Evaluation Methods of Adsorbents for Air Purification and Gas Separation at Low Concentration: Case Studies on Xenon and Krypton

Abstract

The development of gas separation processes dealing with very low concentration ranges is a rapidly growing domain with key applications such as trace detection, air purification from harmful pollutants, etc. Yet, the design of efficient technologies in this field is hampered by the lack of robust strategies to predict the gas selectivity of optimal adsorbents from simple pure gas adsorption data. Here, the selectivity predicted using different methods, namely Henry's method and the ideal adsorbed solution theory (IAST), are compared with the true selectivity obtained using breakthrough experiments. As a case study, these methods are discussed when applied to Xe/Kr separation in two different process conditions using different adsorbents (an active carbon and two silver-doped adsorbents). Typical data show that Henry's method, in which selectivity is assumed to correspond to the ratio of Henry's constant measured for each gas of the mixture, should be considered with caution as it is very sensitive to the pressure range considered but also the number of points used for affinity assessment. IAST is found to be more accurate provided its applicability to predict gas coadsorption from pure gas adsorption data is first established. However, even when applicable, the case of very low concentrations remains a problem as it leads to very large uncertainties in the selectivity predicted using IAST. We discuss how typical errors in assessing the selectivities using the different methods lead to nonoptimal adsorbent choices for a given separation process. Finally we demonstrate that Agloaded zeolite shows xenon capacities and Xe/Kr selectivities that surpass all other materials.

Thermodynamic exploration of xenon/krypton separation based on a high-throughput screening

“…Very recently, Daglar et al 50 reviewed high-throughput computational screening of MOF materials for gas separations, showing that most studies in the field -especially those at larger scale -have focused primarily on thermodynamic data based on single-component properties, and in particular low-pressure selectivity and saturation uptake. In a series of case studies on the Xe/Kr separation at low concentration, Montpezat et al 51 demonstrated the important limitations of relying on the IAST model and approaches based on the Henry's constants, especially for very low concentrations. This is part of what prompted us to look further into the thermodynamics of Xe/Kr separation, in a systematic manner.…”

High‐Silica CHA Zeolite Membrane with Ultra‐High Selectivity and Irradiation Stability for Krypton/Xenon Separation

“…High‐silica CHA is more adsorption selective for Xe over Kr, evidenced by the higher adsorption heat of Xe over Kr (Figure S3a). This can be well explained by the confinement effect [29] and the stronger interaction [30] (thermodynamic control). An unfavorable (high) Xe‐adsorption selectivity would compromise a Kr/Xe diffusion selectivity so that current polycrystalline membranes exhibit a low or moderate Kr/Xe selectivity (0.98–45, Table S1) [16, 31] .…”

Curating Metal-Organic Frameworks to Compose Robust Gas Sensor Arrays in Dilute Conditions

“…1c and used for our demonstration in Sec. 3 are subject to error emanating from (a) our manual extraction of adsorption data from plots found in the literature, (b) the lack of adsorption data at extremely low pressures (i.e., possibly inappropriate fitting to adsorption data that is outside the Henry regime), (c) the number of data points we used in the fitting procedure, 119 (d) slight differences in temperature at which the adsorption was measured. Third, our demonstration assumed that the gas composition space was two-dimensional-i.e., that no gases other than CO 2 and SO 2 adsorb in the MOF.…”