Molecular Dynamics Study Of Several Fundamental Thermophysical Problems In Natural Gas Exploitation And Utilization

During the process of mining, refinement and storage of natural gas, lots ofproblems, which are related to thermophysics, should be addressed timely. For instance,these problems include predicting the phase behavior of geological fluid under hightemperature and high pressure, solving the clogging and corrosion problems in theequipment and pipelines caused by the deposition of the impurities in natural gas as theenvironment changes during the natural gas production, studying the surfacecharacteristics of natural gas in the porous media, and researching the thermophysicalproperties and interfacial characteristics of natural gas hydrates. However, there aremany limitations of investigating the above issues by conventional methods, such asexperimental method, theoretical analysis and so on. The reason is that the experimentconditions and theoretical basis are limited by temperature, pressure, and molecularspecies.In this dissertation, several fundamental thermophysical problems in natural gasexploitation and utilization are investigated by molecular dynamics simulation (MD).During the process of researching, we establishes solvation models of solid impurities innatural gas system based on MD, simulates the nucleation processes of element sulfur inthe S/H2S system, uncovers the microscopic mechanism of interactions between thefluidmoleculesand surface particles of nano-channels, and discusses the thermal properties ofmethane hydrate and the microscopic configuration of water molecules in the methanehydrate system. The main research contents and conclusions are listed as follow:At first, twodissolution models of solid impurities are established based on MDmethods. The solubility of CO2in cryogenic methane and the solubility of sulfur inhydrogen sulfide are discussed respectively according to the solvation models. Theresults show that the solid-liquid dissolution model proposed in this dissertation couldaccurately calculate the solubility of CO2in cryogenic methane under150K. And thesimulation results of H2S solvation model indicate that the solvation shell model of H2Sproposed in this dissertation could calculate the dissolution behavior of sulfur in H2Swhich conforms to experimental data. Without considering the sulfur dissolving inchemical reaction, the simulation results are less than the experimental data.Then the sulfur nucleation in S/H2S system is investigated by reactive force fieldfor the first time. And the results elucidate the growth phenomena of the element sulfur deposition at the initial stage. There are two ways of nucleation: the snow ball effect andthe coalescence of small sulfur clusters into big clusters. At the beginning of thenucleation, the snow ball effect is predominant. Once the cluster exceed its critical state,both two ways works together and accelerates the nucleation of clusters. During thewhole process, the sulfur polymer (the sulfur allotropes) plays the role of catalysis in thenucleation and the decomposition of H2S.Next, the model of fluid molecules interplaying in the nano-channel is establishedfor discussing the thermophysical properties of fluid molecules under the influence ofsolid particles of nano-channel. The adsorption and separation properties of the mixedfluid of CH4/H2S in rutile TiO2(110) lattice nano-channel is studied. The resultsindicate that the adsorption of CH4on the TiO2surface is more than that of H2S. But theTiO2-based nano-channel has a high selectivity for H2S molecules. The electric field canbe used in the adsorption process, which can improve the selectivity of H2S moleculesto a certain degree. The result of the adsorption and separation of the CO2/N2mixedfluid in nano-channels elucidates that the surface of nano-channel has a strongeradsorbability for CO2than that for N2. When the concentration of CO2is heavy, the CO2will appear multilayer adsorption on the surface of nano-channel, so this mode canimprove the absorption efficiency. When the concentration of CO2is low, the CO2willpresent monolayer adsorption on the surface of nano-channel. At this time, the smallsize nano-channel is beneficial to the absorption. The surface of graphite has an intenseseparation of CO2/N2mixture. However, the interactions between the surfaces of thesmall aperture nano-channel can reduce the separation of CO2.In addition, thermal properties of the fluid in different lattice structures ofnano-channel are investigated in theory. The investigation reveals that the thermalproperties of fluid in nano-channel are influenced by the temperature andmicro-structures of the surface. An area ratio Rderived from the surface parameters isdefined to describe the fluid-lattice interaction. For a given material, a high ratio latticecan result in strong adsorbability of the solid surface and increase the number ofabsorbed particles. Meanwhile, the thermal conductivity of fluid particles in thecorresponding nano-channel will be higher than that of other lattices. However, theeffect of the lattices is decreased as the temperature increasing. The interfacialresistance is generated by the fluid–solid interactions, and has an effect on the energytransfer between the fluid particles and surface particles. As the temperature of thesystem is low, interfacial resistance plays a leading role in the heat transfer between the solid and the fluid, and can reduce the thermal conductivity of the fluid. With thetemperature increasing, the interactions between the fluid molecules and the surfaceparticles become intensive. As a consequence, the influence of the interfacial resistancecan be overcome, and the thermal conductivity of the fluid can be increased.The thermal properties of several sI methane hydrate structures under high pressureare discussed afterward. The results indicate the investigated hydrate structures presentalmost the same crystalline distribution of water molecules, but their thermalconductivities are different. In all the investigated hydrate structures, it is found that theguest-free hydrate owns highest thermal conductivity of the studied systems while thedefect hydrate has the lowest thermal properties. The high pressure can promote thethermal properties of methane and water molecules. And the high temperature canpromote the thermal properties of methane molecules, but it can weaken the thermalproperties of water molecules. There is a coupling effect between methane moleculesand cage structures of hydrates, which can cause the resonant scattering effect ofphonon. And the lattice defect of water molecules in hydrate causes the considerablescattering of phonons.Finally, the configurations of water molecules in the decomposition system ofhydrate and hydrate/ice/water mixture are investigated. It is found that someconfigurations of oxygen atoms in water molecules still remain the similar crystalstructurein the decomposition system of hydrate, while the other properties of watermolecule are similar to the liquid water. It can be speculated that the tetrahedralcoordination of oxygen atoms in water should be one of the factors associated with thememory effect of dissociated water from hydrate. The analysis of configuration of watermolecules in hydrate/ice/water system reveals that the configuration of water moleculesin the hydrate area has a significant difference with that in ice/water mixture. And thiskind of difference keeps sequential change through the interface between the hydratearea and the ice/water mixture.