Origin of Self-preservation Effect for Hydrate Decomposition: Coupling of Mass and Heat Transfer Resistances
Gas hydrates could show an unexpected high stability at conditions out of thermodynamic equilibrium, which is called the self-preservation effect. The mechanism of the effect for methane hydrates is here investigated via molecular dynamics simulations, in which an NVT/E method is introduced to represent different levels of heat transfer resistance. Our simulations suggest a coupling between the mass transfer resistance and heat transfer resistance as the driving mechanism for self-preservation effect. We found that the hydrate is initially melted from the interface, and then a solid-like water layer with temperature-dependent structures is formed next to the hydrate interface that exhibits fractal feature, followed by an increase of mass transfer resistance for the diffusion of methane from hydrate region. Furthermore, our results indicate that heat transfer resistance is a more fundamental factor, since it facilitates the formation of the solid-like layer and hence inhibits the further dissociation of the hydrates. The self-preservation effect is found to be enhanced with the increase of pressure and particularly the decrease of temperature. Kinetic equations based on heat balance calculations is also developed to describe the self-preservation effect, which reproduces our simulation results well and provides an association between microscopic and macroscopic properties.
Pore-scale investigations on the effects of ice formation/melting on methane hydrate dissociation using depressurization
Abstract Ice phase affects the methane hydrate dissociation in the porous medium channels. Thus, the investigations on the roles of ice formation and melting in hydrate dissociation around the freezing point are necessary. A model that considers the interphase heat transfer and microscale effects was developed in our previous work. In this study, we extended the work to investigate the effects of ice formation/melting on methane hydrate dissociation in micro porous media channels. Our investigations showed that water temperature, distribution of water and methane, pressure, cumulative methane generation and residual methane hydrate and heat changes in the system are affected seriously by the ice phase. The formation of ice favors the hydrate dissociation for a while, and then inhibits the dissociation of methane hydrate due to the thickening ice. The pressure variations are indications of the possible reformation of methane hydrate and deformation of porous medium channels. The change in heat as one of the key factors for hydrate decomposition shows a more complex changing peculiarity as a result of the ice formation/melting.
Statistical rate theory of interfacial transport. III. Predicted rate of nondissociative adsorption
The statistical rate theory approach is used to derive the expression for the rate of adsorption of a nondissociating gas on a solid surface. By comparing the predicted adsorption with the available experimental results, one of the central assumptions in the statistical rate theory can be examined. On the basis of these experimental results, with which the theoretical predictions are found to be in close agreement, the assumption would appear to be valid. By being in close agreement with the experimental results throughout the experimental period, the adsorption rate expression derived from the statistical rate theory is an improvement over the presently available theories.