Simultaneous in-situ macro and microscopic observation of CH4 hydrate formation/decomposition and solubility behavior using Raman spectroscopy

In-depth understanding of the CH4 hydrate formation and decomposition processes is essential for energy recovery as well as related transportation/storage technologies. Raman spectroscopy is well suited for hydrate studies of aqueous-hydrate conditions, because it is not susceptible to interference from aqueous environments. In order to obtain more specific and ongoing information on the CH4 hydrate formation and dissociation, two different in-situ Raman spectroscopies with specially designed fiber-optic probes were used in a semi-batch stirred reactor (total volume 365 mL). With these arrangements, it was possible to obtain macroscopic (gas uptake, releasing, CH4 solubility) and microscopic (structural, cage occupancy, water structure) data simultaneously during CH4 hydrate formation and decomposition. Based on in-situ Raman observations, we are able to determine the time-dependent encapsulation of CH4 in large and small cage through hydrate formation and decomposition. In all experiments, the Raman intensity of the OH-stretching band (3000-3800 cm(-1)) was significantly reduced or increased at the point where hydrates were formed or dissociated, which is expected to provide valuable information for further hydrogen-bond research including hydrate study. In addition, in-situ Raman spectroscopy was used to determine the concentration of dissolved CH4 under hydrate formation/decomposition conditions and the results were in good agreement with experimental data in the literature.