Fractional Distillation: How Rare Gases Are Extracted from Air

The air you just breathed contained about 0.93% argon, 0.0005% neon, and 0.000008% xenon.

These are not merely trace elements. They are the invisible enablers of modern technology — from the lasers that etch computer chips to the imaging agents that illuminate human organs for medical scans. However, capturing them requires one of the most energy-intensive separation processes in industrial engineering: fractional distillation.

This article explains how rare gases are extracted and why they matter.

1. What Are Rare Gases?

The rare gases — also called noble gases — occupy Group 18 of the periodic table. They are colourless, odourless, and chemically inert under most conditions. Their properties make them useless for chemistry but indispensable for physics.

Helium (He): The second-lightest element. Boiling point: -269°C. Found in natural gas deposits, not atmospheric air. Used for MRI cooling, leak detection, and as a carrier gas in chromatography.

Neon (Ne): Boiling point: -246°C. Concentration in air: 18 ppm. When electrified, it glows orange-red. Used in neon signs, high-voltage indicators, and excimer lasers for semiconductor lithography.

Argon (Ar): The most abundant rare gas in air (0.93%). Boiling point: -186°C. Used as a shielding gas for welding titanium and aluminium, and as a blanket gas in titanium and silicon production.

Krypton (Kr): Boiling point: -153°C. Concentration in air: 1 ppm. Used in energy-efficient windows (fills the gap between panes), high-performance lighting, and laser fusion research.

Xenon (Xe): Boiling point: -108°C. Concentration in air: 0.087 ppm. The heaviest non-radioactive rare gas. Used as an anaesthetic, in ion thrusters for satellites, and in flash lamps for high-speed photography.

2. Extraction by Fractional Distillation: How It Works

The Principle

Fractional distillation exploits differences in boiling points. Liquid air is warmed slowly, and as each component reaches its boiling point, it vaporises and is collected separately. The process is conceptually simple but operationally complex.

Step-by-Step Process

Step 1: Air compression and purification

Atmospheric air is compressed to about 5-10 bar. Water vapour, carbon dioxide, and hydrocarbons are removed — if they freeze later in the process, they will block equipment.

Step 2: Cooling and liquefaction

The compressed air is cooled through successive heat exchangers, then expanded through a valve (Joule-Thomson effect) to achieve liquefaction temperature. The result is liquid air at about -192°C.

Step 3: Distillation in the double column

Liquid air enters the bottom of a high-pressure distillation column (operating at 5-6 bar). It separates into nitrogen (top) and oxygen-rich liquid (bottom). The oxygen-rich liquid is fed to a low-pressure column (1.3 bar) for further separation.

Step 4: Krypton-xenon concentration

Krypton and xenon have higher boiling points than oxygen, so they accumulate in the oxygen stream of the low-pressure column. A side stream rich in krypton and xenon is drawn off and sent to a dedicated enrichment column.

Step 5: Further purification

The concentrate undergoes catalytic conversion to remove hydrocarbons. This is for safety, as hydrocarbons with liquid oxygen can explode. It then undergoes final distillation to separate krypton from xenon. Modern systems achieve purities above 99.9995%.

3. Alternative Extraction Methods

Cryogenic distillation dominates industrial production, but other methods exist for specialised applications.

Adsorption

Zeolites and metal-organic frameworks (MOFs) can selectively adsorb xenon and krypton at room temperature. Activated carbon, for example, shows a xenon uptake of about 54% by weight at atmospheric pressure. The challenge is lower product purity compared to distillation, and the need for pressure or thermal swing to regenerate the adsorbent.

Membrane separation

Polymer membranes can separate gases based on molecular size and permeability. For rare gases, selectivity is the limiting factor — membranes that pass oxygen readily may also pass krypton, making high-purity separation difficult.

Gas hydrate formation

Under high pressure and low temperature, water forms ice-like cages that trap gas molecules. Xenon forms hydrates more readily than krypton or argon, enabling selective separation. Research shows potential energy savings of 30-35% compared to conventional distillation, but the technology is still emerging.

4. Applications by Gas

Helium

• MRI magnets: Liquid helium cools superconducting magnets to 4 Kelvin (-269°C). A typical MRI system contains 1 500-2 000 litres of liquid helium.
• Semiconductor manufacturing: Helium provides an inert atmosphere for crystal growth and acts as a carrier gas in deposition processes.
• Leak detection: Helium's small molecular size makes it the standard tracer gas for vacuum systems.

