PSA VPSA or cryogenic air separation: choosing an oxygen gas plant by purity, scale, liquid output and energy

There are three ways to build an oxygen gas plant, and they are not competitors across the whole range. A cryogenic air separation unit (ASU) is the only route to liquid oxygen, to oxygen purer than about 96 %, and to argon. Pressure swing adsorption (PSA) and its larger cousin vacuum pressure swing adsorption (VPSA) make oxygen gas at 90 to 96 % at ambient temperature, and are the practical choice for hospitals and most factories that make their own. This page compares the three on purity, scale, liquid output and energy, using named sources, and sets out where each one fits. How each process works is on how oxygen is made.

A conversion helps when comparing quotes. One tonne of oxygen a day is about 29.2 Nm³/h of gas at 0 °C, from the NIST density of 1.429 kg/m³. On that basis, Variny's 5,000 Nm³/h threshold is about 170 tonnes a day.

The purity gap is physics, not engineering quality. A distillation column separates by boiling point, and given enough stages it can separate oxygen from argon as well as from nitrogen. A zeolite separates by how strongly it holds each molecule, and it holds argon about as weakly as oxygen. Argon is 0.934 % of air, so if a PSA plant removed every molecule of nitrogen, its product would still be only about 95.7 % oxygen. That is why medical PSA oxygen is specified as Oxygen 93, 90 to 96 %, in the WHO International Pharmacopoeia (11th edition, 2022), rather than against the higher-purity monograph for liquid-derived oxygen. A PSA plant offered at 99 % or above is not describing pressure swing adsorption on zeolite. The mechanism is explained further on how PSA oxygen generation works.

For most uses the gap does not matter. Furnace enrichment, glass and metal melting, effluent treatment, ozone generation and hospital supply all run on 90 to 96 % oxygen. Where it does matter, IS 309:2023 sets the industrial grades: Grade 3 (N2.0) at 99.0 %, Grade 2 (N2.5) at 99.5 % and Grade 1 (N3.5) at 99.95 %. No adsorption plant reaches any of them.

Energy is the main running cost of every oxygen plant, because the oxygen itself is free. The numbers in the literature are not directly comparable unless the purity, delivery pressure and product state are the same.

Two conclusions survive the differences. First, large cryogenic plants and VPSA are in the same range per tonne of gaseous oxygen; neither is dramatically cheaper to run at its best scale. Second, liquid oxygen costs more energy than gas, because it has to be liquefied, and it then has to be transported. For a buyer, the useful question to every supplier is the same: kilowatt-hours per Nm³ of product, at a stated purity and delivery pressure, measured at the plant's actual operating point rather than its design point.

Choose a cryogenic ASU when demand is large and steady, at the scale of a steelworks, a refinery or a merchant liquid plant; when purity above about 96 % is required on site; when argon or nitrogen is wanted as well; or when liquid product is part of the business. An ASU is a long-term asset, slow to start and stop, and is normally run continuously by a gas company.

Choose VPSA when demand is large for an adsorption plant, purity of 90 to 93 % is acceptable, and power cost dominates the decision. VPSA is common for furnace enrichment, pulp and paper, and water treatment.

Choose PSA when demand is small to moderate, the site needs to be independent of deliveries, and 90 to 96 % purity is acceptable. Hospitals, small foundries, glass works, aquaculture and ozone systems are the typical users. A PSA plant starts quickly and follows demand.

Choose bulk liquid or cylinders instead when demand is intermittent, too small to justify a plant, or needs higher purity than adsorption gives. The method for that decision is on cylinder supply or on-site generation, and the hospital case is treated on liquid medical oxygen in India.

Whatever the primary source, a plant producing gas only needs a back-up. A PSA plant has no stored liquid reserve, so it is paired with a cylinder manifold or a liquid tank for maintenance and breakdowns.

Only an ASU makes both from the same machine. Adsorption plants use different sieves for each gas: zeolite for oxygen, which holds nitrogen, and carbon molecular sieve for nitrogen, which holds oxygen. A site that needs both from adsorption installs a PSA oxygen plant and a PSA nitrogen plant side by side, each with its own adsorber vessels and its own purity and pressure duty.

KIGL builds PSA oxygen and PSA nitrogen plants, gas pipelines and medical gas pipeline systems, and fills oxygen, nitrogen and other gases in cylinders at its own PESO-approved works at Noonmati, Guwahati, and Howrah, Kolkata. KIGL also operates an air separation plant and supplies liquid oxygen and liquid nitrogen. For a site sizing an on-site plant, the PSA oxygen plant page sets out what KIGL supplies.