Self-preservation and structural transition of gas hydrates during dissociation below the ice point: an in situ study using Raman spectroscopy

Abstract

The hydrate structure type and dissociation behavior for pure methane and methane-ethane hydrates at temperatures below the ice point and atmospheric pressure were investigated using in situ Raman spectroscopic analysis. The self-preservation effect of sI methane hydrate is significant at lower temperatures (268.15 to 270.15 K), as determined by the stable C-H region Raman peaks and A_L/A_S value (Ratio of total peak area corresponding to occupancies of guest molecules in large cavities to small cavities) being around 3.0. However, it was reduced at higher temperatures (271.15 K and 272.15 K), as shown from the dramatic change in Raman spectra and fluctuations in A_L/A_S values. The self-preservation effect for methane-ethane double hydrate is observed at temperatures lower than 271.15 K. The structure transition from sI to sII occurred during the methane-ethane hydrate decomposition process, which was clearly identified by the shift in peak positions and the change in relative peak intensities at temperatures from 269.15 K to 271.15 K. Further investigation shows that the selectivity for self-preservation of methane over ethane leads to the structure transition; this kind of selectivity increases with decreasing temperature. This work provides new insight into the kinetic behavior of hydrate dissociation below the ice point.

Figure 1

Raman spectra of the C-H region for methane hydrate obtained at different times during hydrate dissociation at (a) 272.15 K, (b) 271.15 K, (c) 270.15 K, (d) 269.15 K, and (e) 268.15 K respectively and atmospheric pressure.

Figure 2. Raman spectra of the C-H region for methane-ethane double hydrate obtained at different times during hydrate dissociation at 272.15 K and atmospheric pressure.

It should be noted that a large amount of spectra were collected on different locations of the hydrate sample. Only representative spectra are given in the figure to show the variation of Raman spectra with time, which also applies to Figs 3, 4, 5, 6.

Figure 3. Raman spectra of the C-H region for methane-ethane hydrate obtained at different times during hydrate dissociation at 271.15 K and atmospheric pressure.

Figure 4. Raman spectra of the C-H region for methane-ethane hydrate obtained at different times during hydrate dissociation at 270.15 K and atmospheric pressure (The structure transition from sI to sII occurred at around 93 min).

Figure 5. Raman spectra of the C-H region for methane-ethane hydrate obtained at different times during hydrate dissociation at 269.15 K and atmospheric pressure.

Figure 6. Raman spectra of the C-H region for methane-ethane hydrate obtained at different times during hydrate dissociation at 268.15 K and atmospheric pressure.

Figure 7. The variation of A_L/A_S values with time for methane hydrate during dissociation at different temperatures.

Figure 8. The variation of A_L/A_S values with the elapsed decomposition time for methane-ethane double hydrate at different temperatures.

Figure 9. The variation of the / values with the elapsed decomposition time for methane-ethane double hydrate at different temperatures.

Figure 10. Schematic diagram for the dissociation process of methane-ethane double hydrate with the elapsed time.

Figure 11. Schematic diagram of the experimental apparatus.

Figure 12. View of the HPOC.