Dissociation mechanism of gas hydrates (I,II, H) of alkane molecules: a comparative molecular dynamics simulation

Employing NPT molecular dynamics method with consistent valence force field, the dissociation processes of sI, sII and sH gas hydrates are simulated at different temperatures and at a constant pressure of 100 MPa. The dissociation mechanisms of gas hydrates are revealed by analysing the structural snapshots, radial distribution functions and diffusion coefficients at different temperatures. As temperature increases, the diffusion rates of water molecules and guest molecules increase; thus the clathrate skeleton formed by water molecules with hydrogen bonds distorts and breaks down; meanwhile the guest molecules encapsulated in the water cavities are released. The size of guest molecules affects the dissociation behaviour of gas hydrate. In addition, the dissociation behaviour also relies on the structural phase of gas hydrates.     

Ab initio DFT study of structural and mechanical properties of methane and carbon dioxide hydrates

The structural and mechanical properties of methane and carbon dioxide hydrates were investigated using density functional theory simulations. Well-established equations of state of solids and exchange-correlation functionals were used for fitting the unit lattice total energy as a function of volume, and the full second-order elastic constants of these two gas hydrates were determined by energy–strain analyses. The polycrystalline elastic properties were also calculated from the unit lattice results. The final results for methane hydrate agree well with available experimental data and with other theoretical results. The two gas hydrates were found to be highly elastically isotropic, but they differed significantly in shear properties. The presented results for carbon dioxide hydrates are the first complete set reported so far. The results are a significant contribution to the ab initio material characterisation of gas hydrates required for ongoing fundamental studies and technological applications.     

Molecular dynamics simulation of replacement of methane hydrate with carbon dioxide

Molecular dynamics simulation was performed to analyse the phenomena of replacement of methane hydrate with carbon dioxide (CO₂) at 270 K and 5.0 MPa for 5300 ps. The methane hydrate phase was constructed with 16 unit cells of hydrate. Every cage in the hydrate was occupied by one methane molecule. The methane hydrate phase was sandwiched between two CO₂ phases. During the simulation the hydrate partially melted and liquid water phase appeared, and CO₂ dissolved in the liquid water phase. The replacements were observed three times at the hydrate–liquid water interface during the simulation. In the first case, the replacement occurred at a S-cage without changing the structure. In the second case, an M-cage of methane hydrate partially collapsed, and methane and CO₂ molecules exchanged. After the exchange, the cage occupied by CO₂ remained in the M-cage structure. In the third case, a S-cage of methane hydrate partially collapsed, and methane and CO₂ molecules exchanged. After the exchange, the cage occupied by CO₂ changed to an M-cage-like structure.     

Electrodissociation of clathrate-like structures

In the present work, we develop molecular dynamics (MD) simulations in the NPT (isobaric–isothermic) ensemble to analyse the effect of an external electrostatic field over a cubic methane hydrate crystallite. The amplitude of the field is in the range 0.5–3.0 V/nm. For the simulations, we used the SPC/E rigid water model and a single-site model for methane at a temperature of 248 K and a pressure of 20 bar. When the external electrostatic field is applied, the water dipoles are oriented in such a way that the methane molecules can diffuse far away from the water cages, hence the clathrate dissociation takes place. This last phenomenon was observed for intensities above 1.5 V/nm. Taking the final configuration of each run as input, we develop a new set of MD simulations, and we observe that the stable clathrate is not recovered immediately when the external electrostatic field is turned off due to limitations in the simulation time.     

Stability analysis for binary sII hydrogen–promoter hydrates by molecular dynamics simulation

Molecular dynamics simulation was applied for the binary sII hydrogen–promoter hydrates to search the potential promoters to stabilise the hydrogen hydrates. The simulations were performed at 10.1 MPa. The simulation temperature was maintained at 260 K for 100 ps, and then it was increased at the rate of 0.1 TK/s. The cell volumes of the hydrates slowly increased with increasing temperature, and then the cell volumes rapidly increased. The temperature at which the cell volumes rapidly increased is identified as the simulated collapse temperature. The promoter which gives high simulated collapse temperature is judged to stabilise the hydrates. The simulated collapse temperature of the hydrate filled with cyclobutane is the highest among the promoters studied in this work.     

