Activated Carbon for Air Filtration
Granulated activated carbon (GAC) is the main ingredient in filters used to remove gaseous and chemical pollution from air and water. It is a material which has hundreds of other applications, from decolourising raw sugar to treating gastrointestinal symptoms, and its use goes back to ancient Egypt.
Activated carbon removes pollutant molecules such as volatile organic compounds (VOCs) and odour compounds from the air via a phenomenon known as adsorption (note, not absorption which is something different). In adsorption, the pollutant molecule is attracted to the surface of the adsorbent (in this case, GAC) and a weak bond forms between it and the surface. In this way, the pollutant is captured from the flow of air through the adsorbent which is normally packed into a column within the filter system. Sometimes a substance is added to the GAC, such as silver or potassium permanganate, is added to GAC to enhance adsorption by creating stronger chemical bonds between the surface and the pollutant molecules.
A GAC filter needs to be matched carefully to its application and, to this end, there are seven vital properties of the material that you need to check out.
Activated charcoal is derived from carbon-based raw material in which heat or chemical treatment has been used to remove volatile non-carbon components and some of the carbon itself. The resultant material has a spongy structure, full of pores and therefore having a very high surface area. One gram of activated carbon will give you a surface area of around 500 square metres. Commercial grade GAC usually comes from:
- Coconut and other nut shells
- Bituminous and lignite coals
- Sawdust, bark and other wood sources
- Petroleum coke
The internal structure and pore size varies with the source of a GAC, with material derived from nut shells and petroleum coke tending to have smaller pores than material derived from coals.
2. Apparent density
The presence of so many pores within activated carbon means its density is far less than that of solid carbon. The term apparent density refers to the mass of a given volume of particles or granules of activated carbon and is usually between 0.4-0.5 grams per cubic centimetre. Apparent density is related to working capacity of a GAC because the higher it is, the more weight – and the more surface area – can be packed into the fixed volume column of the filter. Of course, other factors such as cost or presence of added moisture (which increases apparent density) also come into play so the relationship between apparent density and performance is not a simple one.
3. Activity level
The activity level of a GAC is related to the surface area it presents per unit weight. The openings in that area must be big enough to actually allow adsorption so pore size (see below) is also a factor. The GAC industry has developed a number of tests using substances of different molecule sizes that are adsorbed on activated carbon, namely: iodine, phenol, methylene blue, carbon tetrachloride (a typical VOC), benzene (another VOC) and the colour in standard blackstrap molasses.
4. Pore size
The pore size and distribution in an activated carbon material can be determined by scanning electron microscope studies. Pores are classed on the basis of size as follows:
- Micropores – less than 2 nm diameter
- Mesopores – between 2 and 50 nm diameter
- Macropores – more than 50 nm diameter
Higher micropore density, as found in coconut shell-derived GAC, is likely to favour adsorption of VOCs which are small molecules. Larger pores are good at absorbing molecular aggregates such as those which give colour to raw sugar.
Hardness and resistance to abrasion are valuable properties in all types of GAC. This is particularly important if the material is going to be subjected to frequent handling. Softer materials will tend to shed carbon particles which are, in themselves, a powerful pollutant.
Reactivation is similar to the process used to create the GAC in the first place, and allows the material to be recycled, which cuts costs. Most often, a reactivated GAC will not be as effective as the ‘virgin’ material. Performance is more predictable if the reactivated material is put to the same use as the virgin version, less so if a different application is involved. For some applications, use of a reactivated GAC may be acceptable, particularly if there is a clear cost benefit involved.
7. Environmental acceptability
There is an increasing trend in all industrial sectors towards the use of renewable resources as alternatives to fossil fuels. Nut shells, including coconut shells, are ‘green’ raw materials, compared to petroleum coke and coal. Companies with a commitment to environmental sustainability should be influenced by this issue if performance of the renewable-based GAC is acceptable for its intended application.