A Molecular Simulation Study on Selective Adsorption of CO₂ from an Industrial Ternary Gas Mixture inside Porous Silica and Kaolinite Adsorbents

The need for energy efficient CO2 capture from industrial flue gases motivated us to study selective uptake of flue gases inside porous silica and kaolinite adsorbents at steam methane reforming (SMR) process conditions using the classical GCMC simulation technique. While CO2 has been competitively adsorbed more efficiently inside the kaolinite slit pores than silica slit pores, CH4 adsorption is only marginally improved inside silica pores as compared to the kaolinite pores. Competitive adsorption of SMR flue gas inside both kaolinite and silica pores at all industrial operating conditions follows: H2O > CO2 > CH4. Interestingly, the selectivity metric (Si,j) pointed out that at a given pressure there prevails a closer CO2 and H2O competitive adsorption inside both adsorbents, while CH4 uptake remained consistently low inside both porous adsorbents for a given flue gas composition. Lower adsorption for CH4 is seen as a combined effect of competition on the available active sites and the adsorption space for CH4 with the simultaneous presence of supercritical CO2 and polar water molecules at vapor state. Present calculations marked strong dependency of Si,j on sorption temperature and feed gas mixture composition given the pore–fluid interactions for a particular adsorption system and that the rise in sorption temperature proportionately increases the equilibrium ratio of mixture state CO2 inside the 31.6 Å silica pore relative to that inside 20 Å silica pore with minimal changes in κ-values of mixture state water vapor. Hence, the combined fluid–fluid and pore–fluid interactions result in drastic variation in competitive water vapor adsorption densities inside a 31.6 Å silica pore at 873.15 K. The κ-value calculations further indicate the comparable hydrophilic nature of both adsorbents; however, higher oxygen and Al surface concentrations on kaolinite pore surfaces resulting in enhanced overall interactions causes SCO2,H2O values to be higher inside kaolinite pores than in silica pores.