(392e) Supercritical CO2 Adsorption In Micro- and Mesoporous Carbon

Alternative energy sources with zero carbon emissions represent ultimate solutions to mitigate and stabilize atmospheric CO₂ concentrations. However, technologies are still in development, which are based on efficient and economic generation of electricity from non-carbon-based energy sources. Carbon capture combined with CO₂ sequestration (CCS) offer the possibility to reduce CO₂ emissions in the near future, while allowing for a continued use of fossil fuels. In this context, the injection of CO₂ (or treated flue gas, e.g., mostly CO₂ and N₂) into unmineable coal beds and gas shale reservoirs to enhance natural gas (e.g., CH₄) production is a very attractive option for CO₂ sequestration. Here, the methane that is adsorbed on the coal’s (or shale’s) porous surface is displaced upon CO₂ injection, due to a stronger adsorption of the latter as compared to the former. Additionally, the recovery of the value-added CH₄ is economically attractive as it allows in principle to offset the cost of the CCS operation [1].

CO₂ sequestration in carbon-based geologic formations is not yet a mature technology, in spite of the growing number of pilot and field tests worldwide that have shown its potential and highlighted its difficulties. As a matter of fact, a major obstacle is given by an insufficient understanding of the molecular-scale processes involving CO₂ adsorption in the micro- and mesoporous organic matter at pressures and temperatures representative for reservoir conditions. Current fundamental investigations of gas adsorption in micro- and mesoporous carbon involve the characterization of carbon-based samples by experimental methods, understanding of the electronic structure of functionalized carbon surfaces by density functional theory (DFT), and the thermodynamic property predictions using a Monte Carlo (MC) method within the Grand Canonical ensemble. In the present study, these three aspects are combined; thus, allowing for a more comprehensive characterization of the adsorption process.

The complex pore structures of coal as well as other carbon-based porous materials have frequently been modeled as a collection of independent, non-interconnected slit pores with smooth, homogeneous graphitic walls. The same approach is followed in this study. Density Functional Theory (DFT) calculations with van der Waals-inclusive corrections have been performed to investigate the electronic structure of the graphitic surfaces in addition to the adsorbed phase of molecular CO₂. Grand canonical Monte Carlo (GCMC) is used to connect the electronic properties of the adsorbent/adsorbate system with the macroscopic thermodynamic properties of adsorption. A one-center Lennard-Jones potential model is employed to calculate the interactions between fluid molecules and between fluid molecules and pore walls. Additionally, the TraPPE model that includes three Lennard-Jones potential sites with discrete partial charges is adopted to capture the CO₂ phase behavior more accurately.

The implementation of the GCMC method yields the adsorption isotherms of a given adsorbent-adsorbate interaction in micro- and meso- slit pores: excess adsorption isotherms are predicted and the effects of temperature and pore size are investigated. The simulation results are compared to experimental excess adsorption isotherms of CO₂ on an activated carbon at supercritical and near-critical conditions [2]. To this aim, the continuous experimental pore size distribution is substituted and mimicked by a discrete distribution, and the excess adsorption isotherm is calculated as the weighted average of the excess isotherms obtained in single pores of different size. Useful insights are obtained with respect to the interpretation of the experimental data at both near- and supercritical conditions, and considerations are presented regarding the treatment of the excess adsorption in micro- and mesopores.

References:

[1] Mazzotti M., Pini R., Storti G., Enhanced Coal Bed Methane recovery. Journal of Supercritical Fluids, 2009, 47 (3): 619-627.

[2] Pini R., Ottiger S., Rajendran A., Storti G., Mazzotti M., Reliable measurement of near-critical adsorption by gravimetric method. Adsorption, 2006, 12: 393-403.