Online Observation of Structure Information of Mixed-Gas Hydrate Containing High Concentration of CO2 Based on Raman Spectroscopy

ABSTRACT The research on the micro crystal structure of coal mine gas (CMG) hydrate is especially significant for the technology of gas hydrate separation. Raman spectroscopy is an effective method for on-line analysis of gas hydrate microstructure. Using Raman spectroscopy to observe the microstructure of mixed-gas hydrate crystals formed by three kinds of gas samples under two driving forces, which contain a high concentration of carbon dioxide. This experiment obtained information about the hydrate crystals, including large and small cage occupancy, and obtained the hydration number indirectly based on the statistical thermodynamic model of van der Waals and Platteeuw. The results show that the large cages of hydrate phases are nearly fully occupied by carbon dioxide and methane molecules together, with the occupancy ratios between 97.70% and 98.70%. Most of the guest molecules in large cages are carbon dioxide (75.82% ∼ 94.00%) and only a few are filled with methane (4.70% ∼ 31.50%). However, the small cage occupancy ratios are generally low, in the range from 18.28% to 66.06%, and the guest molecules are all methane. The cage occupancy, both large and small, which methane occupied, increased as the methane concentration in the gas sample increased. However, the large cage occupancy, which methane occupied, is lower than the small cage. In addition, the hydration number of three kinds of gas samples in different systems is from 6.38 to 7.32. The occupancy of small cavities increased as the methane concentration in the coal mine gas sample increased, while the hydration number decreased.