以分子動力學模擬探討甲烷水合物之自我保護效應

Exploring Self-Preservation in Methane Hydrate Dissociation via Molecular Dynamics Simulation

趙紘毅(Hung-I Chao)

指導教授 ： 林祥泰

國立臺灣大學/工學院/化學工程學研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.00807

摘要

甲烷水合物是一種由水與甲烷氣體在低溫高壓之下形成之晶形化合物，水分子會形成籠狀結構抓住甲烷。由於其內部有甲烷，所以也被視為未來可能之替代能源選擇。在此研究中，我們使用分子動力學模擬來測量甲烷水合物的熱力學性質以及觀察自我保護效應。我們使用TIP4P-Ice 做為水分子新的力場，並檢測了熔解溫度曲線和熔解熱來確定新力場是否能準確的預測這些熱力學性質。而自我保護現象是描述在甲烷水合物相不穩定而冰相穩定之溫度區間內，例如常壓下，溫度242K 至 271 K 之間有異常緩慢的熔解速率。有研究提出這可能是因為熔解的甲烷水
合物其表面形成一層薄薄的非晶相的冰，這層冰抑制了甲烷水合物進一步的熔解。
在本研究中，我們的結果支持TIP4P-Ice 力場在估算甲烷水合物的熔解溫度曲線和熔解熱上有不錯的表現。而我們對自我保護現象的模擬，可以觀察到在我們的系統中255 K 至265 K 的溫度區間內也發生異常緩慢的熔解速率。藉由對系統的密度分析、F4 order parameter、ring 的分析、diffusivity 的分析，我們發現在熔解速率的低點，其熔解的表面會形成平坦的密度峰值且其F4 值會維持在0.4 左右而垂直Z 軸的diffusivity 會較XY 平面上的diffusivity 低兩個數量級。這或許是外層水分子會阻礙水合物熔解的結構性質。根據這些結果，表示分子動力學模擬可以定性的描述甲烷水合物的熔解溫度曲線以及熔解熱。而且能在分子模擬中再現甲烷水合物熔解時所發生的自我保護現象之趨勢，但更深入的機制探討仍待更進一步的研究。

關鍵字

甲烷水合物 ； 熔解 ； 自我保護現象 ； 分子動力學模擬

並列摘要

Methane hydrate is a nonstoichiometric crystalline compound composed of water and methane at low temperatures and high pressures. Molecular Dynamics (MD) simulation has been a simulation tool used to unveil the molecular level details of methane hydrate, in this research, we use MD simulation to examine two important thermodynamic properties of methane hydrates, dissociation temperature and heat of
dissociation, and its self-preservation phenomenon simulated on the basis of the combination of Tip4p-Ice force field for water and OPLS-AA for methane. The self-preservation phenomenon shows anomalously slow methane dissociation rate on 242 K to 271 K. Self-preservation phenomenon was probably caused by amorphous quasi-liquid layer on the surface of dissociated methane hydrate decomposed from methane hydrate.
Our results supported that the TIP4P-Ice and OPLS-AA force fields could reproduce good evaluation comparing with the experimental dissociation temperatures and heat of dissociation of methane hydrate. We discovered that there was also an anomalously slow dissociation rate on 255 K to 265 K in this research, and there is flat density peak on the surface showing in density profile, and the F4 value of surface water drop down to about 0.4, and the parallel diffusivity is smaller than vertical diffusivity of surface for two degree. These structural properties might result in anomalously slow dissociation rate.
Molecular dynamics simulation can evaluate key thermodynamic properties of methane hydrate with qualitatively accuracy, and also reproduce the similar trend of self-preservation
during dissociating process, but the mechanism still need further study.

並列關鍵字

methane hydrate ； dissociation ； self-preservation ； molecular dynamics simulation

參考文獻

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