Characterization of microscopic nature of methane hydrate decomposition by different molecular models

doi:10.11949/0438-1157.20190684

Abstract

Abstract:

The microscopic nature of methane hydrate decomposition is simulated in NVE ensemble by SPC/E-UA, SPC/E-AA, TIP4P-UA, and TIP4P-AA molecular models. The equilibrium temperature and decomposition heat under different models are calculated and comparatively analyzed. The hydrate-liquid water interface is distinguished by calculating order parameter F₃, and the time-varying and space-varying hydrate-liquid boundary is monitored. The variations of potential energy at different zones are compared which indicate the decomposition process has significant non-equilibrium heat transfer characteristics. The spatial density distribution of hydrate water and methane is contrastively analyzed and the variation of the number of methane molecules that escape from the methane hydrate during decomposition is determined. It s found that the escape velocity decreases gradually with decomposition. The activation energies required for decomposition under different models are calculated and compared with the experimental results. By comparing the mean square displacements and diffusion coefficients in different decomposition regions, it s found that the decomposition exhibits significant non-equilibrium mass transfer characteristics. The study shows that the methane molecular model has little influence on decomposition, while the water molecular model has observable influence. The SPC/E water model can better reproduce the experimental values of equilibrium temperature, activation energy and decomposition heat in methane hydrate decomposition under NVE ensemble.

Key words: molecular simulation, decomposition, hydrate, potential function, interface, equilibrium temperature

摘要：

采用SPC/E-UA、SPC/E-AA、TIP4P-UA和TIP4P-AA分子模型研究了NVE系综下甲烷水合物的微观分解特性。对比分析了不同模型下水合物的平衡温度及分解热。研究了F₃随空间及时间的变化，对液态水-水合物的界面进行了界定。比较了不同区域的势能的变化规律，发现分解过程具有显著的非平衡传热特性。分析了水和甲烷密度的空间分布规律及逸出的甲烷分子数随时间的变化规律，发现甲烷逸出速度逐渐减小。对不同模型下分解所需的活化能进行了计算，并与实验值进行对比。对比不同区域均方位移及扩散系数发现分解呈现显著的非平衡传质特性。研究表明甲烷分子模型对分解影响甚微，而水分子模型影响较为明显，在NVE系综下通过SPC/E模型表征的平衡温度、活化能及分解热与实验值更加吻合。

关键词: 分子模拟, 分解, 水合物, 势函数, 界面, 平衡温度

CLC Number:

O 641

Jia LI, Zhenju LIANG, Zhaoliang WANG, Jian ZHAO, Dawei TANG. Characterization of microscopic nature of methane hydrate decomposition by different molecular models[J]. CIESC Journal, 2020, 71(3): 955-964.

李佳, 梁贞菊, 王照亮, 赵健, 唐大伟. 不同分子模型对甲烷水合物分解微观特性表征[J]. 化工学报, 2020, 71(3): 955-964.

Figures/Tables 9

Fig.1 Initial configuration of simulation system

Table 1 Parameters of water and methane models

模型	原子类型	ε/(kcal/mol)	σ/?	Q^-/e	Q⁺/e
SPC/E	H	0	0	0	0.4328
SPC/E	O	0.1553	3.166	-0.8476	0
TIP4P	H	0	0	0	0.5242
TIP4P	O	0.16275	3.16435	-1.0484	0
TKM-AA	C	0.3261	3.640	-0.560	0
TKM-AA	H	0	0	0	+0.140
OPLS-UA	CH₄	0.29387	3.730	0	0

Fig.2 Equilibrium temperature under different models

Fig.3 F3 distribution along Z axis

Fig.4 Time dependence of F3 in methane hdyrate

Fig.5 Variation of potential energy of single methane molecule with time

Fig.6 System configuration and density distribution

Fig.7 Number of methane molecules fleeing out of methane hydrate as function of time

Fig.8 Mean square displacement of methane at different zone under SPC/E-AA model

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