Characterization of clathrate hydrates formed with CH₄ or CO₂ plus tetrahydropyran

This paper reports the characterization of clathrate hydrates containing CH₄ or CO₂ and tetrahydropyran (THP) by means of phase equilibrium and powder X-ray diffraction (PXRD) measurements toward hydrate-based gas separation technology. The equilibrium conditions were measured by isochoric method in the range of temperature from 281.0 to 294.8 K and pressure from 0.54 to 3.81 MPa. The presence of THP increases the phase equilibrium temperatures of CH₄ or CO₂ hydrates at least 8 K at a given pressure. Crystallographic structure of these hydrate were identified to be structure II by the PXRD measurements, and the cage occupancy of CH₄ or CO₂ were analyzed using the Rietveld method. The PXRD data as well as phase equilibrium conditions showed that CH₄ was more incorporated into hydrate than CO₂ in THP hydrate systems. From these results, the relations among the equilibrium conditions, lattice constants and cage occupancy are discussed in terms of gas separation performance.