Optimized cesium and potassium ion-exchanged zeolites A and X granules for biogas upgrading

Partially ion-exchanged zeolites A and X binderless granules were evaluated for CO₂ separation from CH₄. The CO₂ adsorption capacity and CO₂-over-CH₄ selectivity of binderless zeolites A and X granules were optimized by partial exchange of cations with K⁺ and Cs⁺, while retaining the mechanical strength of 1.3 MPa and 2 MPa, respectively. Single gas CO₂ and CH₄ adsorption isotherms were recorded on zeolites A and X granules and used to estimate the co-adsorption of CO₂–CH₄ using ideal adsorbed solution theory (IAST). The IAST co-adsorption analysis showed that the partially ion-exchanged binderless zeolites A and X granules had a high CO₂-over-CH₄ selectivity of 1775 and 525 respectively, at 100 kPa and 298 K. Optimally ion-exchanged zeolite X granules retained 97% of CO₂ uptake capacity, 3.8 mmol g⁻¹, after 5 breakthrough adsorption–desorption cycles while for zeolite A ion-exchanged granules the reduction in CO₂ uptake capacity was found to be 18%; CO₂ uptake capacity of 3.4 mmol g⁻¹. The mass transfer analysis of breakthrough experimental data showed that the ion-exchanged zeolite X had offered a higher mass transfer coefficient, (κ) through the adsorption column compared to zeolite A; 0.41 and 0.13 m s⁻¹ for NaK_4.5Cs_0.3X and CaK_2.5Cs_0.2A, respectively.