Comparative study of biogas treatment in dry and wet conditions using modified activated carbons of CNR-115 family

This study investigates the effect of water vapor on the adsorption performance of modified activated carbons (ACs) from the CNR-115 family (CNR-115-ox and CNR-115-ox-am), derived from the commercial AC (CNR-115), for CO${}_2$/CH${}_4$ separation. Breakthrough curve analysis at 0.1 MPa and 303 K showed that CO${}_2$ consistently exhibited higher breakthrough and saturation times than CH${}_4$, indicating stronger interactions with adsorbents of the CNR-115 family. The presence of water vapor in experiments with a 39%/59%/2% (vol.) CO${}_2$/CH${}_4$/H${}_2$O mixture significantly impacted CO${}_2$ adsorption, enhancing its retention and selectivity. Under dry conditions, CNR-115-ox demonstrated higher separation efficiency due to its large BET surface area, while CNR-115-ox-am showed increased CO${}_2$ adsorption capacities and selectivity under wet conditions. This improvement in performance under moist conditions is likely due to bicarbonate formation, which promotes CO${}_2$ retention. Moreover, experiments under wet conditions revealed a roll-up effect for CH${}_4$ and a slight roll-up for CO${}_2$, caused by co-adsorption of H${}_2$O. The findings emphasize the importance of considering water vapor in biogas upgrading processes to optimize adsorbent design and improve CO${}_2$ separation efficiency. Furthermore, the increased CO₂ adsorption per unit area under both dry and wet conditions for CNR-115-ox-am, despite its lower BET surface, underscores the dominant influence of surface chemistry over porosity in gas adsorption. This improvement, driven by nitrogen functionalization, enhances both selectivity and adsorption efficiency, making CNR-115-ox-am a promising material for biogas upgrading applications.