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.