Replacement micro-mechanism of CH4 hydrate by N2/CO2 mixture revealed by ab initio studies

• CO2 is most likely to replace CH4 occupying large cages.
• N2 is most likely to replace CH4 occupying small cages.
• The substitution of CH4 by CO2 is thermodynamically favorable.
• The substitution of CH4 by N2 is kinetically favorable.

Ab initio calculations and molecular dynamics simulations were performed to investigate the thermodynamic and kinetic feasibility of the replacement of CH4 from its hydrate by N2/CO2 mixture. Substitution of CH4 in small and large cages with N2 and CO2 has negative Gibbs free energy, whereas the substitution by CO2 in small cages is structurally unfavorable. The calculations show that the replacement process should be the substitution with CO2 in large cages followed by N2 in small cages, featuring with the produce of mixed N2-CO2 hydrate. Further, the simulations results indicate N2 exhibits a faster diffusion motion than CO2 in hydrate, implying the substitution by N2 is more kinetically favorable to occur. Our results highlights that the replacement process should include the thermodynamically dominated CO2 substitution simultaneous to the kinetically dominated N2 substitution.

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