Molecular simulation of cage occupancy and selectivity of binary THF–H2 sII hydrate

The hydrogen capacity of the binary THF–H₂ sII hydrate is determined by the cage occupancy and by the selectivity of guest molecules. Grand canonical Monte Carlo (GCMC) simulation is used to study the cage occupancy and selectivity of guest molecules from the equilibrium configuration of the binary sII hydrate. The cage framework is regarded as a rigid body and the number of guest molecules is varied to preserve the grand canonical ensemble. The occupancy and selectivity were investigated at a temperature of 270 K for pressures ranging from 0.1 to 200 MPa. It was found that most large cages select THF as guest molecules while small cages include only hydrogen molecules. Multiple occupancy of hydrogen, up to four molecules in large cages and two molecules in small cages, was found as the pressure increases. GCMC results show that the hydrogen capacity is approximately 1.1 wt% at 200 MPa.     

Molecular dynamics simulation of diffusion of hydrogen in binary hydrogen–tetrahydrofuran hydrate

The binary structure II hydrogen–tetrahydrofuran (THF) hydrate was studied with molecular dynamics simulation. The simulations were carried out at 300, 310 K and 10.1 MPa, and with various contents of hydrogen and THF. The migrations of hydrogen molecules from cage to cage were observed. The migration process of hydrogen was also analysed, and the diffusion coefficients of hydrogen in the hydrate were calculated. The calculated diffusion coefficients qualitatively agreed with the experimental data. Double and quintet occupancies of hydrogen molecules were observed in the small and large cages, respectively, without changing the hydrate structure.     

Analysis of crystal growth of methane hydrate using molecular dynamics simulation

Methane hydrate is a crystalline compound with methane molecules enclosed in cages formed by hydrogen-bonded water molecules. Understanding the mechanism of nucleation and crystal growth from methane vapour and liquid water is important for all hydrate applications. However, processes near the water/methane interface are still unclear. In this work, we focused on the crystal growth of methane hydrate seeds located near the water/methane interface. We performed molecular dynamics (MD) simulation and analysed the crystal growth of the hydrate seed at the interface. New cages formed in the liquid water phase were stabilised when they shared faces with the hydrate seed. We also investigated the crystal growth rate as the time development of the number of methane molecules trapped in hydrate cages, based on the trajectory of the MD simulation. The calculated growth rate in the direction that covers the interface was 1.38 times that in the direction towards the inside of the water phase.     

Molecular dynamics simulation on the decomposition of type SII hydrogen hydrate and the performance of tetrahydrofuran as a stabiliser

Molecular dynamics simulation is used to study the decomposition and stability of SII hydrogen and hydrogen/tetrahydrofuran (THF) hydrates at 150 K, 220 K and 100 bar. The modelling of the microscopic decomposition process of hydrogen hydrate indicates that the decomposition of hydrogen hydrate is led by the diffusive behaviour of H₂ molecules. The hydrogen/THF hydrate presents higher stability, by comparing the distributions of the tetrahedral angle of H₂O molecules, radial distribution functions of H₂O molecules and mean square displacements or diffusion coefficients of H₂O and H₂ molecules in hydrogen hydrate with those in hydrogen/THF hydrate. It is also found that the resistance of the diffusion behaviour of H₂O and H₂ molecules can be enhanced by encaging THF molecules in the (5¹²6⁴) cavities. Additionally, the motion of THF molecules is restricted due to its high interaction energy barrier. Accordingly, THF, as a stabiliser, is helpful in increasing the stability of hydrogen hydrate.     

The thermal properties of binary structure sI clathrate hydrate from molecular dynamics simulation

In this work, we present temperature dependence of lattice parameter and normalised lattice parameter in the atmospheric pressure and 120 bar and also pressure dependence of unit cell volume and normalised unit cell volume at 150 and 250?K for variety guests with different size, polarity and guest–host hydrogen bonding capability such as trimethylene oxide (TMO), ethylene oxide (EO), formaldehyde (FA), cyclobutane (CB), cyclopropane (CP) and ethane (Et) in the large cages with CH₄ in small cages of sI clathrate hydrates by molecular dynamics simulations. The obtained values of lattice parameters for the guest species are compatible with the experimental values. These clathrate hydrates are simulated with TIP4P/ice four-site water potential. Herein, isobaric thermal expansivity and isothermal compressibility are calculated at a temperature range of 50–250?K and a wide pressure range. These structural properties have been compared for guests which they are isoelectronic and have similar masses but with different size and polarity. We use molecular dynamics simulations to relate microscopic guest properties, like guest–host hydrogen bonding to macroscopic sI clathrate hydrate properties. The temperature dependence of thermodynamic properties such as constant-volume and constant-pressure heat capacity is presented in the atmospheric pressure for these guest species.     

