Simulation Of The Growth Process And Calculation Of Thermal Conductivity Of Natural Gas Hydrate Based On Molecular Dynamics

Natural gas hydrates have huge reserves in nature and will be an important energy source in the future.As with hydrate mining techniques,nucleation and growth of hydrates has also attracted extensive research.When studying hydrates,scientists found that 1m~3 of natural gas hydrate can store 150-170m~3(standard state)natural gas,and can be stored under lower pressure and higher temperature without explosion.It is expected to become a new type of natural gas storage and transportation.In this paper,molecular dynamics(MD)was used to simulate the growth process of SI and SII hydrates,and the thermal conductivity of hydrates was calculated by equilibrium molecular dynamics.The factors and mechanisms affecting the growth of hydrates and the thermal conductivity of hydrates are explored,which provides a certain temperature and pressure guidance for the industrial production and transportation of hydrates.In this paper,the growth process of SI hydrate was simulated by using three-phase system.At temperature of 270 K and pressure of 30,50,70,and 90 MPa,respectively,the growth rate of the hydrate increases as the pressure increases.The change of methane concentration in the aqueous solution during the growth of hydrate is programmed to change with time.It is found that the higher the pressure,the faster the diffusion rate of methane molecules in aqueous solution,and the higher the average concentration,which lead to the faster the growth rate of hydrate.Under the conditions of pressure of 50MPa and temperature of 240,250,260,270 and 280K,respectively,the average concentration of methane in solution was the highest at260K,and the growth rate of hydrate was the fastest.The growth process of methane SII hydrate and methane-tetrahydrofuran SII hydrate was simulated by using three-phase system.The simulation found that the growth rate of the SII type hydrate was slightly slowed down after the addition of tetrahydrofuran in the aqueous solution.Secondly,the microscopic process of hydrate cage formation was observed by VMD.It was found that the formation was not unidirectional,but the process of repeated generation,collapse and rebirth.It was observed that tetrahydrofuran was adsorbed into the hydrate cage after forming a bowl-shaped structure,indicating that the hydrate is attractive to the guest molecule.The potential of mean force of hydrates to methane molecules along the growth direction of hydrates in equilibrium state was calculated by umbrella sampling method.The results showed that the potential of mean force was lowest at the hydrate cage at the hydrate-water solution interface,which proved that half-cage structure can adsorb the methane in the solution.Finally,the thermal conductivity of methane hydrate was calculated by equilibrium molecular dynamics and Green-Kubo formula.Under the condition of263.15K,the calculated results agree well with the experimental data.The relationship between the thermal conductivity of hydrate and temperature was explored.In the simulated temperature range,the thermal conductivity of hydrate decreases as the temperature increases.And the ctystal characteristics of methane SII hydrate is more obvious than that of the methane SI hydrate.The atomic phonon state density was calculated as a function of temperature.It is found that the higher the temperature,the lower the frequency corresponding to the peak value of the phonon state density of the oxygen atom in the water molecule,and the translational heat transfer of water molecule is weakened,resulting in a decrease in thermal conductivity.Secondly,the relationship between the thermal conductivity of hydrate and the guest molecule was explored.The results show that compared with the methane molecule,the larger tetrahydrofuran molecule enhances the intermolecular coupling vibration,which leads to decrease of the thermal resistance,the rises of the Debye temperature of the hydrate and the increase of the thermal conductivity of hydrate.Finally,the relationship between the thermal conductivity of hydrate and the cage occupancy rate is explored in three cases.The calculation results show that the thermal conductivity decreases with the decrease of the occupancy rate.This is due to the decrease of the occupancy rate and the phonon excited by the methane molecule amount reduced.At the same time,the thermal conductivity of the hydrate calculated under the same occupancy rate but with different structure is also different.The influence of the calculation error is excluded,indicating that the influence factor of the occupation rate on the thermal conductivity is not single,and the influence of the lattice defect caused by empty cages should also be considered.