Neon

• Excimer lasers: Neon is part of the gas mixture that produces deep-UV light for semiconductor lithography. These lasers etch features measured in nanometres.
• Neon signs: Classic orange-red glow comes from neon discharge.
• Cryogenic refrigeration: Neon's low boiling point makes it useful in closed-cycle refrigerators reaching 30-40 Kelvin.

Argon

• Welding: Argon shields titanium, aluminium, and stainless steel welds from atmospheric contamination.
• Titanium and silicon production: Both metals react with oxygen and nitrogen at high temperature. Argon provides an inert blanket throughout processing.
• Double-pane windows: Argon fills the gap between panes, reducing heat transfer better than air.

Krypton

• Energy-efficient windows: Krypton has lower thermal conductivity than argon, allowing thinner window units with the same insulating performance.
• High-intensity lighting: Krypton-filled incandescent bulbs run hotter and brighter than argon-filled ones.
• Laser fusion: Krypton-fluoride lasers are candidates for inertial confinement fusion research.

Xenon

• Medical anaesthesia: Xenon is an ideal anaesthetic — rapid onset, minimal side effects, and eliminated unchanged by the body. The limiting factor is cost.
• Satellite propulsion: Ion thrusters use xenon because it is heavy, easy to ionise, and chemically inert.
• Medical imaging: Xenon isotopes are used as contrast agents for CT lung imaging.
• Semiconductor manufacturing: Xenon is used in ion implantation and deep-UV lithography.

5. Materials for Rare Gas Applications

The technologies that use rare gases often require specialised materials — from the metals that contain them to the components that interact with them. Stanford Advanced Materials (SAM) supplies high-purity materials for these applications:

For Semiconductor Manufacturing

• Sputtering targets (Ti, Ta, Cu, Al) for thin film deposition
• Evaporation materials for metallisation layers
• High-purity metals for chamber components

For Medical and Imaging

• Scintillation crystals for radiation detectors
• High-purity metals for imaging system components
• Ceramic substrates for medical devices

For Lighting and Display

• Phosphor materials for specialty lighting
• Evaporation materials for display coatings
• High-purity metals for electrode fabrication

For Aerospace and Propulsion

• Refractory metals (W, Mo, Ta) for high-temperature applications
• Rare earth metals for specialty alloys
• Ceramic composites for thermal protection

For Research and Development

• High-purity elements in various forms (powders, wires, foils, rods)
• Alloys and compounds for experimental work
• Nanomaterials for advanced research

All materials are available with certification of analysis and full traceability.

6. FAQ: Purity and Handling

Q: Why does purity matter in these applications?
A: In semiconductor manufacturing, trace impurities can ruin entire production batches. In medical applications, purity affects patient safety. In research, reproducibility depends on known composition.

Q: What forms do materials come in?
A: SAM supplies materials in multiple forms: powders, wires, plates, foils, rods, sputtering targets, and custom shapes depending on application requirements.

Q: Do you offer custom specifications?
A: Yes. From small-batch R&D quantities to large-volume production, we work with customers to meet specific purity, form, and packaging requirements.

Q: What documentation comes with materials?
A: Each shipment includes certification of analysis. Batch-specific traceability is maintained for quality audits and regulatory compliance.

About Stanford Advanced Materials (SAM)

Stanford Advanced Materials (SAM) supplies over 10 000 advanced materials to aerospace, medical, semiconductor, and research industries worldwide. Founded in 1994 and headquartered in Santa Ana, California, we offer high-purity metals, alloys, ceramics, sputtering targets, and rare earth materials in various forms — from R&D quantities to full-scale production. With warehouses in the UK, Canada, Europe, and Asia-Pacific, we deliver reliably, anywhere.

References

Häussinger, P., Glatthaar, R., Rhode, W., et al. (2001). Noble Gases. Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH.

Kerry, F.G. (2007). Industrial Gas Handbook: Gas Separation and Purification. CRC Press.

Smith, A.R., & Klosek, J. (2001). A review of air separation technologies and their integration with energy conversion processes. Fuel Processing Technology, 70(2), 115-134.

Thallam Thattai, A., et al. (2016). Experimental investigation of gas hydrate formation for xenon recovery. Chemical Engineering Journal, 302, 74-82.

Banerjee, R., et al. (2008). Metal-organic frameworks for xenon and krypton separation. Science, 319(5865), 939-943.

Baker, R.W. (2002). Future directions of membrane gas separation technology. Industrial & Engineering Chemistry Research, 41(6), 1393-1411.

U.K. Geological Survey. (2023). Mineral Commodity Summaries: Helium, Argon, Neon, Krypton, Xenon.