Molecular Insights into the Nucleation and Growth of CH4 and CO2 Mixed Hydrates from Microsecond Simulations

Abstract

Hydrates of CH4/CO2 gas mixture are widely involved in hydrate-based applications, such as CH4/CO2 separation and exchange in exploitation of natural gas hydrate resource; nevertheless, their formation mechanisms remain elusive. In this study, microsecond simulations are performed to investigate the nucleation and growth of CH4/CO2 mixed hydrates from two-phase systems of water and CH4/CO2 gas mixture. The simulation results show that CH4-occupied small 512 cages initiates hydrate nucleation in a local liquid phase with a high gas concentration, where CO2 and CH4 cooperatively adsorb to hydrogen-bonded water rings toward hydrate-like ordering and cage formation. The difference in hydrophobicity between CH4 and CO2 affects the stability of nanobubbles of CH4/CO2 gas mixture in water and nucleation rate, and a high content (>75%) of CO2 accelerates nucleation due to its high solubility. The formation kinetics reveals the preferential uptake of CH4 into CH4/CO2 mixed hydrates during nucleation and the transi...