Diffusion Of Methane Molecules Research Articles

Effect of moisture content on methane diffusion in kaolinite by molecular dynamics simulations

The diffusion behaviours of methane in kaolinite with water contents ranging from 0-5 wt% are analysed by molecular dynamics (MD) simulations. The results indicate that methane molecules can jump between adjacent holes in the kaolinite matrix. The methane diffusion coefficient is very low (3.28 × 10−9 m2/s) and linearly increases with the increasing moisture content. As the moisture content decreases, the value of the first RDF peak between methane and oxygen is larger, indicating that with a lower moisture content, the interaction energy between methane and oxygen in kaolinite is stronger. The interaction between methane and water linearly increases with the moisture content; by contrast, the interaction energy between kaolinite and water and between kaolinite and methane linearly decreases with the increasing moisture content. The diffusion of methane molecules adsorbed on the surfaces can also be accelerated by the fast diffusion of water molecules in the middle interlayer nanopores of the kaolinite. [Received: May 12, 2019; Accepted: September 17, 2019]

Experimental study of frozen gas hydrate decomposition towards gas recovery from permafrost hydrate deposits below freezing point

Enormous natural gas hydrate reserves have been identified in the global permafrost and oceanic regions. How the existence of ice influences the hydrate dissociation is a fundamental issue related to the development of the permafrost-associated gas hydrate resources. The dissociation behaviors of frozen methane hydrate in hydrate-ice-gas saturated porous media below freezing point are studied in a high pressure reactor. The hydrate-bearing permafrost layer is simulated through decreasing the temperature of the methane hydrate samples to −0.60 °C, and then they are safely dissociated by depressurization and thermal stimulation. Results show that the disturbance on gas and water in the pores can induce secondary hydrate formation during the cooling of the system. The gas recovery rate by depressurization below freezing point is extremely slow, which is caused by the obstructive effect of ice on the diffusion of methane molecules. The self-preservation of frozen gas hydrate could be effectively broken under wellbore heating. Heat is primarily transferred by conduction mode across the hydrate and ice particles. The solid ice tends to absorb heat first and then melts into liquid water. The dissociated fluids play a role in promoting the heat transfer through thermal convection. Moreover, the net energy gain is found to be much higher than the energy consumption, and desirable energy efficiency has been obtained. Sensitivity analyses indicate that higher heat injection rate and lower ice saturation will be more favorable for the exploitation, while the production pressure seems to have negligible influence on the overall production efficiency.

Different Mechanism Effect between Gas-Solid and Liquid-Solid Interface on the Three-Phase Coexistence Hydrate System Dissociation in Seawater: A Molecular Dynamics Simulation Study

Almost 98% of methane hydrate is stored in the seawater environment, the study of microscopic mechanism for methane hydrate dissociation on the sea floor is of great significance to the development of hydrate production, involving a three-phase coexistence system of seawater (3.5% NaCl) + hydrate + methane gas. The molecular dynamics method is used to simulate the hydrate dissociation process. The dissociation of hydrate system depends on diffusion of methane molecules from partially open cages and a layer by layer breakdown of the closed cages. The presence of liquid or gas phases adjacent to the hydrate has an effect on the rate of hydrate dissociation. At the beginning of dissociation process, hydrate layers that are in contact with liquid phase dissociated faster than layers adjacent to the gas phase. As the dissociation continues, the thickness of water film near the hydrate-liquid interface became larger than the hydrate-gas interface giving more resistance to the hydrate dissociation. Dissociation rate of hydrate layers adjacent to gas phase gradually exceeds the dissociation rate of layers adjacent to the liquid phase. The difficulty of methane diffusion in the hydrate-liquid side also brings about change in dissociation rate.

Formation of methane nano-bubbles during hydrate decomposition and their effect on hydrate growth.

Molecular dynamic simulations are performed to study the conditions for methane nano-bubble formation during methane hydrate dissociation in the presence of water and a methane gas reservoir. Hydrate dissociation leads to the quick release of methane into the liquid phase which can cause methane supersaturation. If the diffusion of methane molecules out of the liquid phase is not fast enough, the methane molecules agglomerate and form bubbles. Under the conditions of our simulations, the methane-rich quasi-spherical bubbles grow to become cylindrical with a radius of ∼11 Å. The nano-bubbles remain stable for about 35 ns until they are gradually and homogeneously dispersed in the liquid phase and finally enter the gas phase reservoirs initially set up in the simulation box. We determined that the minimum mole fraction for the dissolved methane in water to form nano-bubbles is 0.044, corresponding to about 30% of hydrate phase composition (0.148). The importance of nano-bubble formation to the mechanism of methane hydrate formation, growth, and dissociation is discussed.

