The broad breakthrough curves indicated the lower degree of utilization of the adsorption bed, and at the breakthrough points, most of the adsorption capacity has not been utilized [50].
The time to 2% CO2 breakthrough was found to increase as the temperature increased because the chemical adsorption is more favorable at higher temperatures. Before the breakthrough points,
CH4 >98% purity was obtained. The adsorption capacity at the breakthrough point and the saturation capacity of the impregnated AC are reported in Table 4. Under atmospheric pressure, CH4 may marginally compete with CO2 molecules for the adsorption sites on the AC-DEA because of influences of both kinetics (the differences in diffusion rates of different molecules) and thermodynamics (the differences in adsorption capacities of difference molecules at
equilibrium). That is, (1) CH4 (molecular diameter of 3.8 Å) has higher steric hindrance than CO2 (molecular diameter of 3.3 Å), which makes it more difficult for CH4 molecules to diffuse to the active sites (the location of amine groups on the adsorbent surface), and (2) the active sites are specific for CO2 chemisorption via the carbamate-forming reaction.