Why can't a PSA plant produce 100% oxygen?

What air is made of

Dry air is 78.08% nitrogen and 20.95% oxygen by volume, and argon is the next largest component at 9,340 ppm, according to NASA's Earth fact sheet. A PSA plant separates these gases by adsorption: its zeolite preferentially adsorbs nitrogen while letting oxygen pass through, and once the zeolite is saturated the vessel pressure is lowered so that the nitrogen is released and the bed can be used again.

Why argon sets the ceiling

HTM 02-01 (2026) states that PSA can produce oxygen concentrations of up to 95%, that the remaining content is mainly argon with some nitrogen, and that the maximum achievable concentration is unlikely to exceed 97/98%. The European Pharmacopoeia describes oxygen from a single-stage adsorption plant as having a nominal content between 90.0% and 96.0%, the remainder argon and nitrogen, and a Global Fund purity note (2024) states that the purity of such plants does not exceed 96% because of the limits of the technology.

How higher purities are made

• Two-stage adsorption: EDQM explains that a second adsorption stage designed to remove argon led to Oxygen 98, monograph 3098, with a nominal 98.0% varying between 96% and 100%.
• Cryogenic distillation: oxygen of at least 99.5% (Ph. Eur. monograph 0417) is made by cryogenic distillation of air and is delivered as liquid or in cylinders.

Purity and flow

Within the single-stage range, a higher purity setting means less flow from the same equipment. The ULTRACON concentrator table lists a lower output at 95% than at 93% for every size: the largest, 14D, gives 111.4 Nm³/h at 93% and 90.4 Nm³/h at 95%. ULTRAOX SC and ST plants deliver 94% (±1%).