Xenon (Xe), a noble gas with atomic number 54, is known for its chemical stability and distinct physical properties. Xenon is remarkably scarce, constituting only about 0.087 parts per million of the Earth’s atmosphere. This rarity makes its recovery a complex and energy-intensive industrial process. The gas’s unique characteristics make it necessary for high-tech applications. Xenon is used as a propellant in satellite ion thrusters, as an inhalation anesthetic, and in specialized high-intensity discharge lamps. Its entire supply is sourced from the atmosphere.
Cryogenic Air Separation: The Starting Point
The process of obtaining Xenon begins with large-scale cryogenic air separation. Air is first purified to remove moisture, carbon dioxide, and hydrocarbons, and then cooled below -180 degrees Celsius until it fully liquefies. This liquid air is separated into its major components—Nitrogen, Oxygen, and Argon—using fractional distillation columns. This primary process produces bulk industrial gases, recovering Xenon and Krypton only as minor byproducts.
The separation relies on the different boiling points of the components. Xenon has a relatively high boiling point (165 K) compared to Oxygen (90 K), making it one of the least volatile components. Consequently, Xenon, along with Krypton, naturally concentrates in the Crude Liquid Oxygen (LOX) stream. This LOX stream is found in the sump, or bottom, of the low-pressure distillation column, and serves as the feed material for the next stage of extraction.
Isolating the Krypton-Xenon Concentrate
The next phase involves diverting a small purge stream of the Xenon-enriched liquid oxygen from the main air separation unit. This liquid stream, containing Xenon and Krypton in trace concentrations, is fed into a specialized secondary distillation system. These auxiliary columns strip the bulk oxygen from the rare gases, significantly concentrating the target elements.
This concentration step must manage the co-accumulation of trace hydrocarbons, such as methane, which pose a safety risk in the oxygen-rich environment. To manage this hazard, the feed stream often passes through a hydrocarbon removal system, such as a catalytic reactor or an adsorption unit. The secondary distillation column further enriches the mixture, producing a crude concentrate. This raw product is a combined Krypton-Xenon concentrate, predominantly noble gases but still contaminated with small amounts of oxygen, nitrogen, and residual hydrocarbons.
Refining and Achieving Final Xenon Purity
The final stage involves separating the Krypton-Xenon mixture and purifying the Xenon to commercial standards. The concentrate is introduced into a final fractional distillation column operating under specific cryogenic conditions. This column leverages the difference in boiling points between Krypton (120 K) and Xenon (165 K) to achieve separation. This rectification process yields two crude streams: one rich in Krypton and one rich in Xenon.
To reach the high purity levels required for applications like semiconductor manufacturing or medical use, the crude Xenon stream undergoes additional chemical purification. This involves passing the gas through specialized adsorption beds containing materials like molecular sieves or activated carbon. These beds capture and remove residual trace contaminants, including oxygen, nitrogen, and hydrocarbons. Through these stringent purification steps, manufacturers achieve final product purities that often exceed 99.999%.