In-Depth Study of the Influence of Host−Framework Flexibility on the Diffusion of Small Gas Molecules in One-Dimensional Zeolitic Pore Systems

Molecular-dynamics simulations are performed to understand the role of host-framework flexibility on the diffusion of methane molecules in the one-dimensional pores of AFI-, LTL-, and MTW-type zeolites. In particular, the impact of the choice of the host model is studied. Dynamically corrected Transition State Theory is used to provide insights into the diffusion mechanism on a molecular level. Free-energy barriers and dynamical correction factors can change significantly by introducing lattice flexibility. In order to understand the phenomenon of free-energy barriers reduction, we investigate the motion of the window atoms. The influence that host-framework flexibility exerts on gas diffusion in zeolites is, generally, a complex function of material, host model, and loading such that transferability of conclusions from one zeolite to the other is not guaranteed.

Experimental and Modeling Study on Decomposition Kinetics of Methane Hydrates in Different Media

The decomposition kinetic behaviors of methane hydrates formed in 5 cm3 porous wet activated carbon were studied experimentally in a closed system in the temperature range of 275.8-264.4 K. The decomposition rates of methane hydrates formed from 5 cm3 of pure free water and an aqueous solution of 650 g x m(-3) sodium dodecyl sulfate (SDS) were also measured for comparison. The decomposition rates of methane hydrates in seven different cases were compared. The results showed that the methane hydrates dissociate more rapidly in porous activated carbon than in free systems. A mathematical model was developed for describing the decomposition kinetic behavior of methane hydrates below ice point based on an ice-shielding mechanism in which a porous ice layer was assumed to be formed during the decomposition of hydrate, and the diffusion of methane molecules through it was assumed to be one of the control steps. The parameters of the model were determined by correlating the decomposition rate data, and the activation energies were further determined with respect to three different media. The model was found to well describe the decomposition kinetic behavior of methane hydrate in different media.

Kinetics of Methane Hydrate Formation from Polycrystalline Deuterated Ice

The kinetics of methane hydrate formation was investigated by in-situ time-of-flight neutron powder diffraction. Samples were prepared from deuterated ice particles (< 0.25 mm) and transformed to clathrate hydrate by pressurizing the system with methane gas. The rates of sI methane hydrate formation were measured in-situ under isothermal conditions with a methane pressure of 1000 psi (6.9 MPa). Kinetic data were analyzed in terms of a shrinking core model, including possible contributions of nucleation, methane diffusion, and interface reaction. The data support the hypothesis that methane hydrate formation reaction from ice particles is diffusion-controlled. The reaction starts quickly at the nucleation stage, which propagates to form a hydrate layer that covers the ice particle. Further reaction is limited by the growth of the hydrate layer and inward diffusion of methane molecules through the hydrate layer to the unreacted ice core. The reaction rate at the interface between hydrate and unreacted ice p...

Isotopic composition of soot carbon from a methane flame

1. It is shown that the soot recovered on the deposition surface of a diffusion jet of methane is enriched in the heavy isotope of carbon the more, the greater the height of this surface is located. This enrichment is explained by the high rate of diffusion of methane molecules containing the light carbon isotope, which leads to enrichment of the methane in the heavy isotope. 2. The soot, formed from methane in the flame of a plane burner, is enriched in the light carbon isotope, which is explained by the isotopic kinetic effect of the growth process of soot particles. 3. The soot, deposited on the surface at the height of its maximum yield in a diffusion jet of methane, is enriched in the heavy carbon isotope the more so the closer it is located to the axis of the jet. 4. The zone of soot formation of a diffusion laminar jet expands from below upwards, due to the involvement in the soot formation process of new layers of gas. In the lower part of the jet, this expansion takes place because of the outside layers of gas, and in the zone of main soot formation because of the inner layers of gas.