(286b) Investigation on Thermophysical Properties of  Structure II Clathrate Hydrates Using Molecualr Dynamics Simulations

The fundamental properties of sII hydrates are important for investigation of sII gas hydrates occur in petroleum pipelines and nature. Here, we report thermophysical calculations of thermal expansion coefficient, compressibility and heat capacity for C₃H₈, THF pure and binary sII hydrates with second-help CH₄ or CO₂ molecules using molecular dynamic simulation under the wide and relevant range of pressure-temperature conditions. An annealing simulation was performed to determine initio configurations of the hydrates with THF guest molecule. Combined with a kind of effective connection method, lattice parameters of the hydrates were modified and found to be agreed with those from experimental measurements at low temperatures. The expansion coefficient and heat capacity of C₃H₈ pure hydrate are comparable but slightly larger than those of THF pure hydrate, while the deviation of compressibility between the two above appears to be considerable due to the difference of guest-host interaction. However, when small cages of the THF hydrates are occupied by the second-help molecules, the variation is weakened because the subtle guest-guest interactions can offset the unfavorable configuration of unstable THF hydrate caused by defect structure. Compared with CH₄ molecule, CO₂ molecule filling in the small cage can increase the expansion coefficient and compressibility as well as decrease heat capacity of binary hydrates as it shows in the sI hydrate. The calculated bulk modulus for C₃H₈ pure and binary hydrates with CH₄ or CO₂ molecule varies between 8.7GPa and 10.6GPa at 287.15K between 10 and 100MPa. The simulations show that the thermophysical properties of hydrates are largely dependent on the enclathrated compounds and the presented results provide a much needed atomistic thermoelastic characterization of sII hydrates and essential input for the large-scale applications of hydrate prevention and production as a potential energy.