In-situ Raman and kinetic study on the methane hydrate formation and decomposition

Experiments of CH4 hydrate formation and decomposition were carried out in a semi-batch stirred reactor under constant temperature and pressure. In this study, we report microscopic observations such as the preferential enclathration of guest molecules by coupling an in-situ Raman spectroscopy at agitating condition in the stirred reactor. Simultaneously, the macroscopic observations (gas uptake or releasing measurements) were also reported under the same conditions. It was observed that at the beginning of hydrate growth phase, the Raman peak intensity of methane occupying the large cage (51262) and small cage (512) gradually increased after abrupt decline of dissolved methane. The encapsulation rate of large cages was faster than that of small cage throughout the hydrate formation process. In the decomposition step, the large and small cage dissociated gradually, but the dissociate rates (the ratio of Raman peak intensities of large and small cages with respect to time) were similar at the initial stage of decomposition, then large cages were dissociated more rapidly. In all experiments, we found that the Raman intensity at 3200 cm-1 is more increased along with the higher content of free water, which might will provide valuable information for further hydrogen-bonded researches including hydrate study. With the aid of in-situ Raman spectroscopy, this study successfully observed the process of hydrate formation/decomposition and further behavior of host and guest molecules under the agitated systems.