What is it?
A cryogenic air separation plant takes ordinary air and splits it into pure oxygen, nitrogen and argon. “Cryogenic” simply means very, very cold: the plant chills air to about −185 °C until it turns into a liquid.
Air is a mix of gases: roughly 78% nitrogen, 21% oxygen and about 1% argon. Each one boils at a different temperature, and that difference is the whole trick.
How does it work?
Think of a pot holding a mix of liquids: the one with the lowest boiling point turns to vapor first. The plant makes air cold enough to become a liquid, then lets it boil gently in a tall tower so each gas comes out on its own.
The main equipment, step by step
| Step | Equipment | What it does (in plain words) |
|---|---|---|
| 1 | Air filter | Cleans dust out of the air, like a vacuum cleaner filter. |
| 2 | Main air compressor | Squeezes the air to high pressure (about 5–6 bar), which makes it easier to cool and liquefy. |
| 3 | Pre-cooling unit | Cools the hot, squeezed air back down with chilled water. |
| 4 | Purifier (molecular sieve) | Removes water vapour and CO₂, which would freeze into ice and block the cold equipment. |
| 5 | Main heat exchanger | Passes warm incoming air next to cold outgoing products, so the air gets very cold for free. |
| 6 | Expansion turbine | Lets some air expand quickly, which drops its temperature further, like a spray can getting cold. |
| 7 | Distillation column (“cold box”) | A tall tower where liquid air is separated: nitrogen rises to the top, oxygen collects at the bottom, argon is drawn from the middle. |
| 8 | Storage tanks and product compressors | Store the products as liquid in insulated tanks, or send them out as gas by pipeline or cylinder. |
The process in more detail
The whole plant does four jobs, one after another.
Warm endPrepare the air
Air is pulled in through filters and squeezed by a big compressor. Squeezing heats the air up, so it is cooled back down with water. Then it passes through beds of special beads (molecular sieves) that soak up moisture and carbon dioxide. This matters because at very low temperatures, water and CO₂ would freeze solid, like ice in a pipe, and block the plant.
ChillingMake it very cold
The clean air now enters the main heat exchanger, a block of thin metal channels. Cold products leaving the plant flow on one side, warm air coming in flows on the other, and heat passes between them. It is like holding a cold drink in a warm hand: the hand cools and the drink warms. A small part of the air is sent through an expansion turbine, where it expands suddenly and loses even more heat. Together these steps bring the air close to its liquefying point.
DistillationSeparate the gases
The very cold air goes into the distillation tower, usually two columns stacked on top of each other (a high-pressure column below and a low-pressure column above). Inside are trays or packing that let rising vapor and falling liquid touch again and again. Each touch sorts the mix a little more. Nitrogen, which boils at the lowest temperature, ends up as vapor at the top. Oxygen ends up as liquid at the bottom. Argon collects in the middle and is drawn off to a separate small column to be purified.
OutputDeliver the products
The pure products travel back through the heat exchanger, giving their cold to the incoming air, and leave as gas. Some are kept as liquid instead and stored in insulated tanks.
All the cold equipment sits inside a tall insulated steel box called the cold box, packed with insulation so that outside heat cannot leak in.
Gas or liquid: how products leave the plant
Products leave in one of two forms, depending on how far the customer is.
| Form | How it travels | Best for |
|---|---|---|
| Gas | Pipeline, or compressed into cylinders | A steel plant or factory right next door that needs a large, steady flow |
| Liquid | Insulated road tankers to storage tanks at the customer | Hospitals, food plants and workshops further away |
Liquid is popular for transport because it takes far less space: one litre of liquid oxygen turns into roughly 860 litres of oxygen gas when it warms up.
Staying safe
These plants are very safe when run properly, but the gases need respect. Three hazards matter most:
Too much oxygen = fire risk
Air rich in oxygen makes things burn much faster and hotter. Oil, grease and sparks are kept well away from oxygen equipment.
Too much nitrogen = no air to breathe
Nitrogen has no smell or colour, and a leak can push oxygen out of a closed room without anyone noticing. Oxygen monitors and good ventilation protect workers.
Extreme cold = burns
Liquid gases at nearly −200 °C can burn skin and make steel brittle. Workers wear gloves, face shields and special clothing.
Where the gases go
Oxygen
Hospitals, steel making, glass and welding.
Nitrogen
Food packaging, keeping products fresh, electronics, and as a safe “blanket” gas in chemical plants.
Argon
Welding shields and filling light bulbs.
Squeeze the air, clean it, make it super cold until it becomes liquid, then let each gas boil off at its own temperature.
Quick glossary
- Cryogenic
- Extremely cold, below about −150 °C
- Boiling point
- The temperature at which a liquid turns into gas
- Distillation
- Separating a liquid mix by boiling it, since each part boils at a different temperature
- Cold box
- The insulated steel box that holds all the very cold equipment
- Molecular sieve
- Beads with tiny pores that trap water and CO₂ from the air
- Expansion turbine
- A spinning machine where gas expands and gets colder
- Heat exchanger
- Equipment that passes heat from a warm stream to a cold one without mixing them
- Purity
- How clean a product is, for example 99.5% oxygen