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In this study, a series of molecular dynamics (MD) simulations are performed for methane-water and methane-brine systems in the range of 263.15 K∼283.15 K, 1 MPa∼10 MPa with the ion mass fraction up to 25 wt%. The interfacial tension (IFT) and density profile, relative adsorption, charge distribution, ions hydration, interfacial thickness are predicted, and the interfacial features are analysed at a molecular level. The results show that the IFT decreases with pressure and is enlarged with the presence of ions, which is positively correlated with ion concentration. Moreover, the methane accumulation and ions depletion at the interface are observed. The relative adsorption of methane and ions at the Gibbs dividing surface and the interfacial thickness are estimated quantitatively. It is shown that the methane relative adsorption is positive and increases at a higher pressure and lower temperature, while the relative adsorption of ions is negative. For the two systems, the interfacial thickness increases with pressure and ion concentration, but the variation trend with temperature is related to the pressure. The interfacial features found in the present study may help understand the interaction among gas, water, and ions, as well as the phase transition behaviour of natural gas hydrate. [ABSTRACT FROM AUTHOR]

Gas hydrates have an important role in environmental and astrochemistry, as well as in energy materials research. Although it is widely accepted that gas accumulation is an important and necessary process during hydrate nucleation, how guest molecules aggregate remains largely unknown. Here, we have performed molecular dynamics simulations to clarify the nucleation path of methane hydrate. We demonstrated that methane gather with a three-body aggregate pattern corresponding to the free energy minimum of three-methane hydrophobic interaction. Methane molecules fluctuate around one methane which later becomes the central gas molecule, and when several methanes move into the region within 0.8 nm of the potential central methane, they act as directional methane molecules. Two neighbor directional methanes and the potential central methane form a three-body aggregate as a regular triangle with a distance of ~6.7 Å which is well within the range of typical methane-methane distances in hydrates or in solution. We further showed that hydrate nucleation and growth is inextricably linked to three-body aggregates. By forming one, two, and three three-body aggregates, the possibility of hydrate nucleation at the aggregate increases from 3/6, 5/6 to 6/6. The results show three-body aggregation of guest molecules is a key step in gas hydrate formation. Gas hydrates have an important role in environmental and astrochemistry, as well as energy materials, but the hydrate nucleation process is not fully understood. Here, the authors use molecular dynamics simulations to show that methane molecules gather with a three-body aggregate pattern. [ABSTRACT FROM AUTHOR]

• Solid surface can enhance hydrate decomposition, and hydroxylated-silica surface has better enhancement effect. • Molecular diffusion increases with increasing temperature, CH 4 diffuses more easily in pores parallel to the surface. • Decomposition temperature and wettability of pore surface affect the formation rate and morphology of nanobubbles. • Bulk spherical bubbles cause uneven decomposition, resulting in a 4.3 times difference in the decomposition rates. Natural gas hydrates (NGHs) exist mainly in marine muddy porous media, and it is of great significance to understand the hydrate decomposition in microporous sediments for hydrate exploration. In the paper, molecular dynamics (MD) simulations have been performed for CH 4 hydrate decomposition in the slit nanopores of graphite and hydroxylated-silica surfaces. As expected, the higher the temperature, the faster the decomposition rate. Compared with the hydrate decomposition in the free system, the solid surface can enhance hydrate decomposition; the hydroxylated silica surface has better enhancement effect, and this effect is more obvious at lower temperatures. At 330 K, 340 K, 350 K and 360 K, the decomposition rates of hydrate in HS-H systems are 50.8%, 36.4%, 14.7% and 11.7%, respectively, which are higher than those in G-H systems. In the slit-nanopores, hydrate decomposition starts from the surface and then grows towards the bulk region, the hydrate near the pore surface decomposes faster. However, due to the strong ability of the hydrophilic surface to adsorb water molecules, the hydrophilic-silica pore has a more vital role in enhancing hydrate decomposition near the surface in the early stage. The higher the temperature, the stronger the diffusion ability of methane molecules and water molecules, and methane molecules diffuse more easily in pores in directions parallel to the surface than perpendicular to it. Due to the difference in wettability, the contact angles of nanobubbles on the surface are different, resulting in different forms of nanobubbles. For the G-H system, both surface cap bubbles and bulk spherical bubbles were formed at higher temperatures, while only surface cap bubbles were formed at lower temperatures. For the HS-H system, bulk spherical bubbles were formed over the temperature range studied. The formation conditions of the bulk spherical bubbles are more lenient than surface cap bubbles. The hydrates in the graphene pores proceed in an isotropic and uniform decomposition. At 330 K, the liquid methane concentration of HS-H system (0.03181–0.03397) is higher than that of G-H system (0.02452–0.02559) at t = 4–6 ns, while the decomposition rate is higher than that of G-H system, which is due to the formation of bulk spherical bubbles. Bulk spherical bubbles in the hydroxylated-silica pores leads to the uneven decomposition of the hydrates, this inhomogeneity leads to local differences in hydrate dissociation rate, and the difference in hydrate dissociation rates in different regions can be up to 4.3 times. The above findings provide the theoretical basis for the safe and efficient exploitation and transportation of hydrate in marine sediments. [ABSTRACT FROM AUTHOR]

