CO2 separation and landfill biogas upgrading: A comparison of 4A and 13X zeolite adsorbents

CO2 separation from wet and dry synthetic biogas has been investigated using adsorption on 4A and 13X molecular sieves in dynamic conditions, flowing through fixed beds in an adsorption column connected with an IR detector. The adsorption of CO2 has also been investigated by IR spectroscopy over pressed disks of the zeolites in static dry and wet conditions. 13X molecular sieve has significant adsorption capacity of CO2 from dry biogas, which is further increased by the presence of moisture. This is due to the change of the adsorption mode of CO2 due to the copresence of water. Over wet 13X zeolite adsorption of CO2 mostly occurs in the form of bicarbonate ions interacting with coadsorbed water, while over the dry surface several different kinds of carbonate ions are formed together with molecular adsorbed species. The adsorption capacity of CO2 from biogas is definitely lower over 4A molecular sieve, where coadsorption of methane is also significant. However, regeneration of 4A by purging with nitrogen at r.t. is faster than that of 13X. Over 4A coadsorption of water do modifies the adsorption state of CO2 but has little effect on adsorption capacity.