Molecular dynamics simulation of methane hydrates: Prediction of the phase equilibria using extracted microscopic parameters from SAFT-VR Mie EOS

Abstract

This paper explores the three-phase equilibrium of structure-I methane hydrate by utilizing the extracted Mie potential parameters for non-bonded molecular interactions in the LAMMPS molecular dynamics simulator. An in-house code was developed to numerically solve the SAFT-VR Mie equation of state (EOS). To reduce numerical errors, unlike common practices, we incorporated all partial derivatives analytically in the code. The microscopic simulation parameters for methane and water molecules were selected so that saturation pressure and liquid density data from EOS were fitted on experimental data. Using this approach, the three-phase (V−Lw−H) equilibrium temperatures for pressure values of 30, 70, 100, and 300 bar were calculated. The solubility of methane in the aqueous phase at three-phase equilibrium was also determined. The results were validated against available laboratory data. To evaluate the performance of Mie potential parameters, the MD simulations were repeated using three well-known force fields for methane molecules, namely, Optimized Potentials for Liquid Simulations-United Atom (OPLS-UA), Universal force field (UFF), and General Amber force field (GAFF). The TIP4P/Ice force field was used for water molecules in all cases. The comparison with experimental data reveals that while the accuracy of the UFF force field is the highest, our simulation approach outperforms OPLS-UA and GAFF force fields in predicting methane solubility in the aqueous phase. Besides molecular dynamics simulation, the van der Waals-Platteeuw model was combined with the SAFT VR Mie EOS to account for the hydrate phase. In our case study, the SAFT-VR Mie + vdW-P model proved to be more accurate in predicting the three-phase equilibrium data compared to the MD approach.