Nanobubbles have attracted wide scientific attention due to their large number of potential applications in nanotechnology, and they are ubiquitous during natural gas hydrates (NGHs) mining which is one of the strategic energy resources. Molecular dynamic simulations were performed to investigate CH 4 nanobubble characteristics during NGHs dissociation. The nucleation mechanism and evolution characteristics of nanobubbles vary with hydrate dissociation temperature. The higher the temperature, the earlier the bubble nucleation, the more small methane clusters are produced, and the more complex the evolution path of nanobubbles, the initial position of bubble nucleation is closer to the decomposition interface. The gas reservoir can provide another possibility for methane molecules to overcome solubility barriers to avoid the formation of nanobubbles at lower dissociation temperature. CH 4 nanobubble evolution was found to be governed by the gas supersaturation and the diffusion of CH 4 molecules. The self-diffusion coefficient of CH 4 molecules near the decomposition interface area is small due to the adsorption effect of the incomplete cages, nanobubbles are more likely to form outside the adsorption zone where the CH 4 diffusion coefficient is larger at a lower temperature. A direct proof of speeding up of hydrate dissociation by nanobubble growth was given. The acceleration of dissociation is later than nanobubble growth and the accumulation of nanobubbles on dissociation interface may have a negative influence. The new findings on nanobubble evolution may help design and optimize nanobubble formation techniques in nanotechnology and efficient gas hydrate production protocols. • CH 4 diffusion coefficient increases with distance from the decomposition interface. • CH 4 nanobubble evolution was governed by gas supersaturation and diffusion. • A direct proof on speeding up of hydrate dissociation by nanobubble growth was given. • A mechanism for nanobubble nucleation during hydrate dissociation was proposed. [ABSTRACT FROM AUTHOR]

During hydrate growth with impingement, hydrate cages were found to be occupied simultaneously by two methane molecules, even in the 51262 cage which was thought to be impossible. The diffusion during the methane hopping process is found to be 3~4 order of magnitude faster than that during hopping between one-occupied and empty hydrate cages. [Display omitted] • hydrate growth with impingement has been first investigated using molecular simulation. • 51262 hydrate cages with two methane molecules have been found. • 3~4 order of magnitude faster diffusion has been found caused by two-occupied cages. Clathrate hydrates have attracted much attention in energy and environment technology as well as the nature of icy bodies. We report a molecular dynamics simulation study of methane hydrate growth with impingement which has not been investigated by either experiment or simulation as far as the authors' knowledge. Several unique features have been discovered including dislocation, two occupied hydrate cages and fast methane diffusion. A certain proportion of hydrate cages were found to be occupied simultaneously by two methane molecules, even in the 51262 cage which was thought to be impossible. Methane molecules were demonstrated to hop between two-occupied and one-occupied hydrate cages as well as between gas bubble and nearby hydrate cages when a gas bubble was present. The diffusion rate during the hopping process is in the order of 10-9 to 10-8 m2/s, which is 3~4 order of magnitude faster than that during hopping between one-occupied and empty hydrate cages. [ABSTRACT FROM AUTHOR]

