACT FROM AUTHOR]