CO2 Separations with KFI Zeolites

T. Remy, S. Van Der Perre, Y. Lorgouilloux, C.E.A. Kirschhock, J.A. Martens, G.V. Baron, J.F.M. Denayer

CO₂ separations with KFI zeolites

Several options exist to separate industrial gas mixtures: cryogenic distillation, membranes, absorption or selective adsorption. Selective separation of CO₂ plays an important role within the current issues about global warming and CO₂ mitigation. Typically CO₂ is being removed from gas streams, via amine stripping which is a process with significant drawbacks. Adsorptive or membrane processes are interesting alternatives for CO₂/CH₄ and CO₂/N₂ separations.

The potential of cation-exchanged KFI zeolites (K-KFI, Li-KFI and Ca-KFI) for these processes has been investigated in this study. The KFI structure consists of a three-dimensional network of two larger a-cages (diameter 11.6 �) and six smaller g-cages (6.6 � x10.8 �) per unit cell. The cavities are connected with each other through eight-membered rings with a diameter of 3.9 �.

Adsorption isotherms of CO₂, CH₄ and N₂ were recorded using the gravimetric technique on K-KFI, Li-KFI and Ca-KFI zeolites at 303 K. CO₂ adsorption isotherms are of type I, with a capacity amounting to 4-4.5 mmol/g depending on the cation present. The uptake measurements indicate much lower affinity for CH₄, with a low diffusion through the small 8-membered windows. Water vapour isotherms measured at 313 K indicate a large affinity for water, with a capacity of about 8.5 mmol/g.

The dynamic separation potential was assessed via breakthrough measurement at 308 K for CO₂/CH₄ and CO₂/N₂  (P = 1 bar) and different compositions in order to record the binary isotherm under these conditions. As a result of the limited diffusion of CH4 and the lower affinity for this component, almost immediate breakthrough of methane is observed. In these dynamic conditions, also CO₂ uptake is hindered by diffusion, resulting in broadened breakthrough profiles. The influence of water was investigated by altering the relative humidity from 0 to 20%.