Formation efficiency of CO2 hydrate from submicron ice was investigated by laboratory experiments and numerical models. Since formation rate of hydrate changes as the surface area of reaction interface decreases, temporal variation of formation rate was taken into account in this study. In the experiment, fine ice particles, the diameter of which was about 0.5 μm, generated by an ultrasonic mist generator, was confined in a reaction cell with pure CO2 gas under the condition of 0.7 MPa and 253 K to form CO2 hydrate. The conversion rate of ice to hydrate was obtained as a time history by measuring its weight at different reaction period. In the numerical part, a fugacity-driven formation model incorporated with a shrinking-core model of a single ice particle was applied to a spherical ice particle. Unknown parameters such as formation rate constant and coefficient of diffusion of CO2 in hydrate film were determined by fitting the model to the experimental results. It was found that the most of the formation phenomena was diffusion-limited process and the estimated diffusion coefficient almost agreed with that proposed in the literature for relatively larger ice particles with diameter of about 100 μm.
Clathrate hydrates are crystalline water-based solids in which small molecules are trapped inside cages of hydrogen bonded water molecules. Recently large amount of natural gases such as methane have been found to exist in sand sediments under the seabed and exploitation of those gases as alternative energy sources is expected. The distinctive difficulty of their recovery compared with the conventional oil and natural gases is attributed to the physical characteristics of hydrate which is normally stable at low temperature and high pressure. Therefore, understanding of dissociation characteristics of hydrate is important for the purpose of efficient gas production.