Methane Hydrate: Killer cause of Earth's greatest mass extinction

The cause for the end Permian mass extinction, the greatest challenge life on Earth faced in its geologic history, is still hotly debated by scientists. The most significant marker of this event is the negative δ¹³C shift and rebound recorded in marine carbonates with a duration ranging from 2000 to 19 000 years depending on localities and sedimentation rates. Leading causes for the event are Siberian trap volcanism and the emission of greenhouse gases with consequent global warming. Measurements of gases vaulted in calcite of end Permian brachiopods and whole rock document significant differences in normal atmospheric equilibrium concentration in gases between modern and end Permian seawaters. The gas composition of the end Permian brachiopod-inclusions reflects dramatically higher seawater carbon dioxide and methane contents leading up to the biotic event. Initial global warming of 8–11 °C sourced by isotopically light carbon dioxide from volcanic emissions triggered the release of isotopically lighter methane from permafrost and shelf sediment methane hydrates. Consequently, the huge quantities of methane emitted into the atmosphere and the oceans accelerated global warming and marked the negative δ¹³C spike observed in marine carbonates, documenting the onset of the mass extinction period. The rapidity of the methane hydrate emission lasting from several years to thousands of years was tempered by the equally rapid oxidation of the atmospheric and oceanic methane that gradually reduced its warming potential but not before global warming had reached levels lethal to most life on land and in the oceans. Based on measurements of gases trapped in biogenic and abiogenic calcite, the release of methane (of ∼3–14% of total C stored) from permafrost and shelf sediment methane hydrate is deemed the ultimate source and cause for the dramatic life-changing global warming (GMAT > 34 °C) and oceanic negative-carbon isotope excursion observed at the end Permian. Global warming triggered by the massive release of carbon dioxide may be catastrophic, but the release of methane from hydrate may be apocalyptic. The end Permian holds an important lesson for humanity regarding the issue it faces today with greenhouse gas emissions, global warming, and climate change.     

Decomposition of CH4 hydrate: effects of temperature and salt from molecular simulations

We report a molecular simulation study to investigate the decomposition of CH₄ hydrate. The decomposition is revealed to be stepwise from the outer to inner layers. Upon decomposition, the number of 5¹²6² cages drops faster than that of 5¹² cages. CH₄ molecules are released, dissolved in water, then enter gas phase; meanwhile, CH₄ bubbles may form particularly at a high temperature. Based on the variations of potential energy, order parameter, cage number and density profile of CH₄ at different temperatures (300, 330, 345 and 360 K) and NaCl concentrations (0, 0.6 and 1.8 M), the effects of temperature and salt are comprehensively examined. With increasing temperature, the decomposition in pure water is accelerated, whereas two opposite effects are observed in NaCl solution. At 330 K, the decomposition is retarded at a higher NaCl concentration, as attributed to the reduced CH₄ solubility in NaCl solution and the participation of ions in cage formation; at 360 K, however, the decomposition is accelerated when NaCl concentration increases due to bubble formation. This simulation study provides microscopic insights into hydrate decomposition, which might be useful towards the optimisation of operating conditions for CH₄ production from CH₄ hydrate.     

Structural properties of sH hydrate: a DFT study of anisotropy and equation of state

ABSTRACT

Structure-H (sH) hydrate is one of the canonical gas hydrates with significant potential applications and scarce characterised material properties despite the wide knowledge available on other gas hydrates. In this work we characterise some of the important physical properties of this hydrate at the atomistic level using Density Functional Theory. Two exchange-correlation functionals (revPBE and DRSLL) were used to simulate six sH hydrate systems encapsulating neohexane and different help gas molecules. The important role of dispersion forces is quantified. The density and isothermal bulk modulus of sH hydrate are higher when dispersion interactions are considered. The presence of those interactions imposes a direct relationship between the hydrate density and its bulk modulus, while their absence reveals the bulk modulus dependency on hydrogen bond density. Anisotropy is a distinguishing feature of this hydrate in distinction to nearly isotropic sI and sII hydrates. Structure-H hydrate experiences a compressional anisotropy in which the a-lattice and the c-lattice constants respond differently to applied pressure showing less compressibility along the c-axis. This compressional anisotropy was found dependant on the chemistry of help gas molecules. Taken together, these property characterisation results and analysis are a significant and novel contribution to the material physics of sH hydrates.     

