How oxygen is made: cryogenic air separation PSA, and how the gas gets into a cylinder to IS 309

Oxygen is made from air. There is no mine and no feedstock to buy: what a producer pays for is the electricity to separate the 20.95 % of air that is oxygen from everything else. There are two industrial ways to do it. Cryogenic air separation liquefies and distils air, and is how almost all liquid oxygen and the purest gaseous oxygen are made. Pressure swing adsorption (PSA) uses a zeolite to hold back nitrogen at room temperature, and is how most on-site plants work. This page walks through both, then follows the gas into a cylinder and through IS 309:2023 testing.

Composition: US Standard Atmosphere 1976; carbon dioxide from the NOAA Global Monitoring Laboratory. Boiling points: NIST Chemistry WebBook.

The three columns that matter are nitrogen, argon and oxygen. Their boiling points lie within 13 K of each other, nitrogen lowest, then argon, then oxygen. That small spread is what a distillation column works with, and it is why argon, sitting only 2.9 K below oxygen, is the hardest of the three to separate.

The process dates from the turn of the twentieth century. Carl von Linde patented air liquefaction in 1895 (DRP 88,824) and a rectification process for separating oxygen in 1902 (BP 1740 of 1902); Georges Claude patented an expansion engine for the refrigeration in 1903 (BP 16,208). Modern plants are much larger and far more efficient, but the sequence is the same.

1. Filter and compress. Air is drawn through an intake filter and compressed in stages, with cooling between stages. The intake location matters: hydrocarbons drawn in will end up in the liquid oxygen.
2. Purify the front end. Water vapour and carbon dioxide would freeze solid and block the cold equipment, so they are removed first, today usually in switching beds of molecular sieve. The same beds take out most hydrocarbons, including acetylene.
3. Cool against the products. The clean air passes through a main heat exchanger, cooled by the cold product and waste streams leaving the plant, and is cooled until it is close to liquefying.
4. Make the refrigeration. Part of the air is expanded through a turbine, doing work and cooling sharply. This is Claude's principle; Linde's original machines used expansion through a valve. The turbine makes up for heat leaking into the cold box.
5. Distil. Air enters a double column. In the higher-pressure lower column it separates into nitrogen at the top and an oxygen-rich liquid at the bottom. That liquid feeds the low-pressure upper column, where nitrogen leaves from the top and nearly pure oxygen collects at the bottom. A condenser-reboiler links the two: nitrogen condensing on one side boils oxygen on the other.
6. Recover argon. At larger plants, an argon-rich stream is drawn from the middle of the upper column into a side column and refined. Argon is not separated by PSA in practice.
7. Deliver. Oxygen leaves either as gas, warmed in the main exchanger and sent to a pipeline, or as liquid drawn into vacuum-insulated storage for tankers. Making liquid costs extra refrigeration, so plants that sell mainly liquid have a liquefier.

The danger point in this sequence is the liquid oxygen sump at the bottom of the upper column. Anything less volatile than oxygen collects there, including hydrocarbons that slip past the front end. Acetylene is the most dangerous, because it is barely soluble in liquid oxygen and can come out as solid. PESO's layout rules for acetylene plants respond directly: at least 30 m to oxygen plants and 90 m to air compressor intakes. The full layout table is on acetylene plant layout and safety distances.

A PSA oxygen plant does at room temperature what the column does at cryogenic temperature, by a different mechanism. Compressed, dried and filtered air flows through a bed of zeolite molecular sieve, which adsorbs nitrogen more strongly than oxygen. Oxygen-rich gas passes through. When the bed approaches saturation, the feed switches to a second bed and the first is depressurised, releasing its nitrogen to atmosphere. The two beds alternate continuously.

The zeolite cannot separate argon from oxygen, so argon travels with the product. If every molecule of nitrogen were removed, the product would be 20.948 parts of oxygen to 0.934 of argon, or about 95.7 % oxygen. Real plants run slightly below that ceiling. The WHO International Pharmacopoeia (11th edition, 2022) covers PSA oxygen as Oxygen 93, with an allowed range of 90 to 96 % (93 ± 3 %), as summarised in a Global Fund briefing note of April 2024. The detail of the cycle, feed air quality and running cost is on how PSA oxygen generation works, and the choice between the two technologies is on PSA vs air separation.

Cylinder oxygen is filled in one of two ways. A filling station supplied with liquid oxygen uses a cryogenic pump to raise the liquid to cylinder pressure and an ambient vaporiser to warm it, which avoids compressing gas and gives a very dry product. A station supplied with gas uses an oxygen-service compressor. Either way the gas reaches a filling manifold and the cylinders on it, typically to about 150 bar at 15 °C. A 46.7 litre cylinder at that pressure holds about 10.1 kg of oxygen (NIST density 215.6 kg/m³ at 150 bar and 15 °C), which is about 7.4 m³ at 15 °C and 1 atm, or about 7.7 m³ on the IS 309 basis of 27 °C and 760 mm Hg.

Filling and the cylinders themselves fall under the Gas Cylinders Rules 2016, and the cylinders are periodically examined and hydrostatically tested; see cylinder testing and retest. Industrial oxygen cylinders are Graphite Black, RAL 9011, throughout under IS 4379:2021. Medical cylinders follow IS 3933:2021, with a white shoulder.

KIGL fills industrial oxygen to IS 309:2023 Grade 2 (N2.5), minimum 99.5 %, and medical oxygen, at its own PESO-approved works at Noonmati, Guwahati, and Howrah, Kolkata. KIGL also operates an air separation plant and supplies liquid oxygen. KIGL builds PSA oxygen plants for sites that want to make their own. Cylinder supply is on the oxygen page.