Air Compression
Atmospheric air is drawn in and compressed via multi-stage compressors to a moderate pressure.

Purification
The compressed air is sent through molecular sieve beds (or similar purification units) to remove moisture, carbon dioxide, and hydrocarbons, ensuring the air is clean before cryogenic cooling. 

Pre-cooling and Heat Exchange
The cleaned air passes through a heat exchanger where it is cooled by exchanging heat with the outgoing cryogenic (cold) gases. This lowers the temperature of the air close to its liquefaction point. 

Expansion/Refrigeration
Part of the cold air is expanded in an expander (or via a Joule–Thomson valve), which reduces its pressure and further cools it. This expansion provides the refrigeration necessary to reach cryogenic temperatures. 

Liquefaction
After sufficient cooling, a portion of the air condenses into liquid. This liquid is rich in nitrogen and oxygen, but not yet separated into pure components. 

Fractional Distillation(Rectification)

The liquid air enters distillation columns (typically a double-column system):

In the high-pressure column, part of the liquid is separated into richer nitrogen vapor (top) and oxygen-enriched liquid (bottom)

In the low-pressure column, the oxygen-enriched liquid is further purified. Nitrogen – which boils at a lower temperature – is boiled off, while oxygen concentrates in the liquid phase due to its higher boiling point. 

Product Recovery

Oxygen: The purified oxygen is withdrawn, either as a cold liquid (LOX) or vaporized to gas, depending on demand.

Nitrogen: Nitrogen can be recovered from the top of the distillation column; in some plants, it is delivered as gas or liquefied.

Additional gases (such as argon) may be separated via sidecolumns if required. 

Heat Recovery & Efficiency

The outgoing product and waste streams (oxygen, nitrogen) are used in heat exchangers to cool the incoming air in counter-flow, improving energy efficiency.

Insulated Cold Box

The cryogenic distillation equipment (heat exchangers and columns) is housed in a well-insulated enclosure (cold box) to minimize thermal losses.