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Gas hydrates have attracted substantial attention in energy and environmental research as a medium for energy storage and transport, gas separation, and carbon dioxide sequestration. However, the f...

Clathrate hydrates are ubiquitous in deep-sea sediments and the permafrost on earth, as well as abundant in interstellar environments. Despite many potential applications of gas hydrates for gas storage/transportation, carbon sequestration, and water treatment, the fundamental mechanism for gas hydrate nucleation and growth are still poorly understood due to the difficulty in spatial and temporal measurements that can probe specific structural properties. Here, high resolution measurements for xenon hydrate formation from ice spheres by X-ray computed tomography are reported. For the first time, the nucleation and growth of hydrates can be accurately and directly quantified from microscale measurements, resulting in new estimates for the intrinsic reaction rate constant of xenon hydrate formation, and the effective diffusion coefficient of xenon in the hydrate. The measured activation energy of hydrate formation is 71.11 kJ/mol and the diffusivity of xenon in hydrate ranges from 2.8 × 10–15 to 4.3 × 10–14 m2/s. These results are of fundamental value in developing a comprehensive understanding on the mechanism of gas hydrate formation, which is essential in their application for energy solutions and increasingly important in astrophysical science.

Gas hydrates have gained increasing attention in energy and environmental fields and have potential applications in gas storage and transport, carbon dioxide sequestration, and flow assurance. Nevertheless, the kinetics of hydrate formation are still not well understood. In situ high-resolution X-ray computed tomography measurements were performed to monitor xenon hydrate formation on water droplets and ice spheres. For the first time, three-dimensional thickness meshes were used to quantify and visualize the kinetics of hydrate formation. The evolution of the hydrate morphology was investigated, and the time-dependent kinetic parameters were obtained, including the hydrate shell thickness and inner and outer diameters and the effective volume fraction of hydrate particles. The results indicate that the formation of gas hydrates undergoes an initial reaction-controlled stage followed by a mass-transfer-limited growth stage. For hydrate formation from a water droplet with an initial diameter of 1.66 mm, the hydrate shell thickness was approximately 30 μm and the effective volume fraction of hydrate particles was approximately 11% at 12 h after hydrate formation began. The standard deviation of the shell thickness, which indicates the surface roughness of the hydrate shell, increased with time for hydrate formation from water droplets. The results presented in this study renew our knowledge on the kinetics of hydrate formation, which is essential for their use in flow assurance and other hydrate-related technologies.

Understanding the transport properties of water and guest molecules in dense hydrate structures is pivotal in controlling their formation and stability; this is nevertheless hindered by challenges in characterizing the time-varying diffusion and permeation process. This paper reports experimental results on the mass-transfer characteristics of water and guest molecules across hydrate shells during their formation from water droplets. The time-dependent 3D morphologies and parameters of hydrate shells were in-situ obtained via X-ray computed tomography to reveal the transport mechanism in hydrates. It was found that in the mass-transfer-limited stage, hydrate growth occurred primarily at the hydrate-gas interface, indicating a more mobile characteristic of water molecules in those hydrate shells than gas. The resulting outward permeation of water led to the growth of hydrate protrusions on the outer surface of the hydrate shell, suggesting a transition of the water transport schema through the hydrate shell from diffusion to permeation. Consequently, a diffusion-based shell growth model was developed to quantify the diffusivity of gas and water molecules in hydrates. Combining the measurements in the temperature range of 275.15–283.15 K, the effective diffusion coefficient of gas through the hydrates was estimated to be in the range of 1.34 × 10−14 − 1.90 × 10−13 m2/s, with that of interstitial water in the range of 2.87 × 10−12 − 3.83 × 10−11 m2/s. These results are of fundamental value in developing an improved understanding of the kinetics of hydrate formation from gas-water interfaces, which is essential in the optimization of hydrate-based techniques.

In this article, the hydrates with different saturations were formed in porous media. The permeability of porous media with the presence of hydrate was measured. Comparison of experiment data with that obtained by theory models showed that the change of absolute permeability in the porous media with a hydrate saturation agreed with the capillary center filled model. The result indicates that hydrates can preferentially form in the capillary center rather than coat the inner wall.