(552h) Adsorption Equilibria of CO2, N2, and H2 and Binary Mixtures of CO2+N2 and CO2+H2 on ZIF-8 Pellets at Elevated Pressure

Adsorption is a feasible and economical separation process for pre- and post-combustion carbon capture, for which different gas compositions are relevant. For post-combustion capture, typical flue gas consists mostly of N₂ with relative amounts of up to 15% of CO₂. For pre-combustion capture, several different process paths are feasible. One of these paths is the production of blue hydrogen from natural gas through steam reforming, resulting in a hydrogen-rich gas with relative amounts of 20 % of CO₂.

Different adsorbents such as metal organic frameworks (MOFs) or zeolites are considered for the separation. ZIF-8, a prototypical zeolitic imidazolate framework, combines the structural features of zeolites with the coordination chemistry of MOFs, and represents a candidate adsorbent for the above gas mixtures. The sorption properties of ZIF-8 powder have already been reported in the literature over a wide temperature and pressure range [1]. However, the available data set includes only measurements with pure components, while competitive adsorption measurements involving the gas mixtures are still lacking. Moreover, in industrial adsorption columns, powders are not practical as they lead to high pressure drops and produce fine particles. Hence, shaped pellets are preferred and their sorption properties must be investigated as they likely differ from those of powders.

In this presentation, we will present data on unary and binary adsorption equilibria of CO₂, N₂, H₂, CO₂+N₂, and CO₂+H₂ on commercially available ZIF-8 pellets measured on two different instruments, namely a magnetic suspension balance and a dynamic column breakthrough system. Unary isotherms have been measured from (298 to 393) K and pressures from (0.03 to 3) MPa, while binary isotherms using varying gas compositions are reported at 293 K and pressures from (0.1 to 2.8) MPa. For both sets of measurements, a comprehensive analysis of measurement uncertainty is presented, the latter being particularly important for the weakly adsorbing component in the gas mixture. We applied co-adsorption models to estimate the co-adsorption behavior to yield a set of isotherm parameters and their uncertainty. In this endeavor, we also considered models that predict binary data based on the unary adsorption data. The rich experimental data set and the isotherm model parameters represent a useful point of departure for the appraisal of the potential of ZIF-8 in gas separations, particularly for the application of separating CO₂ from H₂ or N₂ in pre- and post-combustion applications, respectively.

References

[1] Hwang J, Azzan H, Pini R, Petit C. H₂, N₂, CO₂, and CH₄ Unary Adsorption Isotherm Measurements at Low and High Pressures on Zeolitic Imidazolate Framework ZIF-8. Journal of Chemical & Engineering Data 2022;67(7):1674-86.