Particle size effect of CH4 hydrate for self-preservation

In this study, we used time-resolved, energy-dispersive X-ray diffraction to study the dissociation of methane hydrate of various particle sizes into hexagonal ice and methane gas. The samples of polycrystalline methane hydrate were warmed from 135 to 263 K by slow warming at a rate of 1 K/min under atmospheric pressure. For particle sizes below about 250 mum, all the hydrates dissociated by about 210K. However, larger particle sizes had a high temperature regime in which the hydrate fraction decreased very little with increased temperature. For particle sizes of 1000-1400mum, 20% of the hydrate remained even at 263K. This apparent ability to retain hydrate at high temperatures is consistent with the oft-studied self-preservation effect in which the hydrate is encased by ice that greatly hinders the dissociation process due to methane gas diffusion through the ice. Confocal scanning microscope images of methane hydrate surface after partial dissociation revealed small grains of ice covering the CH4 hydrate surface. These results indicate that, for equal volumes of hydrate. larger hydrate particles can retain more CH4 gas at a given temperature than small particles. (C) 2004 Elsevier Ltd. All rights reserved.