Microscopic measurements on methane hydrate dissociation

The decomposition process of methane hydrate was measured by laser Raman and X-ray powder diffraction (PXRD) at 253 K under normal pressure. The results showed that the methane hydrates at the surface dissociated into Ⅰh ice in the initial 30-50 min and then the ice film covered the hydrate phase which triggered the "self-preservation" effect and finally led to a dramatic decrease in dissociation rate of methane hydrate. During the dissociation, the ratio of methane content in large and small hydrate cages obtained from Raman spectra remained stable at around 3.2 which was generally in accord with the ratio of large to small cages in methane hydrate, while the characteristic peaks of hydrate lattice planes in the powder X-ray diffraction patterns decreased in the same profile, suggesting that the methane hydrate dissociated as a whole crystal unit. However, the characteristic peaks of Ⅰh ice lattice planes increased in different ways. The intensity of (002) plane was found to increase linearly up to 370% of its original value in 60 min while the intensity of (100) plane kept steady at about 200% of its original value after the first 20 min, indicating that the ice inclined to grow into horizontal plates instead of columnar growth. Combining with the transport characteristics of water molecules on the ice surface under low temperature, the growth of ice film covered on the inner hydrate cores was suggested to cultivate the "self-preservation" effect. © All Right Reserved.