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

Abstract

In this study, the guest exchange behavior in the sII (CH4 + C3H8) hydrate – (CO2 + N2) replacement at various CO2 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 CO2 concentrations (%CO2) at a higher partial pressure of CO2 (PCO2) 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 CO2 and N2. 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 CO2/N2 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 CO2 sequestration through guest replacement in natural gas hydrates.