A membrane upgrading unit separates methane from carbon dioxide under pressure. For a raw biogas flow of 500 Nm³/h at 55% methane, a datasheet specified 97.5% methane recovery, about 345 Nm³/h of biomethane at 92 to 96% methane and over 8 barg, and an off-gas of about 255 Nm³/h.
The challenge: clean, dry, steady feed
Membranes tolerate little H₂S or moisture, so raw gas is pre-treated.
| Raw gas | Normal | Range |
|---|---|---|
| Flow | 500 Nm³/h | 50 to 120% |
| Pressure | 20 mbarg | 10 to 50 |
| Methane | 55% | 50 to 60% |
| CO₂ | 44% | 35 to 50% |
| H₂S | 200 ppm | 50 to 300 |
The process
- Cooling in an existing shell and tube unit to remove moisture.
- Buffer tank with 30 to 60 minutes’ holding time to smooth flow.
- Blower raising pressure, controlled on pressure and frequency.
- Activated carbon filter protecting the unit from H₂S, replaced when loaded.
- Compressor to working pressure.
- Cooling in a plate exchanger with a chiller, plus free cooling with outside air for up to 35% of the duty in cooler seasons.
- Condensate separator with demister and level monitoring.
- Re-heating using the compressor’s waste heat, to the optimum membrane temperature.
- Membrane filter group separating methane from CO₂.
The product
| Biomethane | Off-gas | |
|---|---|---|
| Flow | 345 Nm³/h | 255 Nm³/h |
| Methane | 92 to 96% | max 3% |
| CO₂ | under 3% | over 95% |
| Pressure | over 8 barg | over 1 barg |
| H₂S | under 10 ppm | under 10 ppm |
The unit can run continuously, 24 hours a day.
What to consider
- Protect the membranes from H₂S and water.
- Decide what to do with the off-gas.
- Collect condensate safely.
- Check the gas specification for injection or vehicle use.
Compare with pressure swing adsorption.
FAQ
What does 97.5% recovery mean?
97.5% of the methane in the feed ends up in the product.
Why compress first?
Membranes separate by pressure difference.
Where does the waste heat go?
To re-heat the gas.
What is in the off-gas?
Mostly CO₂, with a little methane.