Decomposition of CH₄ hydrate: effects of temperature and salt from molecular simulations

We report a molecular simulation study to investigate the decomposition of CH₄ hydrate. The decomposition is revealed to be stepwise from the outer to inner layers. Upon decomposition, the number of 5¹²6² cages drops faster than that of 5¹² cages. CH₄ molecules are released, dissolved in water, then enter gas phase; meanwhile, CH₄ bubbles may form particularly at a high temperature. Based on the variations of potential energy, order parameter, cage number and density profile of CH₄ at different temperatures (300, 330, 345 and 360 K) and NaCl concentrations (0, 0.6 and 1.8 M), the effects of temperature and salt are comprehensively examined. With increasing temperature, the decomposition in pure water is accelerated, whereas two opposite effects are observed in NaCl solution. At 330 K, the decomposition is retarded at a higher NaCl concentration, as attributed to the reduced CH₄ solubility in NaCl solution and the participation of ions in cage formation; at 360 K, however, the decomposition is accelerated when NaCl concentration increases due to bubble formation. This simulation study provides microscopic insights into hydrate decomposition, which might be useful towards the optimisation of operating conditions for CH₄ production from CH₄ hydrate.本研究开展了一项分子模拟工作，以探究甲烷（CH₄）水合物的分解过程。研究发现，该分解过程呈现由外层至内层的分步特征。在分解过程中，5¹²6²型笼的数量衰减速度快于5¹²型笼。甲烷分子会先被释放并溶解于水中，随后进入气相；而在高温环境下，更易形成甲烷气泡。本研究基于不同温度（300、330、345与360 K）及氯化钠（NaCl）浓度（0、0.6与1.8 M）下甲烷的势能、序参数、笼数量与密度分布的变化特征，全面考察了温度与盐度对水合物分解的影响。随着温度升高，纯水中的水合物分解速率会加快，但在氯化钠溶液中则呈现出两种截然相反的效应：在330 K条件下，较高的氯化钠浓度会延缓分解过程，这是由于氯化钠溶液中甲烷的溶解度降低，且离子参与了水合物笼的形成；而在360 K时，随着氯化钠浓度升高，气泡的形成会促进水合物分解。本分子模拟工作为水合物分解过程提供了微观层面的认知，可为优化甲烷水合物制备甲烷的工况条件提供参考。