| Hydrates are ice-like compounds formed by water and guest molecules.On the one hand,it has broad application prospects in the industry,such as gas separation,natural gas storage and transportation.On the other hand,it is necessary to inhibit the formation of hydrate during pipeline transportation.To understand and solve the technical problems of hydrate growth,decomposition and inhibition,it is necessary to have a basic understanding of its molecular mechanism.For the problems of hydrate growth and decomposition,we need to understand the formation kinetics of gas hydrates,and have a prior knowledge of the temperature and pressure conditions of hydrate formation.For inhibition of the formation of hydrate efficiently,the inhibitory ability of different kinetic hydrate inhibitors and the optimal use concentration should be clarified.In this thesis,the methane-water supersaturated solution systems with/without KHIs were constructed.In these systems,the nucleation process of homogeneous solution under different temperatures and pressures was explored and the effects of temperature and pressure on the nucleation rate of hydrate and the growth rate of nucleation was analyzed by the molecular dynamics simulations.Then the effects of kinetic hydrate inhibitors on hydrate nucleation were investigated and the mechanism of the selected KHIs were clarified at the molecular level.The main contents are summarized as below:First,molecular dynamics simulation method was adopted to simulate the effect of temperature on the nucleation of methane hydrate.The nucleation process of hydrates was studied by configuration analysis and calculation of the change of total energy of the system,the mean square displacement of oxygen atoms in water molecules with time,the radial distribution function and the total number of cages in the statistical system with time.According to the simulation results,we found that the number of cages and order degree of system increased as temperature increased in the temperature range studied.The nucleation of the system is optimal at a temperature of 275 K under 15 MPa.Then,the effect of pressure on hydrate nucleation was explored.By comparing conformational changes,the variation of potential energy,the mean square displacement of oxygen atoms,and the total number of cages in the system with time,it is found that the different pressures on hydrate nucleation at 265 K are not much different.However,at 275 K,the pressure has a great influence on nucleation.It shows that with the increase of pressure,the number of nucleation of the system increases first and then decreases slightly.Among them,nucleation at 25 MPa is most ideal.The variation of the number of four types of cages in the system at 25 MPa was also studied.The whole nucleation and growth process of hydrates was understood from a deeper perspective.Finally,the inhibition effects of PVP,PVP-A and PVP-E on the inhibition process of hydrate nucleation were studied by MD simulation.The results show that the inhibitory effect of the three inhibitors is PVP-A > PVP > PVP-E,which is consistent with the experimental results.It has also been found that the effect of the inhibitor is to reduce the saturation of the solution by desorbing the methane molecules in the solution,thereby inhibiting the nucleation of the hydrate.According to our concentration analysis of PVPA,the inhibitory effect of different concentrations was 3.76 wt% ? 2.59 wt% > 1.39 wt%.The inhibitors of higher concentrations showed similar inhibitory effects.Therefore,2.59 wt% is the most cost-effective concentration of PVP-A.The conclusions based on our findings may provide some help in the laboratory's investigation of the preparation conditions of hydrates,preparation of new kinetic hydrate inhibitors,and choosing the optimum concentration of kinetic hydrate inhibitors. |
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