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A commercial-type activated carbon (AC) and three zeolitic materials, namely, zeolite socony mobil-5 (ZSM-5), Silicalite-1 and SAPO-34, were comparatively studied for single and binary equilibrium adsorptions of CO 2 and CH 4 . The adsorption isotherm data were obtained experimentally by volumetric method at room temperature (30 °C) in the pressure range of 0-10 bar. The isotherms were used to determine the selectivity of the absorbents for CO 2 /CH 4 . The single isotherm data showed AC had the highest adsorption capacity for both CO 2 and CH 4 ; however, the CO 2 /CH 4 ideal selectivity of AC was less than that of the zeolitic materials. These results suggest that AC is a suitable material for adsorption of both CO 2 and CH 4 from flue gas, whereas it would not be suitable for selective separation of CO 2 from CH 4 . The zeolitic materials, namely, ZSM-5, Silicalite-1 and SAPO-34, had good adsorption capacity for CO 2 against CH 4 . The molecular sieving property of SAPO-34 showed higher CO 2 selectivity than that of ZSM-5 and Silicalite-1. Further investigations were conducted on SAPO-34 for binary mixtures of CO 2 -CH 4 with different compositions at various total pressures. The results showed that strong adsorption of CO 2 caused significant decrease in CH 4 adsorption, and therefore, the real selectivity of CO 2 /CH 4 was improved more than the ideal selectivity. Real selectivity of CO 2 /CH 4 at 2.5 bar pressure for an equimolar mixture was 20% more than the ideal selectivity and by increasing total pressure up to 4.5 bar its value improved up to 70% more than the ideal selectivity.

The idea of CO 2 capture was proposed through a multistep vacuum swing adsorption (VSA) process from the tail gas of a hydrogen purification unit with the composition of H 2 , 25%; CO 2 , 55%; CH 4 , 17%; CO, 1%; and N 2 , 2 mol %, using a SAPO-34 molecular sieve. The experimental isotherms and dynamic breakthrough curves were obtained on the lab scale to estimate the equilibrium and kinetic parameters of the adsorption. The VSA process was designed and simulated on the basis of dynamic adsorption−desorption mass and energy balances in the gas and solid phases. Then, the purity of the desorbed CO 2 , the recovery of CO 2 , and the productivity of the process with energy consumption were determined at the cyclic steady state condition. The operational conditions such as weight-hourly space velocity (WHSV), feed pressure, vacuum pressure, and purge to feed ratio were investigated and optimized by the method of fractional factorial design to achieve 90% CO 2 purity with more than 70% recovery. The simulation process was performed with conventional adsorbents of activated carbon (AC) and zeolite 5A and the results revealed the priority of SAPO-34 against AC and 5A for CO 2 enrichment.

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