Mechanism of methane hydrate formation in the presence of hollow silica

Effects of ice and supercooled water on the metastability of methane hydrate: DSC analysis and MD simulations

Methane hydrate (MH) has been viewed as a potential abundant clean energy resource worldwide. Its related technologies play important roles in applications of gas and energy storage, flow assurance of natural gas pipelines etc. Unlike the well-researched stability and decomposition of MH at temperatures above 273 K, the metastability of MH below the ice freezing point, i.e. the anomalous slow decomposition out of thermodynamically stable regions, remains to be unravelled. Studies regarding the influences of ice and supercooled water (SW) on the metastable properties of MH led to varied conclusions, i.e. the as-proposed self-preservation effect and metastable MH-SW-gas equilibrium. In this study, a series of DSC experiments were performed to investigate the thermal stability boundaries and the associated metastable behaviours of MH-ice-gas and MH-SW-gas samples in porous medium. The DSC analysis probed accurate thermal stabilities and characterized decomposition behaviors of the samples, contributing to the hypothesis of potential influences from SW and ice on the metastability of MH. MD simulations were also validated and performed. Active guest-host interactions by the SW layers between MH and gas phases were identified, suggesting probable microscopic configurations related to the metastability of the MH-SW-gas system. Indications of the DSC and MD simulation results call for future high-resolution in situ experimental validations.

Enhanced methane gas storage in the form of hydrates: role of the confined water molecules in silica powders

Methane hydrates are promising materials for storage and transportation of natural gas; however, the slow kinetics and inefficient water to hydrate conversions impede its broad scale utilisation. The purpose of the present study is to demonstrate rapid (2-3 h) and efficient methane hydrate conversions by utilising the water molecules confined in the intra- and inter-granular space of silica powders. All the experiments were conducted with amorphous silica (10 g) powders of 2-30 μm; 10-20 nm grain size, to mimic the hydrate formations in fine sand and clay dominated environments under moderate methane pressure (7-8 MPa). Encasing of methane molecules in hydrate cages was confirmed by Raman spectroscopic (ex situ) and thermodynamic phase boundary measurements. The present studies reveal that the water to hydrate conversion is relatively slower in 10-20 nm grain size silica, although the nucleation event is rapid in both silicas. The process of hydrate conversion is vastly diffusion-controlled, and this was distinctly observed during the hydrate growth in nanosize silica.

SF6 Hydrate Formation in Various Reaction Media: A Preliminary Study on Hydrate-Based Greenhouse Gas Separation

SF₆ hydrate formation behaviors in various reaction media, such as bulk water, porous silica gel, and hollow silica, were investigated for hydrate-based SF₆ separation with a primary focus on thermodynamic stability and formation kinetics. The measured three-phase (H-L_W-V) equilibria demonstrated that the types of reaction media used in this study had no effect on the thermodynamic stability of SF₆ hydrates. The dissociation enthalpy (ΔH_d) of SF₆ hydrate was measured using a high-pressure micro-differential scanning calorimeter, and it corresponded well with estimates from the Clausius-Clapeyron equation. The unstirred porous silica gel system showed a larger gas uptake and a higher growth rate at the early stage of SF₆ hydrate formation. However, the gas uptake and growth rate of SF₆ hydrates in stirred bulk water and unstirred hollow silica were significantly increased at a larger temperature driving force or in the presence of sodium dodecyl sulfate. The experimental results obtained in this study will be very helpful for a better understanding of the thermodynamic and kinetic characteristics of SF₆ hydrate formed in various reaction media and in surfactant-added solution, and are expected to contribute to further development of the hydrate-based SF₆ separation process.

Formulating formation mechanism of natural gas hydrates

A large amount of energy, perhaps twice the total amount of all other hydrocarbon reserves combined, is trapped within gas hydrate deposits. Despite emerging as a potential energy source for the world over the next several hundred years and one of the key factors in causing future climate change, gas hydrate is poorly known in terms of its formation mechanism. To address this issue, a mathematical formulation is proposed in the form of a model to represent the physical insight into the process of hydrate growth that occurs on the surface and in the irregular nanometer-sized pores of the distributed porous particles. To evaluate the versatility of this rigorous model, the experimental data is used for methane (CH₄) and carbon dioxide (CO₂) hydrates grown in different porous media with a wide range of considerations.