Molecular Dynamics Simulation Study On The Effects Of Cyclic Promoters On Hydrate Formation And Decomposition

A large number of natural gas hydrates exist in the deep ocean and permafrost,which is the most potential new energy source in the 21st century and its exploitation and utilization have received extensive attention from scholars all over the world.Using the hydrate method to store and transport natural gas is safe and economical,but the hydrate has a strong dependence on temperature and pressure,and the induction time of the formation process is long.These properties make it difficult to quickly synthesize and stably store natural gas hydrate.At present,the scientific community has little research on the influencing factors and microscopic mechanism of hydrate formation and decomposition process,which largely restricts the development of natural gas storage and transportation technology by hydrate method.Molecular dynamics simulation can explain macroscopic physical phenomena on the atomic scale,which provides necessary theoretical basis for experimental research.In this paper,the molecular dynamics simulation method is used to study the single-phase and two-phase hydrates of methane（CH₄）,tetrahydrofuran（THF）and 1,3-dioxolane（DIOX）,and the decomposition process of hydrates is carried out.The simulation study,and the formation process of the mixed system of methane and water were also studied.The molecular dynamics simulation results of the decomposition process of hydrate are as follows.Methane molecules will accumulate in the system after the decomposition of the single-phase hydrate and form bubbles,which is consistent with the results of existing simulation studies.In addition,the structural stability increases as the occupancy rate of the crystal holes increases.When the occupancy rate was too low,the interaction energy between the host and guest molecules was low,and the hydrate cannot remain stable and decompose.When the occupancy was the same,the stability of the hydrate was related to the relative size of the guest molecule and the crystal cavity.In the two-phase system,due to the existence of the phase interface,the methane molecules need to pass through the liquid film to enter the gas phase after the decomposition of the hydrate.The decomposition rate was proportional to the temperature.The decomposition rate of methane hydrate was 11.92（?）/ns at 293K and 20.26（?）/ns at 303K.And the decomposition rate is inversely proportional to the proportion of promoter molecules.The higher the proportion of promoter molecules,the slower the hydrate decomposition.Studies have found that the mass transfer resistance of the liquid film was an important factor affecting the decomposition rate of the hydrate surface.During the nucleation of methane hydrate,the water molecules around the guest molecules formed polygonal meshes through hydrogen bonds,and these meshes were continuously arranged and reorganized,eventually formed polyhedral cages.For the CH₄+H2O system,the growth mode was type I.The specific process was that methane dissolves in water and first forms 5¹² cages with water molecules.A single 5¹² cage has a shorter lifespan,which need to be combined with 6²5¹² cages to form water molecule clusters that can grow stably,and the number of 5¹² cages was affected by the number of6²5¹² cage during the growth process.When the cyclic promoters were added,the sustainable growth of cage clusters appeared earlier in the system,and the induction time of the nucleation process was shortened.The promotion effect of DIOX was better than that of THF,which reduced the induction time from 561ns to 120ns.At the same time,this type of promoter can form 6⁴5¹² cages with water,changing the growth mode of the system from type I to type II.The research results provided an important scientific reference for the storage and transportation of natural gas by the hydrate method.