Weidmann Arthur B, Franco Luís F M, Sum Amadeu K, Pessôa Filho Pedro A
Universidade de São Paulo (USP), Departamento de Engenharia Química, Escola Politécnica, São Paulo, São Paulo, Brazil.
Universidade Estadual de Campinas (UNICAMP), Faculdade de Engenharia Química, Campinas, São Paulo, Brazil.
J Chem Phys. 2024 Nov 7;161(17). doi: 10.1063/5.0234866.
Molecular simulations are a powerful tool to understand phenomena and obtain properties of gas hydrate systems. The direct coexistence method (DCM) in the NVT or NPT ensembles, the most commonly used method to determine hydrate dissociation temperatures, can be computationally expensive due to the need for several long simulations. Through an extensive set of simulations, we report here the details of the DCM within the NPH (isobaric-isenthalpic) ensemble, which require fewer and shorter trajectories. The dissociation pressure of methane hydrates is obtained for pressures of 4, 8, 15, 30, and 50 MPa. The values are in agreement with other literature simulations and experimental data. The results are further validated with the calculation of the enthalpy of dissociation, with a value of 50 kJ/mol of methane, also in agreement with the literature. The complexity of a multiphase and multicomponent system presents challenges lacking in simpler water/ice systems. These are found to be dependent on energy conservation. The optimal set of parameters to achieve it is also reported, including a smaller time step and the use of double precision, along with an analysis of some factors that could affect the convergence of the method. Although these parameters require more computational cost, the NPH ensemble is successful in providing the dissociation temperature of gas hydrates in fewer simulations than other ensembles and with productions lasting only 500 ns.
分子模拟是理解天然气水合物系统现象并获取其性质的有力工具。在NVT或NPT系综中使用的直接共存法（DCM）是确定水合物分解温度最常用的方法，但由于需要进行多次长时间模拟，计算成本可能很高。通过一系列广泛的模拟，我们在此报告NPH（等压-等焓）系综中DCM的详细情况，该方法所需的轨迹更少且更短。我们获得了4、8、15、30和50MPa压力下甲烷水合物的分解压力。这些值与其他文献模拟和实验数据一致。通过计算分解焓进一步验证了结果，甲烷的分解焓值为50kJ/mol，也与文献一致。多相和多组分系统的复杂性带来了简单水/冰系统中不存在的挑战。发现这些挑战取决于能量守恒。还报告了实现能量守恒的最佳参数集，包括较小的时间步长和使用双精度，以及对一些可能影响该方法收敛性的因素的分析。尽管这些参数需要更多的计算成本，但NPH系综成功地以比其他系综更少的模拟次数提供了天然气水合物的分解温度，且生产时间仅为500纳秒。