[Display omitted] • Maximum energy storage density (57.4 kWh/m3) of hydrate cold storage was obtained. • Increasing the solution concentration boost the cold charge capacity by 2–3 times. • Rapid dilution of dissociated solution was key to enhancing the cold discharge. • Self-regulation of driving force was relied on temperature and concentration. Low energy storage density, intermittent phase changes, and heat transfer barriers have posed significant challenges in the implementation of hydrate energy storage systems. Based on the heterogeneous nucleation mechanism for tetrabutylammonium bromide (TBAB) hydrate phase change energy storage, a novel cold storage system with internally circulating gas disturbance was constructed for energy evaluation and thermodynamic evolution. The hydrate cold storage efficiency, response time, dynamic driving force, unique force pattern of the coils and guest molecule diffusion were analyzed for the first time. The bubbles generated by gas internal circulation provided numerous nucleation sites for hydrate formation, efficiently promoting the energy storage process. The disturbances caused by the bubbles also rapidly transmitted the energy generated by the phase change. A hydrate storage density of 57.4 kWh/m3 for this system was the maximum among known hydrate storage systems. Notably, a change in solution concentration (from 10 wt% to 40 wt%) resulted in a 2–3-fold increase in the cold charge capacity. Rapid dilution of local solutions and rapid release of cold were the ways to promote the cold discharge. Less damage to the stress structure of the coil was due to the loose accumulation of hydrate particles under gas disturbance. The microbubbles generated by the gas disturbance enabled heterogeneous nucleation and heat-mass transfer in the hydrate cold storage. The gas disturbance-based hydrate energy storage process holds significant guiding value for various applications such as refrigerated transport, building cooling systems, and grid peak shaving. [ABSTRACT FROM AUTHOR]

Tetrabutylammonium bromide (TBAB) semi-clathrate hydrate possesses a unique clathrate structure for capturing and sequestering small-molecule gases, such as CH 4 , H 2 and, CO 2 and the advantage of phase change energy storage. Elucidating the diversified reactions and determining the optimal phase change characteristics of TBAB hydrate is crucial to discovering and overcoming specific limitations hindering the large-scale applications. In this study, in situ macroscopic visualization, three-dimensional reconstruction via X-ray Computed Tomography, and differential scanning calorimeter measurements were performed to discover, verify, and analyze the distinct characteristics of TBAB hydrate phase change, including phase separation, three-dimensional structure, and thermal effects. The optimal concentration of TBAB hydrate energy storage was found and verified experimentally for the first time. A peculiar liquid layer (liquid-liquid phase separation) caused by hydrate-solution phase separation was observed after multiple formation-dissociation cycles. The liquid layer separated a region that optimized the hydrate phase change. The solutions with different initial concentrations could evolve an optimized phase-change region of the same concentration (37 wt%) via multiple cycles. The region of the same concentration was shown to be the result of optimal aggregation of molecules. In proving the particularity of the concentration in the optimized region, TBAB hydrate dissociation exhibit thermal effects that can be easily ignored (TBAB molecules ionization and ionic bond synthesis), which affected the TBAB hydrate latent heat measurement. Finally, the optimal concentration of TBAB hydrate phase change was determined as 37 ± 1 wt%, and the characteristics of the concentration, including the volume fraction, complete conversion of water molecules and TBAB molecules, and latent heat, were confirmed by the liquid layer. The experimental results obtained for the first time in this study are important guidance for TBAB hydrate in the field of gas capture and energy storage. [Display omitted] • Solid-liquid phase separation leads to liquid-liquid phase separation. • Guest molecule optimum aggregation hypothesis of memory effect was proposed. • Changes in ionic bonds play an important role in TBAB hydrate phase change. • Hydrate volume fraction could be up 99% by XCT 3D reconstruction. • 37 wt% was considered to be the optimal concentration. [ABSTRACT FROM AUTHOR]

In gas capture/storage and energy storage, tetrabutylammonium bromide (TBAB) hydrates have been regarded as superior carriers owing to their unique molecular selectivity and high latent heat. Hydrate conversion amount is a significant parameter in evaluating gas selection and cold storage characteristics. This study proposes a method for calculating hydrate conversion amount proposed based on the correlation between electrical conductivity and solution concentration. The correlation index (R2) between electrical conductivity and TBAB solution concentration was above 0.99 at concentrations less than below 5 wt%. The conversion amounts of different hydrate types were obtained using a unique hybrid calculation method. The differences in hydrate conversion amount, conversion ratio, and memory effect were compared at different mass fractions. The results indicated that the hydrate morphology and accumulation patterns were influenced by the TBAB solution ionization degree. Regular hexagonal TBAB hydrate crystals were observed at a low ionization degree. The maximum conversion ratio of TBAB hydrate reached 96% (1 °C/30 wt%). The variation trend of TBAB hydrate phase equilibrium temperature was consistent with that of the electrical conductivity of the solution. The investigation of the memory effect revealed that the phase change process of TBAB hydrate is more predictable in high-concentration initial solutions. [Display omitted] • Regular hexagonal hydrate was observed for the first time. • Insufficient ionization of TBAB slowed hydrate conversion. • TBAB hydrate phase equilibrium temperature was consistent with ionization degree. • Temperature, induction factor, and residual solution volume affected hydrate conversion. [ABSTRACT FROM AUTHOR]