Extraction of Xenon Using Enriching Reflux Pressure Swing Adsorption

Measuring radioactive xenon isotopes in the atmosphere is an important technique used for monitoring within the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Several process steps are needed to capture and prepare a xenon sample suitable for transfer to a nuclear detector. These processes include air drying, xenon collection, concentration, and purification. The ideal xenon sampling system would be small, have low power consumption, not use consumables (such as helium gas or liquid nitrogen), and be fully automated, robust, and dependable. Due to the low concentration of xenon in the air, the most energy-intensive part of the gas processing is the first stage collection and concentration. Pacific Northwest National Laboratory has developed a batch-mode enriching-reflux pressure-swing-adsorption (ER-PSA) process that shows promise as a xenon collection and concentration technique. The development of this process was guided by finite element models of the process chemistry. The modeling showed that minimizing the xenon dispersion in the adsorption columns is critical to achieving a good separation. Prior to hardware construction, models were used to determine appropriate trap dimensions, adsorbent particle size, and optimum process variables such as cycle steps, step times, and flow rates. A benchtop ER-PSA unit was constructed, and testing under various operating conditions has begun. The results obtained in these initial parametric experiments have provided insight into the physics of the process and have refined the mathematical models. The combination of modeling and experiment provides a better understanding of process physics than either experiment or models alone. The benchtop system has demonstrated room-temperature collection of xenon with a xenon enrichment of 1000 (starting from air) and a modest pressure ratio of 7–10, resulting in an energy-efficient separation technique. The technique does not use exotic adsorbents, cryogenic temperatures, or high-pressure compressors, enabling a small, low-power, robust system. This ER-PSA process is an enabling technology for small energy-efficient xenon collectors, desirable qualities for next-generation international xenon systems. 2010 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies