Adsorption equilibria and kinetics of CO₂, CH₄, CO, N₂, O₂, and H₂ on silica-based adsorbents for H₂ enhancement processes from steel off-gas for the direct reduced iron process

The direct reduced iron (DRI) process is highly advantageous for reducing carbon emissions in the steel mill industry. A well-designed adsorption technology can provide H-2 mixtures from H-2-containing off-gases for the DRI process and concentrate CO2 for a carbon capture process simultaneously. The adsorption behaviors of steel off-gas components, CO2, CH4, CO, N-2, O-2, and H-2, were studied at 293, 308, and 323 K and at pressures of up to 1000 kPa on two different silica-based pellets. Their weak adsorption affinity and high CO2 selectivity enables relatively easy desorption without vacuum energy. The CO2 and CH4 isotherms were correlated with the dual-site Langmuir model, whereas the single-site Langmuir model adequately predicted the other isotherms. The order of adsorption capacities of both adsorbents was CO2 >> CH4>CO>N-2 approximate to O-2 >> H-2, and the heat of adsorption of CO2 was notably higher than those of the others. The silica gel with the higher Al content and lower surface area (SG1) showed slightly higher CO2 isotherms up to 100 kPa, whereas the other (SG2) exhibited a greater adsorption capacity at higher pressures. For the other adsorbates, higher isotherms were observed for SG2 at all pressures. In the adsorption kinetic analysis using a non-isothermal adsorption model, the diffusional time constants and kinetic parameters showed a consistent dependence on pressure and temperature. The adsorption rate followed the order of N-2>CH4 >= CO >> CO2. The isotherm and uptake curves for CO2 between the silica gel pellets and particles revealed that macropore diffusion affected the overall adsorption kinetics of CO2 owing to adsorption heat resistance.