(454d) Integrating Material and Process Design for Kinetic Separation

Although equilibrium adsorptive separation has become one of the commonly used industrial techniques for gas and liquid separation, kinetic adsorptive separation has not received equal attention. The first known commercial application of kinetic separation is oxygen and nitrogen using Carbon Molecular Sieve ^[1]. Several recent studies have focused on biogas purification (CH₄/N₂) using kinetic separation ^{[2, 3]}. Recently with the development of highly tunable MOFs and ZIFs there is a great potential to exploit kinetic separation for difficult mixtures such as propane and propylene^[4]_.

Kinetic separation exploits the differences in diffusion coefficients of the different adsorbents in the mixture irrespective of their equilibrium loading amounts. This may even result in the selective adsorption of the weakly adsorbing component for a certain duration of time^[5]. Using the traditional Fickian model for gas diffusion is however not adequate for predicting this transient overshoot. We have used the Maxwell-Stefan diffusion equation which has been proposed and extensively validated with crystal diffusion experiments by Krishna et al ^[5]. A few past studies have attempted to screen materials and subsequently develop PSA cycles for kinetic separations incorporating this more accurate description of crystal diffusion.

There is, however, no straight forward method of screening adsorbents for kinetic separation from diffusion coefficients and isotherm data alone. This is mostly due to the complex coupling between the kinetics and equilibrium. This talk will present a parametric study of the influence of isotherm shape and diffusion coefficient on the uptake within a single crystal and light component transient overshoot. This will be used to screen materials which have better potential for kinetic separation. The selected adsorbents will then be incorporated into a PSA cycle to determine the effectiveness of the primary screening using crystal level data. Our goal is a systematic method to co-design materials and adsorption cycles for kinetic adsorption processes.

References

1. Kapoor, A., & Yang, R. T. (1989). Chem. Eng. Sci., 44(8), 1723-1733.
2. Erden, L., Ebner, A. D., & Ritter, J. A. (2018). Energy Fuels, 32(3), 3488-3498
3. Effendy, S., Xu, C., & Farooq, S. (2017). Ind. Eng. Chem. Res., 56(18), 5417-5431.
4. Pimentel, B. R., & Lively, R. P. (2016). Ind. Eng. Chem. Res., 55(48), 12467-12476.
5. Krishna, R., & van Baten, J. M. (2018). Microporous Mesoporous Mater., 258, 151-169.