Filtering VOCs and formaldehyde from air

Volatile organic compounds (VOCs) are usually removed from air with activated carbon without impregnation. The compounds stick physically to the carbon. In industry, VOCs often come from solvents and chemical compounds. For formaldehyde there is a separate medium.

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Activated carbon for VOCs

A filter with activated carbon or zeolite removes 90 to 99 per cent of the VOCs from the exhaust gas, according to the Dutch government's Informatiepunt Leefomgeving (iplo.nl). Carbon works at an inlet concentration of 30 to 5,000 mg/Nm³ (milligrams per cubic metre of air, converted to a fixed temperature and pressure). That concentration may be at most a quarter of the lower explosive limit. Hot air must first be cooled to 80 °C or lower, and humid air must be dried beforehand.

With large amounts of solvent, it can pay to recover it. The carbon is then regenerated and the solvent is released in concentrated form.

Can a carbon filter get hot?

Yes. Adsorption releases heat. With certain compounds, such as aldehydes, ketones, some organic acids and turpentine, the bed can get locally hot or even catch fire. That is why a carbon filter needs temperature monitoring. A rising pressure drop across the bed points to clogging by dust.

A scrubber or carbon?

If the VOCs dissolve well in water, as alcohols and acetone do, a scrubber can also capture them. Many VOCs dissolve poorly in water. For those, a scrubber that uses water works poorly. More on Activated carbon or scrubber.

Formaldehyde

Formaldehyde is captured with an alumina chemisorption medium with potassium permanganate. The same medium also binds H2S, SO2 and mercaptans. For a filter with a polymer adsorbent, iplo.nl gives a residual emission of 1 mg/Nm³ for formaldehyde: that is how much is still in the air after the filter. There are also filters for benzene and formaldehyde.

VOCs in buildings

Offices and hotels near airports. Around an airport the outdoor air can carry kerosene odour and VOCs. That is why buildings near an airport use activated carbon gas filters in their air handling.

Commercial buildings. In offices and other commercial buildings, the concern is fine dust, NOx, SO2 and VOCs. There, fine dust filters and carbon filters sit together in the air handling unit. There are also bag and compact filters with carbon media, for dust and odour in one filter. See Activated carbon filters and Dust filters and HEPA.

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