In Situ Raman Spectroscopic Studies on Small-Cage Occupancy of Methane in the Simple Methane and Methane plus Deuterated Tetrahydrofuran Mixed Hydrates

To investigate the small-cage occupancies of methane (CH4) in the structure-I and structure-II hydrates, under the three-phase equilibrium conditions of dathrate hydrate, aqueous solution, and fluid CH4 phases, in situ Raman spectra of the guest molecules in the simple CH4 hydrate and the CH4 + deuterated tetrahydrofuran (THF-d(8)) mixed hydrate with the stoichiometric THF-d(8) composition were measured. The three-phase equilibrium conditions of CH4 + THF-d(8) mixed hydrate phase, THF-d(8) aqueous solution phase, and fluid CH4 phase were measured in a temperature range of (278 to 294) K and pressure range up to 2.6 MPa. The small-cage occupancy of CH4 in the structure-II CH4 + THF-d(8) hydrate increases with increasing pressure (and also temperature) up to 3 MPa. At pressures higher than 3 MPa, the small-cages of structure-II hydrate are mostly filled up with the CH4 molecules. In the simple CH4 hydrate, the small-cage occupancy is almost independent of the pressures of (5 to 60) MPa measured in the present study. The three-phase equilibrium pressures at which the small cages are filled up with the CH4 molecules would be lower than ever expected by the statistic-thermodynamic model.