Analysis of three-phase equilibrium conditions for methane hydrate by isometric-isothermal molecular dynamics simulations

Abstract

To develop prediction methods of three-phase equilibrium (coexistence) conditions of methane hydrate by molecular simulations, we examined the use of NVT (isometric-isothermal) molecular dynamics (MD) simulations. NVT MD simulations of coexisting solid hydrate, liquid water, and vapor methane phases were performed at four different temperatures, namely, 285, 290, 295, and 300 K. NVT simulations do not require complex pressure control schemes in multi-phase systems, and the growth or dissociation of the hydrate phase can lead to significant pressure changes in the approach toward equilibrium conditions. We found that the calculated equilibrium pressures tended to be higher than those reported by previous NPT (isobaric-isothermal) simulation studies using the same water model. The deviations of equilibrium conditions from previous simulation studies are mainly attributable to the employed calculation methods of pressure and Lennard-Jones interactions. We monitored the pressure in the methane phase, far from the interfaces with other phases, and confirmed that it was higher than the total pressure of the system calculated by previous studies. This fact clearly highlights the difficulties associated with the pressure calculation and control for multi-phase systems. The treatment of Lennard-Jones interactions without tail corrections in MD simulations also contributes to the overestimation of equilibrium pressure. Although improvements are still required to obtain accurate equilibrium conditions, NVT MD simulations exhibit potential for the prediction of equilibrium conditions of multi-phase systems.

Molecular Dynamics Simulation Study on the Growth of Structure II Nitrogen Hydrate

“…Nevertheless, the direct phase coexistence method by MD simulations exhibits potential for the prediction of three-phase coexistence line of hydrate systems. This methodology has also been used to calculate equilibrium conditions for hydrate-forming system. − …”

A comprehensive review on molecular dynamics simulation studies of phenomena and characteristics associated with clathrate hydrates

“…Therefore, a wide variety of potential models for components has been proposed. As an example, water models to study the characteristics of gas hydrates are: SPC [32][33][34][35][36][37], SPC/E [38][39][40][41][42][43], F-SPC [44], SPC/Fw [45,46], TIPS2 [47], TIP3P [48], TIP4P [4,[49][50][51][52][53][54][55][56][57][58][59], TIP4P/2005 [60][61][62][63][64], TIP4P-Ew [22,26,65], TIP4P/Ice [27,[66][67][68][69][70][71][72][73][74][75][76]…”

“…Second, NVT reduces the number of time steps in simulations. Therefore, it is more suitable to study the phase coexistence of gas hydrates [75]. Moreover, the equilibrium condition can be reached more accurately when the temperature and total energy become constant [408].…”

A Molecular Dynamics Study on Xe/Kr Separation Mechanisms Using Crystal Growth Method

“…They obtained the occupancy of pure Xe molecules in the hydrate cage and observed the different hydrate structure when guest molecule composition changed. Although previous studies have provided many explorations of the methods to obtain hydrate equilibrium data using MD simulations, most of them focus on the simulation of pure gas. ,− These works have made great contributions to the exploration of the molecular behaviors of gas hydrate growth.…”