In situ observations of CH4 hydrate dissociation using X-ray diffraction were carried out at atmospheric
pressure and at both 168 and 189 K. Dissociation rates of the hydrate and the rate of transformation into
hexagonal ice were measured using time-resolved energy-dispersive X-ray diffraction. The dissociation of
CH4 hydrate had an initially fast regime followed by slower dissociation. Thus, the data support a previously
suggested two-step process. In addition, we observed dynamic behavior of the X-ray diffraction intensities of
ice Ih, which implies a transient crystal structure at the beginning of the dissociation. Our analyses indicates
that the first step, which lasted several tens of minutes, was the formation of an ice Ih layer around the CH4
hydrate, and the second step was relatively slow because the CH4 had to diffuse through the thickening ice
layer. This second step determined the hydrate lifetime. The resulting diffusion coefficients were estimated
at 2.2 × 10-11 m2/s at 189 K and 9.6 × 10-12 m2/s at 168 K.
Clathrate hydrates of methane-ethane mixed gases have two crystal structures depending on their composition. To study their compositions and cage occupancies and how their structure is determined, we synthesized hydrate samples from methane-ethane mixtures. Analysis of the samples using X-ray diffraction, Raman spectroscopy, and gas chromatography revealed their structures, compositions, and cage occupancies. Experimentally, hydrate structure II existed in samples formed when the gas equilibrated with hydrates was approximately 2% C 2 H 6 (molar fraction) whereas both structures I (sI) and II (sII) coexisted for 12 to 22% C 2 H 6 . The structures below 2% and above 22% of C 2 H 6 existed only as sI hydrates. Volume ratios of both structures were obtained from the ratio of the peak intensities of the C-C stretching peaks in the Raman spectra. In the transition zone containing both structures, the volume ratio of the sII structure gradually decreased with increasing C 2 H 6 concentration. Cage occupancies of guest molecules in the hydrate cages were determined by the relative intensity ratio of Raman spectra. C 2 H 6 molecules occupied only large cages in both structures, whereas CH 4 molecules occupied the remaining cages. The experiments agree with the structure and molecular distributions that were predicted by minimizing the Gibbs free energy of the sample. This model calculation provides insight into the structural transition mechanism.
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