Abstract
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.
Highlights
Gas hydrates could show an unexpected high stability at conditions out of thermodynamic equilibrium, which is called the self-preservation effect
Gas hydrates are a class of inclusion compounds formed by the physically stable interaction between water and guest molecules, in which guest molecules occupy the cages built by hydrogen-bonded water molecules[1]
A two-step dissociation model was first reported by Handa[4]: the destruction of the host lattice followed by the desorption of the guest molecules
Summary
Gas hydrates could show an unexpected high stability at conditions out of thermodynamic equilibrium, which is called the self-preservation effect. An anomalous phenomenon was commonly found that some kinds of hydrates can prevent themselves from further decomposing above their melting points but below the melting point for water, and show an unexpected stability This stability under non-equilibrium conditions is thought to be caused by a layer of ice formed when the hydrate melts, and the ice layer coats the hydrate surface to seal it from further dissociation. A consecutive desorption and melting (CDM) model was proposed by Windmeier and Oellrich[17] in their theoretical study They suggested that the desorption of guest molecules near the interface is the reason for hydrate decomposition, which leads to the melting of the superheated empty hydrate lattice. Considering the heat transfer during the hydrate melted, Baghel et al.[22] performed micro-canonical MD simulations at temperatures higher than 273 K, and fitted their results to the heat balance equations
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