The difference in one paragraph
An adsorbent bed fills with impurity and must be cleaned before it can be used again. A PSA cleans the bed by dropping its pressure and purging it with a little product hydrogen; this is fast, so cycles last minutes, but the purge and blowdown gas leaves as low-pressure tail gas and takes hydrogen with it. A TSA cleans the bed by passing hot gas through it; heating and cooling a bed is slow, so cycles last hours, but strongly held impurities such as water are driven off completely and very little hydrogen is lost.
PSA and TSA compared
| Criterion | Pressure swing adsorption (PSA) | Temperature swing adsorption (TSA) |
|---|---|---|
| Regeneration method | Pressure reduction and product purge | Heating with regeneration gas, then cooling |
| Typical cycle time | Minutes | Hours |
| Impurity level in feed | Per cent levels: a reformer feed is typically 70 to 80 mol% hydrogen | Trace levels: parts per million up to saturation with water |
| Impurities removed | Carbon dioxide, methane, carbon monoxide, nitrogen, water | Water; other strongly adsorbed trace species |
| Typical hydrogen recovery | 70 to 90 %, depending on bed count, tail gas pressure and purity | Very high; loss is limited to any regeneration gas that is not recycled |
| By-product stream | Low-pressure tail gas, normally used as fuel | Small wet regeneration stream, often cooled, knocked out and recycled |
| Energy input | None beyond feed compression; the cost is lost hydrogen | Regeneration heater duty |
| Typical adsorbents | Layered alumina or silica gel, activated carbon, molecular sieve | Molecular sieve, activated alumina |
| Usual applications | Steam methane reformer hydrogen, refinery off-gas, syngas | Gas dehydration; drying electrolyser hydrogen after a deoxidiser |
When PSA is the right choice
Choose PSA when the feed contains impurities at per cent levels and several of them at once. Hydrogen from a steam methane reformer, after shift conversion, carries carbon dioxide, methane, carbon monoxide and water. A layered PSA bed removes all of them in one unit and delivers 99.9 to 99.999 mol% hydrogen close to feed pressure. The price is recovery: part of the hydrogen leaves in the tail gas. In a reformer plant that tail gas fuels the reformer burners, so the hydrogen in it is not wasted. See PSA hydrogen purification.
When TSA is the right choice
Choose TSA when the hydrogen is already nearly pure and the job is to remove a small amount of a strongly held impurity. Hydrogen from a water electrolyser is the clearest case. It leaves the stack saturated with water and carrying a trace of oxygen. A catalytic deoxidiser converts the oxygen to water, and a TSA dryer then removes the water. Arranged as a DeOxo dryer package, this takes hydrogen from about 99.9 % to 99.999 % purity. A PSA in this duty would throw away hydrogen that cost electricity to make. See TSA hydrogen purification.
Common mistakes when choosing
- Treating recovery as a fixed property of PSA. Recovery depends on the number of beds and equalisation steps, the tail gas pressure and the purity demanded.
- Specifying a TSA dryer without stating the oxygen content of the feed. Oxygen sets the deoxidiser temperature rise and adds water load to the dryer.
- Ignoring the regeneration gas route on a TSA. Whether wet regeneration gas is recycled, vented or sent to fuel decides the real hydrogen loss.
- Forgetting turndown. Electrolysers following renewable power run at part load for long periods, and the purification package must stay on specification there.
For membrane and cryogenic options alongside adsorption, see the hydrogen purification comparison.
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