(647d) On the Use of Absolute and Excess Isotherms in Adsorption Processes: H2 PSA As a Case Study.

The vast majority of adsorption isotherms in literature are reported as excess isotherms. This is generally due to the fact that the skeletal solid volume required to calculate the excess isotherms can be measured relative to helium, leading often to define excess adsorption as the only directly measurable quantity¹. Molecular simulations, process modelling and any thermodynamically consistent model, on the other hand, require the use of absolute isotherms^2,3.

The perceived advantage in measuring and reporting excess isotherms has driven the development of adsorption dynamics models where the mass balance is adapted to allow the use the parameters resulting from the direct fit of the excess isotherms.

This study explores the implications of using this approach compared to the rigorous one where a thermodynamically consistent model based on the absolute adsorbed amount is used. We apply both methods to the case study of a H₂ PSA from steam methane reforming as this is one of the separations where the distinction between absolute and excess adsorbed amounts is significant². Production of ultrapure H₂ (>99.9% mol) is in fact a very challenging separation where model accuracy is key for the reliability of process predictions⁴.

All fundamental adsorbent quantities (porosities, densities) as well as equilibrium isotherms were measured experimentally in our laboratory to minimize the uncertainty of the model comparisons. High pressure isotherms for H₂, CO₂, CH₄ and N₂ on the NIST NaY reference material RM8850^5,6 were measured using our custom-made Adsorption Differential Volumetric Apparatus (ADVA). The resulting excess and absolute adsorption isotherms at different temperatures, fitted using a Langmuir model, were used in the dynamic simulation of the H₂ PSA process. In addition, high-pressure binary H₂/CO₂ breakthrough experiments were carried out to validate the accuracy of the model predictions. Finally, full process simulations were implemented to compare the two methodologies in terms of the predicted purity and recovery for the separation.

[1] S. Sircar, Gibbsian Surface Excess for Gas Adsorption - Revisited, Ind. Eng. Chem. Res. 38 (1999) 3670–3682. https://doi.org/10.1021/ie9900871.

[2] S. Brandani, et al., Net, excess and absolute adsorption and adsorption of helium, Adsorption. 22 (2016) 261–276. https://doi.org/10.1007/s10450-016-9766-0.

[3] A.L. Myers, P.A. Monson, Physical adsorption of gases: the case for absolute adsorption as the basis for thermodynamic analysis, Adsorption. 20 (2014) 591–622. https://doi.org/10.1007/S10450-014-9604-1.

[4] M. Luberti, et al., Design of a H₂ PSA for cogeneration of ultrapure hydrogen and power at an advanced integrated gasification combined cycle with pre-combustion capture, Adsorption. 20 (2014) 511–524. https://doi.org/10.1007/S10450-013-9598-0.

[5] H. Giang, et al., Porosity, Powder X-Ray Diffraction Patterns, Skeletal Density, and Thermal Stability of NIST Zeolitic Reference Materials RM 8850, RM 8851, and RM 8852, 126 (2021). https://doi.org/10.6028/jres.126.047.

[6] H.G.T. Nguyen, et al., A reference high-pressure CH₄ adsorption isotherm for zeolite Y: results of an interlaboratory study, Adsorption. 26 (2020) 1253–1266. https://doi.org/10.1007/s10450-020-00253-0.