English Niall J, Johnson J K, Taylor Charles E
U.S. Department of Energy, National Energy Technology Laboratory, P.O. Box 10940, Pittsburgh, Pennsylvania 15236, USA.
J Chem Phys. 2005 Dec 22;123(24):244503. doi: 10.1063/1.2138697.
Nonequilibrium molecular-dynamics simulations have been carried out at 276.65 K and 68 bar for the dissolution of spherical methane hydrate crystallites surrounded by a liquid phase. The liquid was composed of pure water or a water-methane mixture ranging in methane composition from 50% to 100% of the corresponding theoretical maximum for the hydrate and ranged in size from about 1600 to 2200 water molecules. Four different crystallites ranging in size from 115 to 230 water molecules were used in the two-phase systems; the nanocrystals were either empty or had a methane occupation from 80% to 100% of the theoretical maximum. The crystal-liquid systems were prepared in two distinct ways, involving constrained melting of a bulk hydrate system or implantation of the crystallite into a separate liquid phase. The breakup rates were very similar for the four different crystal sizes investigated. The method of system preparation was not found to affect the eventual dissociation rates, despite a lag time of approximately 70 ps associated with relaxation of the liquid interfacial layer in the constrained melting approach. The dissolution rates were not affected substantially by methane occupation of the hydrate phase in the 80%-100% range. In contrast, empty hydrate clusters were found to break up significantly more quickly. Our simulations indicate that the diffusion of methane molecules to the surrounding liquid layer from the crystal surface appears to be the rate-controlling step in hydrate breakup. Increasing the size of the liquid phase was found to reduce the initial delay in breakup. We have compared breakup rates computed using different long-range electrostatic methods. Use of the Ewald, minimum image, and spherical cut-off techniques led to more rapid dissociation relative to the Lekner method.
在276.65 K和68巴的条件下，进行了非平衡分子动力学模拟，以研究被液相包围的球形甲烷水合物微晶的溶解过程。液相由纯水或甲烷 - 水混合物组成，甲烷含量范围为水合物相应理论最大值的50%至100%，液相大小约为1600至2200个水分子。在两相系统中使用了四种不同大小的微晶，其大小范围为115至230个水分子；这些纳米晶体要么是空的，要么甲烷占有率为理论最大值的80%至100%。晶体 - 液体系统通过两种不同的方式制备，一种是对大块水合物系统进行受限熔化，另一种是将微晶植入单独的液相中。对于所研究的四种不同晶体大小，其分解速率非常相似。尽管在受限熔化方法中与液体界面层松弛相关的延迟时间约为70皮秒，但未发现系统制备方法会影响最终的解离速率。水合物相中甲烷占有率在80% - 100%范围内时，溶解速率基本不受影响。相比之下，发现空的水合物簇分解得明显更快。我们的模拟表明，甲烷分子从晶体表面扩散到周围液体层似乎是水合物分解的速率控制步骤。发现增加液相大小可减少分解的初始延迟。我们比较了使用不同长程静电方法计算的分解速率。相对于Lekner方法，使用埃瓦尔德（Ewald）、最小镜像和球形截断技术会导致更快的解离。
