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Molecular dynamics simulation of high-pressure phases of gas hydrate solid solution

天然气水合物固溶体高压相的分子动力学模拟

基本信息

批准号：
16540439
负责人：
AKAMATSU Tadashi
金额：
$ 1.34万
依托单位：
Kochi University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2004
资助国家：
日本
起止时间：
2004 至 2005
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-16540439/
关键词：
mixed gas hydrate high-pressure phase solid solution molecular dynamics method molecular dynamics simulation MD lattice parameter 個溶体

项目摘要

The molecular dynamics (MD) method was applied to CH4-CO2 mixed hydrates with the structures sII (cubic) and filled ice (orthorhombic), in order to reproduce and predict the compositional dependence of crystallographic and thermodynamic properties.The following three patterns of sII mixed hydrate crystals were prepared for the simulation :1. The CH4 is completely partitioned into S-cage, and CO2 into L-cage.2. The CH4 and CO2 are evenly distributed to S- and L-cages (Disordered structure).3. The CO2 is completely partitioned into S-cage, and CH4 into L-cage.The system contains 192 gas molecules and 1088 H2O molecules. The MD calculation was carried out under constant temperature and pressure conditions (P=0.25 GPa, T=100,250,and 300 K). The characteristic results for sII mixed hydrate are as follows :1. The lattice parameter varies systematically with the CO2 concentration [=CO2/(CH4+CO2)] in S- and L-cages. The concentration of CO2 in S-cage largely affects the lattice parameter.2. Under low temperatures (100 and 250 K), there occurs a preferred orientation of CO2 molecules, which lowers the symmetry of the crystal from cubic to tetragonal.3. The structure with the enrichment of CO2 into L-cage is more stable than the disordered structure, where CH4 and CO2 are evenly distributed to both cages.The system with filled ice structure employed in this study contains 64 gas molecules and 128 H2O molecules. (Filled ice structure has only a kind of site that can contain gas molecules.) The MD calculation was carried out under constant temperature and pressure conditions (P=5 GPa, T=100 and 250 K).This filled ice structure is maintained only with the CO2 concentration less than 20%. In this range, the lattice parameters a, b, and c vary systematically with the CO2 concentration.

将分子动力学方法应用于具有SiI(立方体)和填充冰(正交)结构的CH4-CO2混合水合物，以再现和预测结晶学和热力学性质的组成依赖关系。制备了以下三种混合水合物晶体用于模拟：1.CH4完全分成S笼，CO2完全分成L笼。甲烷和二氧化碳均匀分布在S笼和L笼中(无序结构)。二氧化碳被完全分配到S笼中，甲烷被完全分配到L笼中，该体系包含192个气体分子和1088个水分子。分子动力学计算是在恒温恒压条件下(P=0.25 Gpa，T=100,250,300K)进行的。SiI混合水合物的特征结果如下：1.在S笼和L笼中，晶格参数随CO_2浓度[=CO_2/(CH_4+CO_2)]系统地变化。S笼中CO_2浓度对晶格参数有很大影响。在低温下(100 K和250 K)，出现了CO2分子的择优取向，使晶体的对称性从立方降低到四方。CO2向L笼内富集的结构比CH4和CO2均匀分布在笼内的无序结构更稳定，本研究采用的填充冰结构体系包含气体分子和128H2O分子。(填充的冰结构只有一种位置可以包含气体分子。)MD计算是在恒定的温度和压力条件下(P=5 GPaT=100和250K)进行的，只有当CO2浓度低于20%时，这种填充的冰结构才能保持。在此范围内，晶格参数a、b和c随CO2浓度系统地变化。