# PSA vs membrane oxygen generators

A 2001 review of air separation technologies (Smith and Klosek, Fuel Processing Technology) gives adsorption plants (usually PSA or VSA) a purity limit of 95 vol.% and polymeric membranes about 40 vol.%, noting that membrane systems are usually limited to producing oxygen-enriched air.

The review groups both under non-cryogenic processes, but they separate air in different ways. In an adsorption plant, air passes through a bed of zeolite that retains nitrogen, and an oxygen-rich stream leaves the bed; regenerating the bed by reducing the pressure (PSA or VSA) is usually the process of choice, because of its faster cycle time and simplified operation. A polymeric membrane relies instead on oxygen and nitrogen diffusing at different rates through a membrane that separates a high-pressure and a low-pressure stream; most membrane materials are more permeable to oxygen than to nitrogen.

## How the 2001 review compares them

| Smith and Klosek (2001) | Adsorption (usually PSA or VSA) | Polymeric membrane |
|---|---|---|
| Typical product | Oxygen, typically 93 to 95 vol.% | Oxygen-enriched air |
| Purity limit (maximum purity economically produced) | 95 vol.% | About 40 vol.% |
| Economic range | Below 150 short tons per day (sTPD) | Below 20 sTPD |
| Status in 2001 | Semi-mature | Semi-mature |
| Start-up time | Minutes | Minutes |

The review adds that all of these technologies can produce lower purities than their limit, by adjusting process conditions or by blending purer oxygen with air, and that the less efficient but less complex adsorption and membrane systems are favoured at the lower production rates.

## Practical differences named in the review

- Membranes run as a simple, continuous process at near ambient conditions: an air blower overcomes the pressure drop, and a vacuum pump typically maintains the pressure difference across the membrane and delivers the oxygen.
- Carbon dioxide and water usually appear in the membrane product, because most membrane materials are more permeable to them than to oxygen. The review places membranes in applications up to 20 tons per day where these contaminants can be tolerated.
- Near-ambient operation gives membranes a fast start-up, which the review finds especially attractive where oxygen use is discontinuous.
- In adsorption plants, bed size is the controlling factor in capital cost, and capital rises more rapidly with production rate than in cryogenic plants.

These figures date from 2001. The review itself noted that improvements in membrane materials could make membranes attractive for somewhat larger oxygen requirements.

## Medical oxygen

For medicinal use, The International Pharmacopoeia (13th edition, 2025) defines Oxygen 93% as oxygen produced from ambient air by pressure swing adsorption (PSA) or vacuum swing adsorption (VSA). See PSA vs cryogenic air separation for the third route.

## Where Ultra Controlo plants fit

ULTRAOX SC and ST hospital plants deliver oxygen at 94% (±1%) according to their datasheets, using ULTRACON PSA units with two columns. The ULTRAOX VSA industrial line uses vacuum swing adsorption and delivers 75% to 95% oxygen at 50 kPa.

### Which starts faster, a PSA plant or a membrane system?

The 2001 review by Smith and Klosek gives a start-up time of minutes for both adsorption and membrane systems, against hours for cryogenic plants.

### Does membrane oxygen contain moisture?

Smith and Klosek (2001) note that carbon dioxide and water usually appear in the oxygen-enriched air from membranes, because most membrane materials let them through faster than oxygen.

## Sources

- [Smith A.R., Klosek J., A review of air separation technologies and their integration with energy conversion processes, Fuel Processing Technology 70 (2001) 115-134, sections 2 and 4 and Table 1 (copy hosted by US DOE NETL)](https://www.netl.doe.gov/sites/default/files/2023-08/Smith%202001%20Review%20of%20Air%20Separation%20Technologies.pdf)
- [The International Pharmacopoeia, 13th edition (2025), Medicinal oxygen](https://digicollections.net/phint/202502/index.html#d/b.6.1.226)
- [ULTRAOX SC datasheet, p. 2](https://www.ultracontrolo.com/products/ultraox)
- [ULTRAOX ST datasheet, p. 2](https://www.ultracontrolo.com/products/ultraox-st)
- [ULTRACON datasheet, p. 2](https://www.ultracontrolo.com/products/ultracon)
- [ULTRAOX VSA datasheet, p. 2](https://www.ultracontrolo.com/products/ultraox-vsa)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/compare/psa-vs-membrane-oxygen-generators/