Molecular Dynamic Simulations Of CO₂ Hydrate Formation

As the effective energy-saving and environmental protection technology,CO₂hydrate has attracted increasing attention in the fields of cold storage air conditioning,fire extinguishing,oil and gas exploitation,etc.,and the research on its formation characteristics is of great importance.However,the research on the formation characteristics of CO₂ hydrate recently is still mainly focused on the traditional experimental stage,which has various limitations that cannot be explored from the microscopic perspective.Therefore,this paper introduces the method of molecular dynamics simulation to explore the formation of CO₂ hydrate from the microscopic perspective.In this paper,Materials Studio was used to establish a reasonable CO₂ hydrate formation system.First,the formation system model was simulated by large parallel computing software Lammps,and the effects of different temperature conditions（240 K,250 K,260 K,270 K and 275 K）and different pressure conditions（100 atm,200 atm,300 atm,400 atm,500 atm and 600 atm）on the formation characteristics of CO₂ hydrate were investigated.After the kinetic calculation,the formation characteristics of hydrate were further analyzed from the perspectives of conformation analysis,hydrogen bond analysis,radial distribution function,mean square displacement,diffusion coefficient,energy change,etc.,and the following conclusions were obtained:The simulation results show that the formation of CO₂ hydrate is a stable and orderly process.As time goes on,the H₂O molecules form molecular cages through hydrogen bonds,CO₂ molecules continuously occupies the central position of the water cage,thus generating complete clathtate hydrate.The growth of hydrate is spreading outward from the vicinity of the existing clathtate hydrate,and the growth is on a scale.The influence of different temperature conditions on the formation of CO₂ hydrate was investigated under the pressure of 300atm.When the temperature was between 240 K and 250 K,the formation rate of hydrate was slow.When the temperature rises to 260 K,CO₂ hydrate is well formed and the rate is accelerated.However,when the temperature is in the range of270 K to 275 K,the original hydrate cage is directly decomposed,and the hydrate cannot be formed normally.The results show that the appropriate degree of supercooling has a crucial effect on the hydrate formation.Too low temperature will affect the normal movement and diffusion of molecules,and too high temperature will aggravate the instability of the system,both of which are not conducive to the rapid formation of hydrate.At the temperature of 260 K and the pressure range of 100 atm to 600 atm,the formation of hydrates in the system was well guaranteed.The second characteristic peak of RDF between atoms is obvious with the increase of pressure under different pressure conditions.With the constant increase of pressure,the MSD of H₂O and CO₂ continues to rise,the diffusion coefficient continues to decrease,and the potential energy of the system drops faster.That is,the higher the pressure,the easier the formation of clathrate hydrate,the faster the formation rate of hydrate.Later,the effects of different metal particles on the formation of CO₂ hydrate were investigated,and the composite models of Cu,Fe and Ag under different mass fractions（0.2%,0.5%,0.8%,1.0%and 1.3%）were simulated.The effects of metal particles on hydrate formation characteristics were further investigated through the parameters of the simulation system,such as conformation,hydrogen bond,radial distribution function,mean square displacement,diffusion coefficient,energy change,etc.,and the following conclusions were obtained:The addition of suitable concentration metal particles can promote the formation of CO₂ hydrate.For CO₂ hydrate-Cu composite system,the hydrate formation rate is accelerated when the mass fraction of Cu is from 0.2%to 1.0%,and reaches the peak when the mass fraction is 1.0%.However,the formation of Cu was inhibited after the mass fraction reached 1.3%.For CO₂ hydrate-Fe composite system,when the mass fraction of Fe increased from 0.2%to 0.8%,the hydrate formation rate accelerated,and showed a tendency of increasing first and then decreasing,reaching a peak when the mass fraction was 0.5%.When the mass fraction of Fe reaches 1.0%and 1.3%,the formation rate of hydrate is lower than that of pure hydrate system.For CO₂ hydrate-Ag composite system,the hydrate formation rate accelerated at a low mass fraction of 0.2%to 0.5%,and gradually showed a downward trend,reaching a peak at a mass fraction of 0.2%.However,when the mass fraction of Ag reaches 0.8%to 1.3%,the formation rate of Ag is lower than that of pure hydrate system,and the higher the mass fraction of Ag,the stronger the inhibition effect.The promoting mechanism of metal particles on hydrate formation characteristics mainly includes three aspects.The first is that the movement intensity of metal particles is large,and Brownian motion is active,which speeds up the orderly combination of CO₂and H₂O in solution,and promotes the rapid transfer of energy.Secondly,metal particles with small size and high surface increase the contact surface with liquid molecules,resulting in increased intermolecular forces,thus improving the flow resistance,increasing the solution viscosity,and making it easier for H₂O molecules to be effectively bonded into clathrate by hydrogen bonds.Thirdly,the heat of formation of the solution system can be eliminated in time by the metal particles with high thermal conductivity,and the simulated formation environment can be optimized to a certain extent,so as to drive the rapid formation of hydrate.However,it has an inhibitory effect when the concentration of metal particles is too high.The Brownian motion in the solution of the solid metal particles with too high concentration is too strong,which aggravates the collision with the boundary hydrate cage.And the high concentration leads to larger clusters that clump together,which also hinders the orderly movement of molecules to a certain extent.