Molecular Simulations Revealing Adsorbent Heterogeneity-Induced Changes in Adsorption and Nonideal Behavior of Captured CO2 from Industrial Flue Gases with Varying Moisture.

Abstract

Selective CO2 capture from dry blast furnace gas (BFG) and moisture-varying steam methane reforming (SMR) flue gases using six heterogeneous silica adsorbents of comparable porosity, viz., StH and EnH, is quantified via classical molecular simulations. In these adsorbents, CO2-philicity from pure and mixture feeds is enhanced (compared to the previously shown silica slit pores) due to the heterogeneity-induced higher surface oxygen-to-surface-modified groups (-OH or O-Na+) ratio at pore surfaces, thereby increasing adsorbent selectivity for CO2 by decreasing the CO2 thermodynamic equilibrium ratio. In addition to increased CO2-philicity of StH and EnH adsorbents, CO2 adsorption in EnH is governed by combined "gating" and "crowding" effects depending on adsorption T and P, feed gas compositions, Si content, and sodium concentration near pore surfaces. However, the presence of water in the SMR mixture leads to competitive adsorption in the pores of StH and EnH, in contrast to dry BFG flue gas adsorption, hence making surface-CO2 interactions more attractive in the case of BFG adsorption than SMR adsorption. A closer arrangement of adsorbed CO2 near the pore surfaces and cluster formations of adsorbed water molecules have been observed in all adsorbents. Non-monotonic adsorption depends on adsorbate-adsorbate-adsorbent, adsorbate-adsorbate, and adsorbent-adsorbate interactions for pure CO2, whereas the latter two interactions dominate in defining H2O adsorption behaviour in all adsorbents. Spreading pressures (Π) are shown to vary with T, P, molar adsorbed area (A'), and excluded molar area (b'), with A'-b' predominantly controlling positive Π variations. The estimated activity coefficient indicates nonideal behaviour of selectively adsorbed CO2 from BFG and SMR mixtures, with the activity coefficient ratio being inversely proportional to selectivity.