Mechanism and kinetics of guest exchange in sII hydrate - Flue gas replacement as revealed by experimental and computational approaches for hydrocarbon recovery and CO2 sequestration

In this study, the kinetics of guest exchange in sII (CH4 (90%) + C3H8 (10%)) hydrate - flue gas (CO2 (20%) + N-2 (80%)) replacement was investigated to elucidate the replacement mechanism and guest exchange characteristics for hydrocarbon recovery and CO2 sequestration. The Rietveld refinement of Powder X-ray diffraction (PXRD) patterns of the replaced hydrates at two pressures (11.0 MPa and 17.0 MPa) demonstrated that sII hydrate - flue gas replacement occurred iso-structurally. The time-dependent guest compositions in the hydrate phase and the cage occupancy of guest molecules were examined to understand the effect of pressure on guest exchange behaviors. The increased CO2 inclusion in both the small (5(12)) and large (5(12)6(4)) cages at a higher pressure resulted in the larger production of CH4 and C3H8 and a lower N-2/CO2 ratio in the hydrate phase. The slower inclusion rate and lower N-2 occupancy in the small (5(12)) cages at a higher pressure during replacement were observed due to the competitive inclusion of CO2 and N-2 in the small (5(12)) cages. The molecular dynamics (MD) simulation demonstrated that the expansion or shrinkage of hydrate cages caused by guest exchange has a significant impact on the lattice parameter of the replaced hydrates and it was influenced by not only the average molecular size of guest molecules in each cage but also the dispersive movement of the guest molecules in each cage.