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Activated carbon filters : untangling the true from the false

Our second article on air purification technologies focuses on activated carbon filters. Find out the whole truth about them.

What is an activated carbon filter?

Activated carbon filters are used by some air purifier manufacturers to trap volatile organic compounds, molecules responsible for odours and some recognized as important indoor air pollutants. Activated carbons, on the other hand, are not used to filter particles.

Activated carbon

Activated carbon is a material consisting essentially of carbonaceous material with a porous structure. It can be produced from any carbon-rich plant organic matter: wood, bark, coconut shells, etc. Once activated (physically or chemically), the surface area of the activated carbon can reach 400 to 2,000 m².g-1. The coal then retains the molecules of gases and liquids. Therefore, the effectiveness of an activated carbon on gases can depend on the ambient humidity. The pore diameter depends on the method of activation and the pores present in the raw material used. Coconut shells thus give micro-pores ( < 2 nm). Woods can have pores larger than 50 nanometres. To purify the air of polluting gases, activated carbons with pores measuring 1 to 2 nanometres are the most suitable. Activated carbons with different pore diameters will have different gas efficiencies. In general, however, the pore size of activated carbons is not regular, so they are called "unstructured" porous materials. Their behaviour is therefore highly variable depending on volatile organic compounds and ambient conditions (temperature and humidity).

How does an activated carbon filter work?

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The activated carbon filter acts by adsorption, and more precisely by physisorption. This means that certain gases present in the air get lodged in the pores of the coal and. The air leaving the filter is then purified of these adsorbed gases.

Adsorption or absorption?

Adsorption should not be confused with absorption. Absorption is a process by which gas or liquid molecules, when brought into contact with a solid material, become incorporated into its entire volume. Adsorption is when this process only affects the surface of the solid.

Filters based on two different forms of activated carbon

A distinction is made between granular activated carbon (GAC) and fibre activated carbon (FAC).

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Granular Activated Carbon (GAC) filters contain granules up to 3 mm in diameter. They have a longer service life and are lighter than activated carbon fibre (AFC) filters. In this form, coal can be combined with other substances. For example, some manufacturers use zeolite with the pellets. They claim it is effective against formaldehyde, an inhalation carcinogenic gas that is very representative of volatile organic compounds (VOCs), but there are no serious scientific studies to prove this. In fact, the main advantage of zeolite is its low cost. German researchers from Stiftung Warentest* have tested air purifiers with filters (Philips, Dyson, Soehnle, Rowenta), with the result that activated carbons capture very little formaldehyde.

Fibre Activated Carbon (FAC) filters are made up of grain sizes between 10 and 50 µm (100 times smaller). They are reputed to be more manageable than GAC filters, and would have a faster adsorption capacity. Researchers at the IBR laboratories suspect that manufacturers prefer this less effective form because it is 2 to 3 times cheaper. Based on their observations on the filtration of isopropanol (another VOC present in gasoline and hydroalcoholic gel), CAF filters would be 26.8% less efficient than GAC filters.

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Their efficiency is dependent on various factors

The activated carbon efficiency can be limited by many factors:

  • The materials used to make activated carbon: marl, wood, coconut
  • The size of the grains and their density
  • The exchanging surface between the carbon and the air flow
  • The seal between the filter and the air duct
  • The pre-filters quality
  • The number of combined filter: the more filter there are, the more the activated carbon filter is protected against particles ; but a high number of filters reduces the air flow or increases the noise and the energy consumption ; the less filter there are, the faster the filter is clogged.
  • The air speed that passes through the filter: the faster it is, the less the activated carbon can purify the air.
  • The level of filter saturation by gas adsorption: gas molecules of pollutants can be re-emitted. Moreover, the saturation speed varies according to pollutants, it makes the calculation of its lifetime.
  • The reversibility of the physical phenomenon: the trapped molecules can be re-emitted if the conditions of temperature and humidity change.
  • Humidity: the activated carbon filter is useless in the case of high humidity and it would tend to adsorb the water molecules first.

According to a recent study, the efficiency of an activated carbon filter may decrease after a few weeks. After 3 or 4 weeks, the activated carbon filter should be cleaned or replaced. This frequency varies according to the level of pollution.

In conclusion, activated carbon filtration is a complex technology that is difficult to master. It depends on many factors and is particularly sensitive to temperature and humidity conditions, which can even lead to the release of trapped pollutants! Hence its uncertain effectiveness, which does not make it a very reliable tool to protect oneself from polluting gases, volatile organic compounds.

Activated carbon filtration is a complex and difficult technology to master. It depends on many factors, hence its effectiveness is often uncertain. *german consumer organisation carrying out comparative tests of products and services on the market.
 

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