Molecular Dynamics Simulations of Methane Hydrate Dissociation Under Temperature Step and Ramping

• Main Author: Cueto Duenas, Dianalaura
• Other Authors: Dunn-Rankin, Derek
• Format: Thesis
• Language: English
• Published: eScholarship, University of California 2021
• Subjects: Environmental engineering; Molecular physics; Dissociation; Hydrate; Methane; Molecular Dynamics; Methane hydrate
• Online Access: https://escholarship.org/uc/item/7xz930bq; https://escholarship.org/content/qt7xz930bq/qt7xz930bq.pdf

Methane hydrates are crystalline solids of water that contain methane molecules trapped inside their molecular cavities. Gas hydrates with methane as a guest molecule form Structure I hydrates with a unit cell containing 46 water molecules arranged on 2 small dodecahedral cages and 6 tetra decahedral large cages. An ideal Structure I methane hydrate unit cell contains 8 methane molecules, with one in each cage. Methane molecules are classified according to whether they occupy the large tetra decahedral or the small dodecahedra cells. The influence of occupation and the difference between the behavior of methane release during the dissociation process for the different cage types is the major interest of this work.To assess and analyze the structure evolution during the dissociation of methane hydrates, a series of molecular dynamics simulations using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) is conducted. The dissociation conditions examined include different heating rates at 0.8 TK/s, 4TK/s, 40 TK/s and 400 TK/s, and different temperature increments, ∆T, as steps of 80 K, 85 K, 90 K, 95 K and 100 K above hydrate equilibrium stability conditions for 5 ns. Both simulated systems were first equilibrated at 270 K and 5 MPA. The potential energy of the system, mean-squared displacement (MSD), and the radial distribution function were analyzed to determine the full process of dissociation, temperature changes, molecular diffusive behavior, and structure evolution. Temperature step results showed the earliest dissociation starting 50 ps into the simulation at a ∆T of 100 K, while at a ∆T of 80 K, dissociation was not observed. There was not a clear dissociation preference observed between large and small cages, so it appears that the dissociation affects the entire structure uniformly when temperature increases are applied throughout the system rather than transported from a boundary. Temperature ramping simulations showed that the dissociation temperature increased with an increased ...