【作者】 李期斌；

【导师】 刘朝；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2014， 博士

【摘要】 在天然气的开采、加工、储运过程中，存在诸多与热物理相关的问题亟需解决。例如：预测地壳中高温高压条件下，天然气流体的相行为特征；天然气中的杂质随环境改变，析出沉积在管道表面，造成设备堵塞及腐蚀问题；天然气在多孔介质中的表面特性；天然气水合物的热物理性质及其界面特性等。由于实验条件和理论基础受温度，压力，天然气分子种类等因素的限制，使得通过常规手段(宏观实验和理论分析)研究上述问题存在很大局限性。本文采用分子动力学方法，研究了天然气开采利用中的若干热物理基础问题。建立分子动力学研究天然气固体杂质溶解模型；模拟了元素硫在S/H2S体系中的核化生长过程；揭示了流体分子在纳米通道中与固体表面粒子作用的微观机理；并对甲烷水合物导热问题及水合物体系中水分子的微观构型进行了讨论。主要研究内容及结论如下：首先，基于分子动力学方法，建立了两种固体溶解模型，并分别对CO2在低温CH4中以及硫在H2S中的溶解机理进行了讨论。结果表明，提出的固液溶解模型，可以准确计算CO2在150K以下CH4中的溶解度。而H2S溶解模型研究结果表明，H2S溶剂化层模型可以得出硫在H2S中的溶解趋势。由于没有考虑到化学溶解作用，计算结果小于实验值。其次，应用化学反应势函数首次研究了硫在硫/硫化氢混合体系中的核化生长过程。得出元素硫沉积初期生长规律，发现硫核化生长的两种方式：雪球效应和硫团簇融合。核化初期，硫团簇以雪球效应生长方式为主。团簇生长超过相应的临界状态后，两种生长方式共同作用，加快硫团簇生长速率。在整个过程中，硫的聚合物(同素异形体)对整个核化过程以及H2S的分解起催化作用。然后，建立了流体分子在纳米通道中的作用模型，探讨流体分子在固体表面粒子影响下热物理相关性质的变化。研究了金红石型TiO2(110)纳米通道对CH4/H2S混合流体的吸附分离效果，结果表明TiO2纳米通道表面上吸附的CH4数量要多于H2S，但TiO2纳米通道对H2S的选择性较高。在吸附过程中施加电场，可以提高TiO2纳米通道对H2S的分离能力。CO2/N2混合流体在纳米通道中，固体表面对CO2的吸附能力强于对N2的吸附能力。当CO2浓度较高时，CO2在纳米通道表面上多层吸附，提高吸附效率。当CO2浓度较低时，CO2在固体壁面呈单层吸附，小孔径纳米通道有利于CO2的吸附。石墨壁面对CO2/N2混合流体具有强烈的分离作用，但在小孔径纳米通道中受到两壁面的共同吸引作用，分离效果反而降低。此外，理论研究了流体在不同晶面结构纳米通道中的吸附和导热性能，发现纳米通道内流体的导热性能受固体表面结构和温度共同影响。基于固体表面晶胞基本参数，定义了一个比值参数R，用于表征固体表面结构对流体相关性质的影响。固体表面材料R的增大将导致固体表面对流体吸附作用增强，固体表面上吸附的流体粒子增多，相应纳米通道中流体粒子的热导率也有所提高。这些现象在低温时表现明显，随温度升高逐渐减弱。流体与固体接触面存在界面热阻，限制固体表面粒子与流体粒子的能量传递。当体系温度较低时，界面热阻对固体表面粒子与流体粒子间的传热起主导作用，明显降低流体的热导率。随温度升高，流体粒子与固体表面粒子相互作用剧烈，克服界面热阻的影响，流体热导率增强。随后，讨论了多种I型甲烷水合物结构在高压下的导热性能，结果表明不同水合物结构中水分子的排布构型几乎相同，热导率各不相同。模拟中，空穴水合物结构的导热性能最好，晶格缺陷水合物结构的导热性能最差。进一步分析表明，高压可以促进水分子和甲烷分子的导热性能。高温能促进甲烷分子的导热性能，但会降低水分子的导热性能。水合物结构中的甲烷分子与水分子笼状结构存在耦合作用，共振散射声子。水合物结构的晶格缺陷导致大量声子散射。最后，研究了水合物溶解体系和水合物/冰/水混合体系中水分子的微观构型。发现水合物溶解后，大部分水分子的氧原子保持与水合物(或冰)晶体结构相近的排布构型，而水分子的其他性质则与常规液态水相近。推测溶解水中氧原子的排布构型可能是导致水合物溶解水记忆效应的原因之一。水合物/冰/水混合体系中，水分子的构型表明，水合物内部与冰/水混合体系中的水分子排布存在明显差异，这种差异在水合物体系与冰/水混合体系的界面连续变化。更多还原

【Abstract】 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.更多还原