Phase equilibrium measurements and crystallographic analyses on structure-H type gas hydrate formed from the CH₄-CO₂-neohexane-water system

Phase equilibrium conditions and the crystallographic properties of structure-H type gas hydrates containing various amounts of methane (CH ₄), carbon dioxide (CO₂), neohexane (2,2-dimethylbutane; NH), and liquid water were investigated. When the CH₄ concentration was as high as approximately 70%, the phase equilibrium pressure of the structure-H hydrate, which included NH, was about 1 MPa lower at a given temperature than that of the structure-I hydrate with the same composition (except for a lack of NH). However, as the CO₂ concentration increased, the pressure difference between the structures became smaller and, at CO₂ concentrations below 50%, the phase equilibrium line for the structure-H hydrate crossed that for the structure I. This cross point occurred at a lower temperature at higher CO₂ concentration. Extrapolating this relation between the cross point and the CO₂ concentration to 100% CO₂ suggests that the cross-point temperature would be far below 273.2 K. It is then difficult to form structure-H hydrates in the CO ₂-NH-liquid water system. To examine the structure, guest composition, and formation process of structure-H hydrates at various CH ₄-CO₂ compositions, we used the methods of Raman spectroscopy, X-ray diffraction, and gas chromatography. Raman spectroscopic analyses indicated that the CH₄ molecules were found to occupy both 5¹² and 4³5⁶6³ cages, but they preferably occupied only the 5¹² cages. On the other hand, the CO₂ molecules appeared to be trapped only in the 4³5 ⁶6³ cages. Thus, the CO₂ molecules aided the formation of structure-H hydrates even though they reduced the stability of that structure. This encaged condition of guest molecules was also compared with the theoretical calculations. In the batch-type reactor, this process may cause the fractionation of the remaining vapor composition in the opposite sense as that for CH₄-CO₂ hydrate (structure-I), and thus may result in an alternating formation of structure-H hydrates and structure-I in the same batch-type reactor.