The Stability of Clathrate Hydrates: Temperature Dependence of Dissociation Pressure in Xe and Ar Hydrate

Abstract

The thermodynamic stability of clathrate hydrates I and II encaging xenon or argon has been investigated by examining the temperature dependence of the dissociation pressure. The evaluation of the stability is made based on the generalized van der Waals and Platteeuw theory developed by Tanaka and Kiyohara [J. Chem. Phys. 98, 4098 (1993)]. In the new treatment, the free energy of formation of hydrates in equilibrium with ice is calculated by taking the coupling of the host lattice vibrations with guests into consideration. The predicted dissociation pressures of Xe and Ar hydrates agree well with experiments in higher temperature range. A poor agreement between experiment and calculation for Ar clathrate hydrate at low temperature is improved by the use of a quantum mechanical partition function for a harmonic oscillator in evaluating the free energy difference between ice and empty hydrate.     

Monte Carlo study of sI hydrogen hydrates

In this study, we perform grand canonical Monte Carlo simulations to evaluate the hydrogen storage capacity of structure I (sI) hydrogen hydrates at pressures up to 500 MPa. Initially, we calculate the upper limit of H₂ content of sI hydrates by studying the hypothetical sI hydrate, where H₂ is the single guest component. It is found that the storage capacity of the hypothetical pure H₂ sI hydrate could reach 3.5 wt% at 500 MPa and 274 K. Depending on pressure, the large cavities of the pure H₂ hydrate can accommodate up to three H₂ molecules while the small ones are singly occupied at most, even at pressures as high as 500 MPa, without any double occupancy being observed. Subsequently, the binary H₂–ethylene oxide (EO) hydrate is examined. In this case, the large cavities are occupied by a single EO molecule while the small cavities can accommodate at most a single H₂ molecule. Such configuration results in a maximum H₂ content of only 0.37 wt%. The hydrogen storage capacity does not improve significantly even in case when EO is replaced by a component with smaller molecular weight.     

Molecular simulation of size-selective gas adsorption in idealised carbon nanotubes

Molecular simulations were used to examine the adsorption of diatomic molecules (nitrogen and oxygen) and similarly sized gases (argon and methane) in pores with van der Waals diameters similar in size to the gas diameters. Idealised carbon nanotubes were used to model generic pores, to better understand the effect of pore diameter on guest adsorption in the absence of defects, specific adsorption sites, or variations in pore diameter that often complicate studies of gas adsorption in other porous materials. Molecular dynamics simulations of open nanotubes show that argon and methane are able to enter tubes whose diameters are slightly smaller than the gas diameters. Diatomic gases are able to enter tubes that are significantly smaller than their kinetic diameters with the molecular axis aligned parallel to the nanotube. The results indicate that size-selective adsorption of these gases is theoretically possible, although differences in pore diameters of only a few tenths of an Angstrom are required. Grand canonical Monte Carlo simulations of a 3.38 Å nanotube indicate significant uptake by argon and oxygen, but not nitrogen or methane. The adsorption of nitrogen and methane gradually increases as the nanotube diameter approaches 4.07 Å, and all gases fully saturate a 4.54 Å nanotube. Of the nanotubes studied, the largest adsorption enthalpy for any gas corresponds to the 4.54 Å nanotube, with significantly lower enthalpies seen in the 5.07 Å nanotube. These results suggest an ideal pore diameter for each gas based on the gas–pore van der Waals interaction energies. Trends in the ideal diameter correlate with the minimum tube diameter accessible to each gas.     

Molecular simulations as a tool for selection of kinetic hydrate inhibitors

Natural gas hydrates are ice-like structures in which water molecules form a cage around gas molecules. They have been a problem in the petroleum industry. The heavy cost of alcohol and glycol injections needed to suppress the formation of hydrates has spurred an interest in so-called “kinetic inhibitors”, able to slow down the hydrate formation rather than prevent it. An earlier work (Kvamme, B. et al. 1997, Mol. Phys., 90, p. 979) proposed a simulation-based scheme to assess the comparative performance of prospective inhibitors and select the best candidates for experimental testing. In this work, we employed molecular dynamics simulations to test several kinetic inhibitors in a multiphase water–hydrate system with rigid hydrate interface. In addition, a long-scale run was implemented for a system where the hydrate was free to melt and reform. Our conclusion that PVCap inhibitor will outperform PVP as a kinetic hydrate inhibitor is supported by experimental data. We demonstrate that numerical experiments can be a valuable tool for selecting kinetic inhibitors as well as provide insight into mechanisms of kinetic inhibition and hydrate melting and reformation.