[ oxygen ]
John Cockerill, operate at pressures between 15 and 40 bar. In these cases, both the oxygen and hydrogen can be piped over long distances from the electrolyser without the need for gas compression.
Other types of electrolyser, such as the Siemens Energy Elyzer PEM unit and alkaline units from Nel or thyssenkrupp nucera, operate at a very low pressure. This avoids the complexity of compliance with the Pressure Equipment Directive( PED).
It is likely that a low-pressure electrolyser will not provide sufficient pressure to drive the oxygen and hydrogen gases to their respective compressors. For this reason, oxygen from these types of electrolyser is generally vented to the atmosphere at a safe height.
It is, therefore, most likely that high pressure PEM or alkaline electrolysers would be used if medicinal oxygen from electrolysis becomes accepted through changes in the relevant Pharmacopeia Monographs in the future.
However, with low-pressure electrolyser operation, hydrogen gas compression is required close to the electrolyser. Safety practices will require an adequate separation distance between the hydrogen compressor and any oxygen compressor. This is to avoid the risk of the two gases leaking, mixing, and causing an explosion.
Manufacturers of low-pressure electrolysers also have plans to introduce systems that will operate at between 5 and 10 bar. Therefore, it is highly likely that in the coming decades, most hydrogen electrolyser schemes will be able to generate oxygen at sufficient pressure for recovery and use in the medical sector or other industrial gas applications.
Pressurised alkaline electrolysis process
O 2
Safety gas analysis H 2 in O 2
Safety gas analysis O 2 in H 2
Condensate trap
Control valve |
Demister |
Demister |
Catalytic de-oxo unit |
Cooling water
Cooling water
Water feed |
Feed water pump |
Purifier |
Gas / lye phase separator |
Regenerative twin bed dryer |
Gas / lye phase separator
Control valve
KOH management
H 2
Lye tank
Pressurised alkaline electrolysis stack
Gas purity analysis
Fresh KOH |
Recirculation pump |
Cooling water |
Filter |
N 2
Rectifier
Transformer
High voltage electricity
© 2025 sbh4 GmbH
40 Hydrogen Tech World | Issue 22 | June 2025