Understanding CH4-CO2 Guest Exchange Mechanism between in sI Clathrate: CH4 Recovery and CO2 Storage Opportunities. - Archive ouverte HAL Accéder directement au contenu
Communication Dans Un Congrès Année : 2022

Understanding CH4-CO2 Guest Exchange Mechanism between in sI Clathrate: CH4 Recovery and CO2 Storage Opportunities.

Résumé

CH4–CO2 replacement in the naturally occurring reservoirs of gas hydrates is an attractive alternative for simultaneous energy extraction and CO 2 sequestration. Despite the numerous advantages of the process, the industrial application of that approach is hindered by the lack of a consistent picture about the diffusion mechanism and the kinetics of the process. Reliable data of the possible amount of methane recovery and CO2 storage capacity in hydrate, which are essential for economic feasibility studies, are also missing. Understanding the replacement mechanism requires evaluating the thermodynamic stability of CH4 and CO2 sI clathrates. Using DFT electronic calculations with improved van der Waals density functional (vdW-DF2) allowed to investigate the interactions associated with CH4 and CO2 adsorption in sI clathrate hydrate. We have calculated the optimum total storage capacity of CH4 and CO2 in sI clathrate cages to be 12.3 and up to 42.9 wt%, respectively. First-principles calculations confirmed that while CO2 prefers to bind to 51262 cage, while CH4 tends to be enclathrated in 512 one. In addition to stability, diffusion plays an important role in the direction kinetics of the replacement process. We have investigated all possible scenarios of solid-state diffusion between the outer mixed hydrate phase layer and pure methane core. Our calculation showed that the isostructural replacement occurs mainly in the large cage (512 62) via ”double occupation mechanism,” which facilitates both CO2 and CH4 diffusion through the large cage channel. The diffusion energy barrier of CH4 and CO2 via such a mechanism is 0.525 and 0.307 eV, respectively, which is significantly lower compared to 1.066 and 0.416 eV proposed by the ”hole-in-cage-wall” scenario. On the othe hand, we found that the replacement process is less probable in small cages in the case of pure CH4–CO2 exchange. In such a scenario, methane can hardly diffuse out of 512 cages via neighboring empty cages and the presence of water vacancies. The estimated diffusion energy barrier of methane between empty small and large cages is 1.156 eV. However, the replacing CO2 molecule can easily diffuse in the small cage benefiting from double occupation with a significantly lower diffusion energy barrier of 0.725 eV compared to 1.133 eV proposed by the ”hole-in-cage-wall” scenario. Finally, we examined the replacement under finite temperatures and realistic pressure conditions and calculated Gibbs free energies and equilibrium constants confirmed the feasibility of the reaction from both thermodynamic and kinetics points of view. The proposed mechanism and results agrees well with experimental observations and can be a basis of diffusion kinetics studies and estimation the economic feasibility of the replacement process.
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Dates et versions

hal-04251813 , version 1 (20-10-2023)

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  • HAL Id : hal-04251813 , version 1

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Ahmed Omran, Nikolay Nesterenko, Valentin Valtchev. Understanding CH4-CO2 Guest Exchange Mechanism between in sI Clathrate: CH4 Recovery and CO2 Storage Opportunities.. Les Journées « HYDRATES 2022 », GDR2026 Hydrates de gaz, Nov 2022, Paris, France. ⟨hal-04251813⟩
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