Activated carbon or scrubber?
An activated carbon filter holds substances on a solid granule. A scrubber dissolves them in a liquid, usually water. Which of the two fits depends on the substance, the concentration and how humid the air is. The figures come from the Informatiepunt Leefomgeving (iplo.nl), the Dutch government's information service on the living environment.
How they work
Activated carbon (adsorption). The substance sticks to the surface of the carbon. With plain carbon this is a weak physical bond. With an impregnation, the carbon also binds a gas chemically. When the carbon is full, it is replaced or regenerated.
Scrubber (absorption). The air comes into close contact with a scrubbing liquid. An acid liquid captures alkaline gases such as ammonia; an alkaline liquid captures acid gases such as H2S. For odour there is an alkaline-oxidising scrubber.
Adsorption and absorption look alike as words, but they are different processes.
Side by side
|
|
Activated carbon filter |
Scrubber |
|---|---|---|
|
Mainly for |
odour, VOCs, mercury vapour, dioxins |
ammonia, odour, H2S, HCl, HF, SO2, VOCs |
|
Efficiency for VOCs |
90 to 99 % |
50 to 99 %, only if the VOCs dissolve in water |
|
Efficiency for odour |
80 to 95 % |
60 to 85 % with water, 80 to 90 % alkaline-oxidising |
|
Efficiency for H2S |
95 to 99 % |
90 to 95 % (alkaline) |
|
Efficiency for ammonia |
no value on iplo.nl |
more than 99 % (acid) |
|
Temperature |
5 to 80 °C |
5 to 80 °C |
|
Humidity |
below 70 % RH, no condensation |
no limit |
|
Dust in the air |
less than 3 to 5 mg/Nm³ |
less than 10 g/Nm³ |
|
What is left over |
saturated medium to replace or regenerate |
waste water that must be treated |
|
Investment per 1,000 Nm³/h |
€ 10,000 to 50,000 |
€ 7,500 to 25,000 standard, € 10,000 to 50,000 acid, alkaline or oxidising |
|
Operating cost per 1,000 Nm³/h per year |
€ 2,500 |
€ 1,000 to 30,000 |
Nm³ is a cubic metre of air converted to a fixed pressure and temperature, so that volumes can be compared. The values depend on the set-up and the operating conditions. Sources: Informatiepunt Leefomgeving, Adsorptiefilter and Gaswasser (in Dutch).
When to choose activated carbon
- For low concentrations and processes that do not run continuously.
- For VOCs that barely dissolve in water.
- As the last stage after another technique, to get below the limit value or as a safety net if an earlier stage fails.
- It can also be added to an existing installation later.
Carbon is less suitable for wet air, a lot of dust and high concentrations: the filter then fills up or clogs quickly.
When to choose a scrubber
- For substances that dissolve well in water, such as alcohols and acetone.
- For high concentrations: a scrubber accepts air with up to 15,000 mg/Nm³ H2S or 20,000 mg/Nm³ HCl. For activated carbon, iplo.nl gives 1,500 mg/Nm³ H2S.
- For humid air.
On the other hand, a scrubber uses water and chemicals, produces waste water and is sensitive to frost and corrosion.
Both
The two techniques are also combined. A scrubber then removes the bulk of the load and a carbon filter behind it captures the rest. Two scrubbers in series, one acid and one alkaline, is another option. If the composition of the air is unknown, a small test filter on part of the air flow shows which medium works and how long it lasts.
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