Investigation on the Structure, Formation Mechanism, and Surface Morphology of CP–CH₄ Binary Hydrate

Abstract

Cyclopentane (CP) is often used as a hydrate former, which shows an excellent promotion effect in hydrate-based desalination. To elucidate the guest molecule distribution and microcosmic mechanism during hydrate formation, the in situ Raman spectroscopy of CP−methane binary hydrates is performed. The Raman spectra for the intramolecular vibration mode of host and guest molecules reveal that sII CP−methane hydrate formed with CP and CH 4 encaged in large and small cages, respectively. But the small cages were not completely filling in. Because of the structure of CP with a five-membered ring, hydrogen-bonded water molecules arranged around CP forming a metastable structure, which accelerates the nucleation rate of hydrate. The peak O−H stretching vibration (R OH−H = I 3170 /I 3400 ) in hydrate phase is an indication of hydrogen-bond ordering. At 3.5 MPa, the CH 4 occupancy in the small cages was 0.9978 and the CP occupancy in the small cages was 0.1751. According to cage occupancies and the thermodynamic model based on van der Waals−Platteeuw, the calculated hydration number was 12.6. Cryo-scanning electron microscopy was applied to observe the surface morphology of hydrate. Energy-dispersive spectroscopy was used to distinguish hydrate from ice by identifying element C.

Review and Outlook of Visual and Geophysical Detections for the Pore-Scale Morphology of Gas Hydrates

“…Scanning electron microscopy (SEM) uses the signals (secondary electrons, backscattered electrons, or X-rays) generated by the interactions between focused high-energy electron beams and materials to characterize the surface morphology of mineral grains or hydrates . Although SEM focuses only on the sample’s surface and provides only 2D images, several studies have used its high-resolution features to display fine surface morphology of different gas hydrates (e.g., methane, isobutane, carbon dioxide, and argon). − However, SEM images describing the hydrate-grain contact relationship are rarely reported. ,,,,− Recently, Sun et al studied the surface morphology of GHBS from the laboratory and the South China Sea. Both the synthesized and natural methane hydrates appear to be grain contacting, but it is hard to determine whether they are frame-supporting or grain-cementing hydrates (Figure ).…”