How does a nitrogen generator work? PSA and membrane explained
A nitrogen generator takes ordinary compressed air, which is about 78% nitrogen. It removes most of the oxygen, water vapour and other gases. Pressure swing adsorption (PSA) generators do this with beds of carbon molecular sieve and reach up to 99.9995% purity. Membrane generators push the air through hollow fibres and suit purities up to about 99.5%.The starting point: compressed air
Every on-site nitrogen generator starts with air from a compressor. Dry air is roughly 78% nitrogen, 21% oxygen and 1% argon, with traces of carbon dioxide and other gases. A generator does not make nitrogen. It separates out what is already there and sends the oxygen-rich remainder back to atmosphere. That has two practical consequences. You need a compressor big enough to feed the generator as well as the rest of the site, and the higher the purity you ask for, the more compressed air the generator uses for each cubic metre of nitrogen it delivers.A note on purity figures: argon behaves like nitrogen in both technologies and passes straight through. Generator purity is therefore quoted as remaining oxygen. A generator rated at 99.5% leaves no more than 0.5% oxygen in the gas, and the rest is nitrogen plus a little argon.
How a PSA nitrogen generator works
A PSA generator has two columns, or beds, filled with carbon molecular sieve (CMS). CMS is a carbon material full of tiny pores. Under pressure, oxygen molecules, which are slightly smaller than nitrogen molecules, enter those pores far faster than nitrogen does.
The cycle in four steps
PSA is the technology to choose when you need high purity. Parker’s NITROSource PSA range, which ALS Industrial install and service across the UK, covers purities from 95% up to 99.9995%, which is 5 parts per million of oxygen. PSA generators are used for food packaging, brewing, laser cutting, heat treatment and electronics, where oxygen levels in parts per million matter.
- Adsorption. Clean, dry compressed air flows up through the first bed. The CMS holds back oxygen, carbon dioxide and moisture. Nitrogen passes out of the top.
- Switch over. Before the first bed is full, valves switch the air to the second bed so the nitrogen supply carries on without a break.
- Regeneration. The first bed is vented to atmosphere. With the pressure gone, the CMS releases the oxygen it was holding.
- Repeat. The two beds take turns, a minute or so at a time, for as long as nitrogen is needed. A buffer vessel smooths out the flow.
How a membrane nitrogen generator works
A membrane generator is a bundle of thousands of hollow polymer fibres inside a tube. Compressed air enters one end and travels down the centre of the fibres. Oxygen, water vapour and carbon dioxide pass through the fibre walls faster than nitrogen and vent away through a side port. Nitrogen-rich gas comes out of the far end. There are no switching valves and no cycle, so a membrane gives a steady flow, starts quickly and runs almost silently. Purity is set by how slowly the air moves through the fibres: slower flow gives higher purity but less nitrogen. Parker describes its HFX membrane nitrogen generators as ideal where 1% to 5% remaining oxygen is acceptable, such as chemical blanketing, fire and explosion prevention, pigging and tyre inflation. Its SmartFluxx and HiFluxx membrane modules reach up to 99.5%.PSA or membrane: how to choose
Choose PSA when
- you need 99.5% purity or better
- the gas touches food, drink or a sensitive process
- you want the lowest air use at high purity
- flow is large or steady through the working day
Choose membrane when
- 95% to 99% purity is enough
- you want a simple unit with very few moving parts
- space, noise or quick start-up matter most
- the generator sits at one point of use
Why the air going in matters
Both technologies are only as good as the compressed air feeding them. Oil aerosol and oil vapour damage CMS and membrane fibres, and the damage is permanent. Water reduces PSA capacity and purity. That is why every generator sits behind a filtration package and, for PSA, a dryer that keeps the pressure dew point low enough. A purity alarm or a gradual fall in output is often the first sign that something upstream has failed. A drain that has stuck or a filter element left past its change date can do the damage within weeks.What needs maintaining
- Filter elements ahead of the generator, normally every 12 months or sooner if the differential pressure says so.
- Valves and controls on PSA generators, which open and close thousands of times a week.
- The oxygen analyser, which needs checking and calibrating so the purity reading can be trusted.
- The dryer and drains upstream, because they protect everything after them.