Study On Effects Of Guest Molecules On Stabilities Of Natural Gas Hydrate

The key technology of natural gas hydrate storage and transportation is to improve the hydrate stability to reduce the storage costs.Thought it is known that the stability of natural gas hydrate can be improved effectively by controlling the composition of guest molecules,the mechanism of guest molecules on hydrate stability remains immature up to now.Molecular dynamics simulation is adopted in this thesis to investigate the mechanism action of guest molecule compositions on the hydrate stability in aspects of guest molecule species,hydrate cavity occupancy and the ratio of different guest molecules,which is aimed at providing technical supports for the industrial application of natural gas hydrate storage and transportation.Firstly,the impact of the guest molecule size on the hydrate structure is summarized,the hydrate structure and crystal parameters are determined,the hydrate models for simulation were built and the criteria for measuring the hydrate stability were chosen.Secondly,hydrate models with different guest molecules were built to research the impact of guest molecules on the hydrate stability in different temperature and pressure conditions,and the radial distribution function,the mean square displacement and the interaction energy of hydrate were analyzed,the results reveal that the interaction energy between the guest molecules and cavities they were filled in influences the hydrate stability and the interaction energy is determined by the ratios of guest molecule diameters to the cavity diameters.Thirdly the effects of cavity occupancy on hydrates with different guest molecules were investigated,results show that the cavity occupancy can influence the hydrate stability through changing the stabilization energy of hydrate,the cavity occupancy range needed to keep the stability of hydrate with different guest molecules are different and the cavity occupancy needed to keep the hydrate stability decreases as the difference of guest molecule diameters with the cavity diameters decreases.Finally the hydrate models with different guest molecule ratios were built to investigate the hydrate stability under different conditions and the mechanism action of multicomponent guest molecules on the hydrate stability was analyzed.Results reveals that as the ratio of large size guest molecules increases,the hydrate structure changes,the combination of guest molecules and water molecules grow tighter and the hydrate stabilization energy decreases,in which case the stability of hydrate is enhanced.The results can be used to provide instructions for optimizing the storage and transportation conditions of natural gas hydrate by improving its stability.