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CO2置换水合天然气和氢气水合物稳定机理的分子模拟
其他题名Molecular simulation on the CO2 replacement of natural gas and stabilization of hydrogen hydrate
耿春宇
学位类型博士
导师温浩
2010-02-09
学位授予单位中国科学院研究生院
学位专业应用化学
关键词甲烷水合物 置换开采 氢气水合物
摘要天然气水合物储量丰富,是一种重要的潜在后继新能源。依此,本论文采用分子模拟的方法,对CO2置换甲烷水合物的过程及置换产物的稳定性进行研究,并进一步对水合物法储氢中氢气分解过程及其稳定性进行研究。主要研究工作及成果如下: (1) 通过量子化学和分子动力学模拟,计算CH4和CO2分子在SI型水合物晶穴中的稳定化能,并比较SI型甲烷水合物、二氧化碳水合物及不同占据方式的甲烷-二氧化碳混合气体水合物的稳定性,发现CH4分子适合存在于512晶穴,CO2分子适合存在于51262晶穴,甲烷-二氧化碳混合气体水合物(CH4占据水合物512晶穴,CO2占据水合物51262晶穴)的稳定性最好。从稳定性的观点可以看出,所得到的置换产物更可能为CH4分子占据512晶穴、CO2分子占据51262晶穴的甲烷-二氧化碳混合气体水合物。 (2) 通过分子动力学模拟从微观分子水平模拟CO2置换甲烷水合物的过程发现,H2O分子氢键网络结构的破坏导致甲烷水合物的分解,并且氢键网络结构的破坏由水合物外层向内层逐渐推进,当甲烷水合物的氢键网络结构被完全破坏后,CH4分子在水中聚集,形成H2O分子状态介于水合物和液态水之间的“中间相”,置换过程在中间相完成。在甲烷-二氧化碳混合气体水合物形成初期,气体分子周围的H2O分子聚集并形成水合物的氢键网络结构,气体分子与H2O分子形成的晶穴逐渐减小,H2O分子扩散逐渐减弱,形成水合物的晶核。相对于CO2分子,CH4分子在水合物形成初期更易生成水合物。 (3) 模拟了氢气水合物的分解过程,并比较氢气水合物与氢气-四氢呋喃双分子水合物的稳定性。研究发现,H2分子的扩散是影响氢气水合物稳定性的重要的因素。由于H2分子的扩散运动使水合物晶格的稳定性下降,促使晶胞尺寸的进一步增大和水合物的分解。由于四氢呋喃分子不能出现在晶穴间的扩散运动,降低了H2分子在晶穴间扩散运动的概率,从而增强了水合物的稳定性。
其他摘要Natural gas hydrate (NGH) is expected to be an important potential future energy source for its large amount below the ocean floor. Accordingly, in this dissertation, the replacement of methane hydrate by CO2 and the stability for its product have been studied by means of molecular simulation. Furthermore, the decomposition and stability of hydrogen hydrate have also been considered. The main content and contributions of this dissertation are shown in the following: (1) The stabilization energies of small and large cavities occupied by CH4 and CO2 have been calculated and the stabilities of methane hydrate, carbon dioxide hydrate and methane-carbon dioxide mixed hydrate with different occupation have also been compared. The results reveal that the CH4 molecule is suitable for the small cavity while CO2 molecule is suitable for the large cavity. Therefore, the methane-carbon dioxide mixed hydrate (where CH4 molecules are encaged in small cavities and CO2 molecules in large cavities) is the most stable one in all above hydrates. Accordingly, the most appropriate product during the replacement is the methane-carbon dioxide mixed hydrate where CH4 molecules are encaged in small cavities and CO2 molecules in large cavities, from the hydrate stability point of view. (2) The replacement of methane hydrate by CO2 has been performed by molecular dynamics. During the replacement, the hydrogen-bond structure of methane hydrate is firstly broken from the outer layer to the inner layer. Then the methane molecules aggregate together in the water, and form a mesophase in which the state of H2O molecules is between the hydrate and liquid water. The replacement is completed in the mesophase. While in the early stage of clathrate formation, clathrate-like cavities are formed with the aggregation of the water molecules around the gas molecules. With the reducing of clathrate-like cavities size and the weaking of the diffusion of H2O molecules, the crystal nuclears will be formed. Comparing with CO2 molecules, CH4 molecules are easily formed in the early stage of hydrate formation. (3) Molecular dynamics simulation is also used to study the decomposition and stability of SII hydrogen and hydrogen-THF hydrate. The results reveal that the diffusion of H2 molecules is an important factor for weakening the stability of hydrogen hydrate. The increasing of cell size and the decomposition of hydrogen hydrate are led by the diffusive behavior of H2 molecules. It is also found that THF molecule cannot move in the hydrate cavity unless the cavity destroying takes place, therefore, the resistance of the diffusion behavior of H2 molecules can be enhanced by encaging THF molecules in (51264) cavities. Accordingly, THF is, as a stabilizer, helpful in increasing the stability of hydrogen hydrate.
页数121
语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/1634
专题研究所(批量导入)
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耿春宇. CO2置换水合天然气和氢气水合物稳定机理的分子模拟[D]. 中国科学院研究生院,2010.
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