Molecular dynamics simulations of the mechanisms of thermal conduction in methane hydrates

Gas hydrates in sustainable chemistry

This review includes the current state of the art understanding and advances in technical developments about various fields of gas hydrates, which are combined with expert perspectives and analyses.

Molecular Simulation Study on the Microscopic Structure and Mechanical Property of Defect-Containing sI Methane Hydrate

The study of changes in the related mechanical property and microscopic structure of methane hydrate during the decomposition process are of vital significance to its exploitation and comprehensive utilization. This paper had employed the molecular dynamics (MD) method to investigate the influence of defects on the microscopic structure and mechanical property of the sI methane hydrate system, and to discover the mechanical property for the defect-containing hydrate system to maintain its brittle materials. Moreover, the stress-strain curve of each system was analyzed, and it was discovered that the presence of certain defects in the methane hydrate could promote its mechanical property; however, the system mechanical property would be reduced when the defects had reached a certain degree (particle deletion rate of 9.02% in this study). Besides, the microscopic structures of the sI methane hydrate before and after failure were analyzed using the F3 order parameter value method, and it was found that the F3 order parameters near the crack would be subject to great fluctuations at the time of failure of the hydrate structure. The phenomenon and conclusions drawn in this study provide a basis for the study of the microscopic structure and mechanical characteristics of methane hydrate.

Molecular dynamics simulation of halogen bonding in Cl2, BrCl, and mixed Cl2/Br2 clathrate hydrates

Clathrate hydrate phases of dihalogen molecules have properties that differ from those of other guest molecules of similar size. The water oxygen–chlorine distances in the structure I (sI) Cl2 hydrate are smaller than the sum of the van der Waals radii of oxygen and chlorine. Bromine hydrate forms a unique clathrate hydrate structure that is not seen in other guest substances. In mixed Cl2/Br2 structure I hydrate, the water oxygen–bromine distances are also smaller than the sum of the oxygen and bromine van der Waals radii. We previously studied the structure of three dihalogen clathrate hydrates using single crystal X-ray diffraction and described these structural features in terms of halogen bonding between the dihalogen and water molecules. In this work, we perform molecular dynamics simulations of cubic sI Cl2, mixed Cl2/Br2, and BrCl clathrate hydrate phases. We perform quantum chemical computations on the dihalogen molecules to determine the nature of σ-hole near the halogen atoms. We fit the electrostatic potential of the molecules to point charge models including dummy atoms that represent σ-holes adjacent to the halogen molecules. Molecular dynamics simulations are used to determine the lattice constants, radial distribution functions, and guest dynamics in these phases. We determine the effect of guest size and difference in halogen bonding on the properties of the clathrate hydrate phase. Simulations for the Cl2, BrCl, and mixed Cl2/Br2 hydrates are performed with small cages of the sI clathrate hydrate phases completely full or filled with experimental occupancies with Cl2 guests.

Molecular Dynamics Study of Thermal Conduction in Carbon Dioxide Hydrates

Equilibrium molecular dynamics simulations that use the Green–Kubo method for sI CO2-hydrate systems from medium to full occupancy were performed to estimate the corresponding thermal conductivities at temperatures that range from 233.15K to 278.15K and pressures that range from 3MPa to 100MPa. Specific potential models for water and CO2were adopted. The effects of guest occupancy ratios and outside thermobaric conditions on CO2hydrate thermal conductivity were studied. The thermal mechanism was also analyzed. The thermal conductivities of hydrates of CH4, C2H6, N2, and O2were estimated. The size ratio of guest diameter to cavity diameter provided an adequate basis for understanding the thermal conductivities of gas hydrates.