Molecular Dynamics Simulation Of Methane Hydrate Formation

As a new type of energy material,natural gas hydrate has become a hot research topic at home and abroad.The formation mechanism of natural gas hydrate is an important theoretical basis for hydrate utilization and development technology.Molecular dynamics interpretation of a series of microscopic phenomena such as nucleation,growth,impingement and interfacial diffusion of methane hydrate is an important scientific issue.In this thesis,the growth and impingement processes of methane hydrate,the effect of thermodynamic inhibitors on the growth and impingement of methane hydrate and the effect of GO on the nucleation and growth of methane hydrate are studied by molecular dynamics simulation.Firstly,the growth process of methane hydrate with impingement is simulated by molecular dynamics method.By tracking trajectory,the impingement interface contains some unique phenomena including dislocation,two occupied hydrate cages and fast methane diffusion is found.A certain proportion of hydrate cages were found to be occupied simultaneously by two methane molecules,even in the smaller 5¹²6² cage.By further studying the states of methane molecules,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 orders of magnitude of diffusion coefficient during the hopping process is in the order of 10^-9 to 10^-8 m²/s,which is 3～4 order of magnitude larger than that during hopping between one-occupied and empty hydrate cages.Secondly,the effects of three thermodynamic inhibitors:Na Cl,KCl and ethylene glycol on hydrate growth and impingement are studied.The research result prove that Na Cl,KCl and ethylene glycol delay the moment of hydrate come into accelerated growth stages,decreasing the growth rate of the hydrate and reducing the concentration of methane in the water solution.The Cl^-ions are observed more frequently to insert into the cages than Na⁺ions during the growth stage.The ethylene glycol molecules tend to occupy 5¹²6² cages and occur in pairs or in clusters.The crystal clusters are more complete of Na Cl and the ions of KCl are more scattered at the impingement interface,and the ions cause some unusual water-occupied defect structures to appear frequently near them.Above defective structures indicate that ions have destructive effect on hydrate structure.The microscopic mechanism of hydrate growth and impingement by salts and ethylene glycol effects and the dual role of the thermodynamic inhibitors:inhibition effect in the growth stage and destroy effect for the impingement interface,has been demonstrated has been revealed.Finally,molecular dynamics simulations are used to investigate the effects of graphere oxide on nucleation and growth of methane hydrate.It was found that under the same oxidation degree,the spatial distribution of oxide groups of GO have little effect on the nucleation and growth of hydrate.Obvious decomposition of hydrate is observed only in a few cases due to randomness of nucleation and methane adsorption on the surface.The adsorption effect of surface competes with the formed cage for methane,reducing the concentration of solvated methane and preventing hydrate formation.The increase in oxidation degree also results in a thicker water-absorbing layer of GO and more hydrogen bonds forming between surface and water,resulting in increased instability of hydrate growth state.This thesis clearly shows the structure picture of the adsorption layer of water and methane on the graphere oxide surface.The effect of GO on the nucleation and growth of methane hydrate provides a detailed mechanism analysis.In summary,the formation mechanism and unique phenomenon of methane hydrate impingement interface are studied in this theses;the effect of thermodynamic inhibitors on hydrate growth mechanism is clarified and the decreasing concentration of methane caused by the hydration of ion is the critical factor that results in the decrease of hydrate growth rate,while the formation of hydrate cage defect structure precipitate or single crystal structure also reflects the ion of hydrate structure damage;the root cause of graphene oxide surface inhibition of hydrate growth is found,which provides theoretical basis and guidance for the practical application of new materials as hydrate inhibitors.