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

• Published in: Scientific Reports
• Main Authors: Zhong, Jin-Rong, Zeng, Xin-Yang, Zhou, Feng-He, Ran, Qi-Dong, Sun, Chang-Yu, Zhong, Rui-Qin, Yang, Lan-Ying, Chen, Guang-Jin, Koh, Carolyn A.
• Format: Text
• Language: English
• Published: Nature Publishing Group 2016
• Subjects: Article; Ice Point; Methane hydrate
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5150642/; http://www.ncbi.nlm.nih.gov/pubmed/27941857; https://doi.org/10.1038/srep38855

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 AL/AS 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 AL/AS 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.