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.
Gas hydrate single-crystal structure analyses.
The first single-crystal diffraction studies on methane, propane, methane/propane, and adamantane gas hydrates SI, SII, and SH have been performed. To circumvent the problem of very slow crystal growth, a novel technique of in situ cocrystallization of gases and liquids resulting in oligocrystalline material in a capillary has been developed. With special data treatment, termed oligo diffractometry, structural data of the gas hydrates of methane, acetylene, propane, a propane/ethanol/methane-mixture and an adamantane/methane-mixture were obtained. Cell parameters are in accord with reported values. Host network and guest are subject to extensive disorder, reducing the reliability of structural information. It was found that most cages are fully occupied by a guest molecule with the exception of the dodecahedral cage in the acetylene hydrate which is only filled to 60%. For adamantane in the icosahedral cage a disordered model is proposed.
NMR Investigation of Methane Hydrate Dissociation
The methane hydrate dissociation mechanism was studied on the molecular scale using 13C magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Analysis of time-resolved 13C MAS NMR spectra for methane Structure I (sI) hydrate dissociation obtained by raising the temperature above the hydrate equilibrium conditions permitted the cage occupancy and pressure to be determined during this process. The relationship between NMR methane gas chemical shift, pressure, and temperature was developed, which allows estimating the system pressure in the sealed glass tube sample at any temperature. The large to small cage occupancy ratio remained constant during hydrate dissociation suggesting that there is no preferential dissociation of hydrate cavities and the whole unit cell decomposes during dissociation. This decomposition rate is virtually the same for both the large and the small cages in methane hydrate on a per cage basis. The similar decomposition rate of both cage types shows that the sI me...