Single-stage vs two-stage PSA oxygen

In a single-stage plant the remainder of the gas is argon and nitrogen. Among the developments behind monograph 3098 Oxygen (98 per cent), first published in Supplement 11.5, EDQM names the introduction of a second-stage adsorption process designed to remove argon. When it opened an extraordinary consultation on 16 April 2020, EDQM noted that two-stage PSA plants capable of producing oxygen with a nominal 98.0% were then available.

How the two grades compare

Monograph 3098 also requires a risk assessment to identify impurities it does not list, and EDQM says that this assessment should take account of the quality of the gas produced under maximum performance conditions.

Purity ceilings in other documents

A Global Fund briefing note on PSA plant oxygen purity (3 April 2024) states that PSA purity does not exceed 96% because of the limits of the technology. HTM 02-01 (2026), the NHS England guidance, says PSA can produce oxygen concentrations of up to 95%, the rest mainly argon with some nitrogen, and that the maximum achievable concentration is unlikely to exceed 97/98%. For two-stage plants, the EDQM history of Ph. Eur. 3098 gives a nominal 98.0%, varying between 96% and 100%, and The International Pharmacopoeia defines Oxygen 98% as not less than 96.0% O2.

Columns and stages

A Global Fund briefing note on PSA, VPSA and VSA (4 March 2025) states that a PSA plant has two sieve bed tanks in its oxygen generator. EDQM describes the second stage of a two-stage plant as an adsorption process designed to remove argon.

INFARMED circular (2024)

An INFARMED circular of 29 October 2024 described 98% oxygen generators as being in strong technological development, with certification processes started.

See what is oxygen 98 and two-stage PSA.