Mechanism Study On The Decomposition And Replacement Of Natural Gas Hydrate Based On Molecular Dynamics Simulation

Natural gas hydrates,with its characteristics of high reserves,wide distribution,high energy density and cleaning,are widely considered as one of the new potential alternative energy resources.Compared with the developed countries,the study of gas hydrate in China is still in the initial stage,especially at the fundamental research fields.With the development of computer technologies,molecular simulation has become a powerful tool for conducting the microscopic studies at molecular scale,which is very crucial for descovering the mechanism problem on the research area of hydrates.In this dissertation,thermodynamic analysis is firstly conducted to illustrate energy-efficient performance of hydrate-base separation processes from different gas sources.Then based on the theory of ideal minimum work,second-law efficiency and the separation factor,a performance evaluation and comparison is conducted between hydrate-based separation processes and some other common separation technologies.It is proved that the ideal minimum work of hydrate-based separation processes is about 3⁷ kJ/mol,which is one of the lowest methods among existing separation technologies such as absorption,adsorption,membrane and thermochemical cycle.However,thermodynamics is mainly based on the macroscopic processes,not used to understand the micromechanics because hydrate-based separation processes is in fact a coupling process of hydrate formation and decomposition.Therefore,in the present paper,the molecular dynamics simulation is applied to investigate the melting of CH₄ hydrate.The dynamics of hydrate decomposition is initially compared within different ensembles.Then at NPT ensemble,the bubble formation and evolution is studied.It is proved that the melting of hydrate is layer by layer,by which about92.6%of total CH₄ molecules enter the gas phase after 10 ns.Methane bubbles formed during decomposition.The CH₄ molecules released firstly aggregated near the interface and form the quasi-sphere bubbles.Then the bubbles evolved into quasi-cylinder,and merged other bubbles to form a continuous and uniform gas film.The bubbles can absorb CH₄ molecules near the interface,and fluctuate in the liquid phase,which can speed the collapse of hydrogen bonds and enhance dynamics of decomposition.In addition,the replacement of CH₄ in natural gas hydrates by CO₂ can not only conduct energy exploitation,but also achieve the storage of greenhouse gases.Thus,it has high potential on the existing research field of gas hydrates.In this paper,molecular simulation is performed on the replacement of CH₄ hydrate by CO₂ to investigate the dynamics of replacement and bubble evolution.The system size,amount of CH₄ hydrates,and pressure is as the same as that in the decomposition,which makes it interesting to compare the two processes.Simulation studies show that,the replacement also moves layer by layer,but much slower compared with the decomposition process.About 23.1%of CH₄ molecules enter the gas phase after 180ns replacement.The mixed bubble composing of CH₄ and CO₂ molecules formed during replacement,which had similar behavior with that in the decomposition except that the uniform gas film and the second bubble did not turn up.It is found that CH₄molecules are sandwiched by some intensive CO₂ molecules in the mixed bubble.It is also observed that during replacement a fraction of CO₂ molecules in the liquid can assist the aggregation of CH₄ molecules at an initial stage and prevent their further escape and reoccupation by encompassing CH₄ molecules afterwards to weaken the reoccupation.In summary,the separation,decomposition and replacement of gas hydrates are complex physical and chemical processes.The studies in this dissertation only try to illustrate the microscopic mechanism of several processes at a molecular level.