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.