Characterization of clathrate hydrate formed in H₂ + CO₂ + tetrahydropyran + water system as carbon capture materials

This paper reports the thermodynamic and crystallographic characterization of the clathrate hydrate formed in H₂ + CO₂ + tetrahydropyran (THP) + water system by phase equilibrium measurements and powder X-ray diffraction measurements with a goal of developing a hydrate-based carbon capture technology in an integrated coal gasification combined cycle. The four-phase [(H₂ + CO₂) gas + water liquid + THP liquid + hydrate] equilibrium conditions were measured with two series of gas compositions (around H₂:CO₂ = 0.7:0.3 and 0.9:0.1 in mole fractions) in the range of 2.64 MPa to 7.51 MPa. At around H₂:CO₂ = 0.7:0.3 in gas phase, the equilibrium temperatures in (H₂ + CO₂) gas + THP + water system are always higher by approximately 9 to 10 K compared to the equilibrium temperatures of H₂ + CO₂ + water system. This means that THP acts as the hydrate thermodynamic promoter. The equilibrium conditions in (H₂ + CO₂) gas + THP + water system, which is only considered with CO₂ partial pressure, greatly agree with those in CO₂ + THP + water system. Therefore, the equilibrium conditions in (H₂ + CO₂) gas + THP + water system would only depend on the partial pressure of CO₂. Moreover, the crystallographic structure of the hydrate formed in H₂ + CO₂ + THP + water system was identified to be structure II, which is the same structure as the hydrate formed in H₂ + CO₂ + water system. These results can contribute to the study of a hydrate-based gas separation and carbon capture.