Evolution of bubbles in decomposition and replacement process of methane hydrate

Abstract

Nanobubbles have been proved to play an important role for the extraction kinetics of CH₄ hydrates. Existing investigations on nanobubbles mainly focus on the hydrate melting. However, the mechanism and effect of bubble evolution during the replacement of CH₄ hydrates by CO₂ are still elusive. Therefore, molecular dynamics simulations on melting and replacement of CH₄ hydrate are performed with the same initial system size, pressure and amount of CH₄ hydrates. It is proved the two processes move layer by layer but with different features. Moreover, despite the different composition of gas phases, behaviours of nanobubbles are similar during the two processes except that the gas film is not formed during the replacement. In addition, a quantitative analysis of molecules in each nanobubble during the decomposition is conducted preliminarily. Also a remarkable effect of mixed nanobubble on the replacement is revealed based on the analysis of bubble behaviour. It is found that CH₄ molecules are surrounded by some dense CO₂ molecules in the mixed bubble during the replacement. Moreover, CO₂ molecules in the solution can assist the bubble formation of CH₄ molecules at an initial stage and prevent their further escape and reoccupation by encompassing them afterwards.

Keywords:

• Molecular dynamics simulation
• methane hydrate
• bubble
• replacement
• carbon dioxide

Acknowledgements

The authors thank Dr. Saman Alavi from University of British Columbia, Dr. Rajnish Kumar from National Chemical Laboratory of India, and Prof. Shiang-Tai Lin from National Taiwan University for valuable discussions. Calculations were performed on the National Supercomputer Center in Guangzhou, China.