Effect of Hydrate Shell Formation on the Stability of Dry Water

Abstract

This study investigates the effect of gas hydrate formation on the stability of dry water (DW) particles when they are exposed to high pressure methane at low temperatures. The DW particles are prepared by mixing water with hydrophobic silica nanoparticles at high speed to form a water-in-air inverse foam. A high pressure autoclave was used to determine the hydrate equilibrium conditions and formation characteristics including hydrate onset time, subcooling temperature, and initial growth rate. In comparison to bulk water, the equilibrium conditions for methane hydrate are shifted to higher temperatures and low pressures, suggesting that the silica nanoparticles promote the hydrate equilibrium conditions. The surface-to-volume ratio between the gas and the water encapsulated by the silica nanoparticles is increased in comparison to bulk water which enhances the kinetics of methane hydrate formation without the need for vigorous mixing. However, after multiple cycles of hydrate formation and dissociation, the hydrate fraction decreases exponentially and approaches 0.22, which is approximately 20% of the hydrate fraction formed during the first cycle. From the data presented, it was concluded that the hydrates form a shell on the DW particles. Dissociation of this hydrate-shell generates a free water phase that cannot be reabsorbed into the DW particles which causes the exponential reduction in the hydrate fraction. PXRD confirms that structure I methane hydrate is formed with a lattice parameter of 1.1827(1) nm. Raman spectroscopy confirms that the hydrate-shell covers the DW particles as evidenced by the presence of two peaks for methane at 2901 and 2913 cm(-1), which indicates that the methane exists in large and small cages, respectively. These results suggest that the particles are covered with a hydrate-shell when methane hydrates are formed. Therefore, the hydrophobic silica is rearranging during hydrate formation, and after dissociation of the hydrate, free water is expelled. This free water cannot absorb back into the particles due to the hydrophobic surface.