Dynamic Adsorption Separation of Low Concentration Xenon/Krypton Mixtures on HKUST-1 Pellets

Abstract

Nuclear data are used to describe the properties of the nucleus and the reactions of the nucleus with other particles. The accuracy and reliability of nuclear data play important roles in nuclear fundamental research and nuclear technology application. The radioactive noble gas Xe and Kr are usually used for neutron activation analysis of nuclear materials. However, it is necessary to separate radioactive Xe from Kr because of the interference of radioactive Kr isotopes during nuclear data measurement. Furthermore, separating Kr from Xe is an important step in removing long half-life radioactive 85Kr during spent nuclear fuel treatment. Therefore, separation of Xe and Kr from Xe/Kr mixtures is of great importance to the accuracy of nuclear data measurement and the capture of radioactive gaseous products from nuclear facilities. The conventional method to separate Xe and Kr is fractional distillation at cryogenic temperatures, which is an energy intensive process and not suitable for low concentration radioactive gas separation. The adsorptive separation of Xe/Kr mixtures using porous materials is considered as an energy-effective and cost-effective alternative. Metal-organic frameworks (MOFs) receive much attention in a variety of applications due to their large specific surface area, adjustable porosity, and easy functionalization. HKUST-1 is a representative MOF composed of dimeric Cu2+ paddlewheel secondary building units with unsaturated metal centers and benzenetricarboxylate linkers, and it is regarded as an attractive adsorbent for gas-adsorption because of its accessible open Cu2+ metal sites in the pore and the ease of industrial-scale preparation. However, HKUST-1 is mostly fabricated in powder form which require well shaped from powders to a suitable particle while maintaining their intrinsic properties and performances in real application scenarios. It is worth noting that unshaped MOF powder is not suitable for adsorption columns or pressure/temperature swing adsorption units, as the powders can lead to issues such as pressure drop and inhomogeneous gas flow. Thus, the shaping of MOFs on a macroscopic level is essential for successful deployment of these materials. Besides, HKUST-1 pellets used for the adsorption separation of low concentration Xe and Kr have not been fully studied. In this work, HKUST-1 powder was shaped with polyvinyl alcohol binder to produce spherical particles. The results of compression tests show that the HKUST-1 spherical particles have relatively high compressive strength. The static adsorption data of Xe(Kr) on HKUST-1 powder and pellet samples show that at low pressure, Xe(Kr) Henry coefficient and Xe/Kr Henry selectivity are almost no change at 0 ℃ or 25 ℃, respectively. The Xe(Kr) isosteric heat on HKUST-1 pellets is smaller than that on HKUST-1 powder. The low concentration Xe/Kr mixtures dynamic separation on HKUST-1 pellets was evaluated at 25 ℃ and 100 kPa by breakthrough experiments. The breakthrough curves reveal that HKUST-1 pellets could efficiently separate Xe from Kr. The dynamic capacity of Xe and corresponding Xe/Kr dynamic adsorption selectivity increase along with the increase of volumetric flow rate when Xe outlet concentration (c/c0) fixed at 0.02, and Xe(Kr) dynamic saturated capacity and corresponding Xe/Kr dynamic adsorption selectivity remain constant at different volumetric flow rates. In order to predict the theoretical breakthrough curves, Wheeler-Jonas model was applied to determine the model parameters at the initial Xe breakthroughs in the adsorption column. The results show that Wheeler-Jonas model is in good agreement with the experimental data when the Xe outlet concentration (c/c0) is less than 0.02.

Keywords

• hkust-1
• pellet
• low concentration gas
• xenon
• krypton
• dynamic separation