CO2 adsorption over modified AC samples: A new methodology for determining selectivity

Activated carbon (AC) based adsorbents having high and stable CO2 adsorption capacity with enhanced CO2 selectivity in presence of CH4 were developed. Alkali modified AC samples were prepared, their CO2 adsorption capacities were measured, a new methodology for selective adsorption capacity determination under multicomponent gas mixture flow was developed, and the results were analyzed to determine the preparation procedure yielding optimum adsorbent design. Two groups of adsorbents were prepared by K2CO3 impregnation on air and HNO3 oxidized forms of a commercial AC followed by calcination at various temperatures. The resulting adsorbents were named according to calcination temperatures as ACxK-calT. The highest CO2 adsorption capacity was measured on AC3K-300 sample as 110 mg/g adsorbent at 1000 mbar CO2 and 25 degrees C. CO2 adsorption was confirmed reversible, whereas CH4 adsorption was found partially irreversible. The highest mass based CO2:CH4 selectivity, ca. 3.7, was achieved over AC2K-200 at 25 degrees C for the 50% CO2-50% CH4 mixture. AC2K-200 was further tested at higher total pressures, for 0-5000 mbar pressure range, at 25 degrees C. CO2 adsorption capacity was measured as 197 mg/g adsorbent at 5000 mbar CO2. Among Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models, D-R was found to be the most successful one explaining CO2 adsorption behavior of AC samples.