(43f) Studying Adsorption Equilibria and Kinetics in Zeolite 13X Beads Using in-Situ and Ex-Situ Techniques

Adsorption-based separation is an effective technology to reduce greenhouse gas emissions [1]. This technology is deployed in many industrial processes, including hydrogen production and flue-gas treatment. The performance of the adsorption unit is driven by adsorption equilibria and kinetics of the shaped adsorbents. These adsorbents typically contain chemical and structural heterogeneities across different length scales that result from the formulation and shaping process. Hence, fundamentally, the design of improved shaped adsorbents relies on understanding the relationship between these heterogeneities and the adsorption equilibria and kinetics, over the range of relevant length scales. Classic approaches to characterisation provide average properties and assume homogeneity of the material to infer parameters. An ability to directly probe adsorption kinetics within shaped adsorbents promises to provide spatiotemporal information that can inform on the ideal formulation and shaping of structured adsorbents [2].

In this study we address this knowledge gap by investigating CO₂ adsorption on commercial zeolite 13X beads for partial pressures ranging from 0.05 to 0.40 bar. For both equilibrium and kinetic adsorption analyses, we used ex-situ manometric and in-situ imaging techniques to collect bulk and local information, respectively. We measured CO₂ adsorption equilibrium ex-situ using a commercial gas sorption analyser. To get local information, we complemented this analysis with in-situ X-ray computed tomography (XCT) measurements (equilibrium and kinetics) from a synchrotron source (µm-scale resolution) and a medical-grade scanner (mm-scale resolution) in custom-made experimental cells [3]. We also collected CO₂ adsorption kinetics ex-situ using a zero-length column (ZLC) set-up [4]. To interpret the results, we used a mathematical model that uses the simplified statistical adsorption isotherm model developed by Ruthven [5,6] to describe CO₂ adsorption, while inter- and intra-crystalline diffusion is described with two mass transfer coefficients accounting for: (i) diffusion in the macropores, via Fickian diffusion, and (ii) diffusion in the micropores, via surface diffusion [7].

The equilibrium results obtained with the ex-situ measurements show general agreement with literature [8] in terms of adsorbed amounts and isotherm shapes. The kinetic results show that the diffusion process is mainly controlled by the diffusion in the macropores. For this benchmark adsorbent, this result is also in agreement with the literature [9]. The corresponding data from the synchrotron imaging support these findings by providing information on the internal adsorption process. We observe stronger X-ray attenuation and distinct rate of CO₂ uptake with increasing partial pressure of CO₂. The signal changes were present everywhere in the adsorbent pellet, indicating full use of the adsorbent. The adsorption and desorption curves capture well the expected trend of faster initial uptake/release, followed by long-time tailing. While the quantitative comparison between in-situ and ex-situ measurements is affected by image noise and other artifacts, the results indicate satisfactory agreement.

Overall, relating the ex-situ and in-situ characterisation techniques for shaped adsorbents shows promise. The synchrotron results provide high spatial and temporal resolution, but extracting quantitative information remains challenging. On the other hand, medical-grade scanners, at the cost of a lower spatial resolution, offer quantitative information, while preserving the high temporal resolution and allowing for the use of larger samples, such as packed-bed adsorption columns [10].

[1] Raganati, F., Miccio, F. and Ammendola, P. (2021) Adsorption of carbon dioxide for post-combustion capture: A Review, Energy & Fuels, 35(16), pp. 12845–12868. https://doi.org/10.1021/acs.energyfuels.1c01618

[2] Pini, R. and Joss, L. (2019) See the unseen: Applications of imaging techniques to study adsorption in microporous materials, Current Opinion in Chemical Engineering, 24, pp. 37–44. doi:10.1016/j.coche.2019.01.002.

[3] Joss, L., & Pini, R. (2017). Digital adsorption: 3D imaging of gas adsorption isotherms by X-ray computed tomography. The Journal of Physical Chemistry C, 121(48), 26903–26915. https://doi.org/10.1021/acs.jpcc.7b09836

[4] Azzan, H., Rajagopalan, A. K., L’Hermitte, A., Pini, R., & Petit, C. (2022). Simultaneous estimation of gas adsorption equilibria and kinetics of individual shaped adsorbents. Chemistry of Materials, 34(15), 6671–6686. https://doi.org/10.1021/acs.chemmater.2c01567

[5] Ruthven, D.M. Simple Theoretical Adsorption Isotherm for Zeolites. Nature Physical Science 232, 70–71 (1971). https://doi.org/10.1038/physci232070a0

[6] Ruthven, D.M. and Loughlin, K.F. The effect of crystallite shape and size distribution on diffusion measurements in molecular sieves, Chemical Engineering Science, 26(5), pp. 577–584 (1971). doi:10.1016/0009-2509(71)86002-5.

[7] Ruthven, D.M. (1984) Principles of Adsorption and Adsorption Processes. Chap. 6, John Wiley & Sons, New York.

[8] Hefti, M. et al. (2015) Adsorption equilibrium of binary mixtures of carbon dioxide and nitrogen on zeolites ZSM-5 and 13X, Microporous and Mesoporous Materials, 215. https://doi.org/10.1016/j.micromeso.2015.05.044.

[9] Hu, X., Mangano, E., Friedrich, D. et al. Diffusion mechanism of CO₂ in 13X zeolite beads. Adsorption 20, 121–135 (2014). https://doi.org/10.1007/s10450-013-9554-z

[10] Pini, R., Joss, L. & Hosseinzadeh Hejazi, S.A. Quantitative imaging of gas adsorption equilibrium and dynamics by X-ray computed tomography. Adsorption 27, 801–818 (2021). https://doi.org/10.1007/s10450-020-00268-7