Carbon dioxide adsorption on nitrogen-enriched gel beads from calcined eggshell/sodium alginate natural composite

Bio-degradable calcined egg-shell/sodium alginate (CES/SA) beads were prepared and functionalized using aqueous ammonia (33%). The tailor-made beads were then used as adsorbent for CO₂ capture in a fixed-bed reactor. The performance of the prepared beads was evaluated under various experimental conditions (i.e. 1 < P < 2.5 bar, 30 < T < 50 °C, 50 < flow rate < 90 ml/min and an inlet CO₂ concentration between 20 to 45 vol%). Modification with ammonia could successfully introduce additional functional groups containing nitrogen onto the surface of the CES/SA (up to 10.56 wt%). The results showed an increase in CO₂ adsorption with ammonia-impregnated CES/SA, compared to the non-impregnated CES/SA. A maximum CO₂ adsorption capacity of 0.2380 mmol/g was obtained for the gel beads with 45% CO₂ concentration at 1 bar and 30 °C. The adsorption capacity decreased with an increase in temperature and increased with an elevation in the inlet CO₂ concentration, pressure and flow rate. Among the investigated isotherm models (i.e. Sips, Freundlich and Toth), the Toth isotherm model best described the adsorption data. The thermodynamic properties using the adsorption isotherm data revealed the entropy change (ΔS > 0) reflected the affinity of the adsorbent with the CO₂ molecules. The CO₂ adsorption/desorption process indicated a drop in the CO₂ adsorption capacity of the gel beads in the second cycle; however, this almost remained constant in the subsequent cycles. This observation pointed out to a chemisorption process for the fresh adsorbent and a physical adsorption mechanism for the subsequent adsorption cycles.