What the package does
Hydrogen from an alkaline or proton exchange membrane (PEM) electrolyser is typically about 99.9 % pure on a dry basis. The impurities are water, because the gas leaves saturated, and a trace of oxygen that crosses from the other side of the cell. A DeOxo dryer removes both. In the deoxidiser, a precious metal catalyst reacts the oxygen with hydrogen to form water. A cooler and separator remove most of the water. A temperature swing adsorption (TSA) dryer removes the rest. The package can take hydrogen from about 99.9 % to 99.999 % purity; the figure for a given project depends on its feed and is confirmed in design.
Input 1: oxygen content, across the operating range
The reaction of oxygen with hydrogen releases about 242 kJ for each mole of water formed. With the heat capacity of hydrogen at about 29 J/(mol·K), each 0.1 mol% of oxygen in the feed raises the gas temperature across the catalyst by roughly 17 °C. Oxygen at 0.5 mol% therefore gives a rise of roughly 85 °C. This sets the reactor design temperature, the cooler duty and the material selection downstream. Oxygen content generally rises at low electrolyser load, so state the value at minimum load as well as at design load.
Each mole of oxygen also forms two moles of water and consumes two moles of hydrogen. At 0.1 mol% oxygen the hydrogen consumed is 0.2 % of the flow.
Input 2: water load
State the feed pressure and temperature, and whether the gas is saturated. As an illustration, hydrogen saturated at 30 °C and 31 bara carries about 0.14 mol% water, or about 1,400 µmol/mol. The same gas at 40 °C carries roughly 75 % more. Lower inlet temperature ahead of the dryer means smaller beds or longer cycles, which is why the cooler and separator are part of the package and chilled cooling is sometimes justified.
Input 3: operating pressure
Pressure sets vessel size and wall thickness, and the water content of saturated gas. State the normal, minimum and maximum pressure in barg at the package inlet, and the pressure drop available across the package. If a compressor is planned, say whether the package sits upstream or downstream of it; each position has consequences for water load and for oil contamination risk.
Input 4: product specification
Quote the standard and grade. For fuel cell vehicles, ISO 14687:2019 Grade D requires a hydrogen fuel index of at least 99.97 %, with water at no more than 5 µmol/mol and oxygen at no more than 5 µmol/mol, among other limits. Five µmol/mol of water corresponds to a dew point of about −65 °C at atmospheric pressure. Industrial users may set different limits. A purity percentage alone does not define the dryer, because it says nothing about which impurity is limited.
Input 5: turndown and operating pattern
State the minimum continuous flow, the number of starts and stops expected, and how long the package may stand idle under pressure. Low flow lengthens adsorption time but can cause poor gas distribution in the beds, and oxygen content is usually highest there. Regeneration heating must complete within the cycle at every load. Packages that follow solar or wind power need a control philosophy for holding, restarting and completing an interrupted regeneration.
Input 6: regeneration gas route
A TSA bed is regenerated with a heated stream of dry hydrogen, which leaves wet. There are three routes for it, and the choice sets the hydrogen loss of the package.
| Route | Hydrogen loss | What it needs |
|---|---|---|
| Cool, separate water and recycle to the dryer inlet | Very low | A cooler, a separator and a pressure difference or blower to drive the recycle |
| Send to a fuel or other low-pressure user | Equal to the regeneration flow, but used | A user that can accept an intermittent, wet hydrogen stream |
| Vent to a safe location | Equal to the regeneration flow | A vent system designed for hydrogen |
What to send with an enquiry
- Electrolyser type, design hydrogen flow in Nm³/h or kg/h, and minimum continuous flow.
- Feed pressure in barg and temperature in °C at the package inlet.
- Oxygen in hydrogen at design load and at minimum load, in mol% or µmol/mol.
- Product specification by standard and grade, or a list of impurity limits.
- Cooling medium and its temperature; electrical supply; area classification.
- Preferred regeneration gas route and any limit on hydrogen loss.
- Applicable design codes and any approved vendor list.
Talk to an engineer about your unit
Tell us the equipment, the duty and what you are seeing. Partial data is normal: send what you have and we will tell you what else matters. We acknowledge every enquiry within 1 working day.