Picture yourself a sponge with ”holes or entries” of different diameter and you have a fair idea of what activated carbon looks like if put under a microscope. Depending on the size of the molecules of the compounds to be removed, it is important to select a type of activated carbon offering just the right amount of pores to adsorb as much of those molecules as possible.
Adsorption mainly takes place in the smaller (micro) pores. Here the biggest attraction forces are concentrating. The amount, in which activated carbon is capable to bind the molecules of a certain compound, is called adsorption capacity or affinity. The adsorption capacity is mainly determined by the molecular structure of the compound: e.g. benzene (C6H6) is very well adsorbed, while carbon monoxide (CO) is not adsorbed at all.
The larger (meso/macro) pores take care of resp. easy access and further transport of the molecules to their final destination: the micro pores.
Highly activated carbon may offer an extremely large total surface area (sometimes over 1.250 m²² per gram!) but have a relatively small amount of micro pores. Such carbons are very suitable to treat large volumes of saturated gas streams. The excess amount of molecules will find their way into the mesopores anyway.
Gas streams having lower concentrations of impurities are best treated with lesser activated carbon grades offering a more balanced pore size distribution.
NOTE: with activated carbon you do not buy a product, YOU BUY ACTIVITY.
Humidity of the gas stream:
Water is easily adsorbed by activated carbon. This does not necessarily mean that it will influence the adsorptive properties, but it is very important to keep the relative humidity of the gas stream below 75 %. At higher values, the carbon will increasingly adsorb water faster than anything else and a small layer of water will form on the surface of the carbon. This water film will hinder the natural attraction forces and will transport the molecules that need to be adsorbed through the carbon bed and out of the filter.
Conditions for optimum use:
- A sufficient amount of carbon is required to obtain almost 100% odour removal. A contact time of 0.2- 0.5 seconds should be maintained and the relative air velocity should be between 5 and 30 cm per second.
- As a rule of thumb the fan capacity should be such that 8- 10 times the volume of the vented space per hour can be treated. Eg- a room of 5 x 5 x 2.5 metres (62.5 m3) requires a fan with a capacity of 500 m3/hr. A carbon filter should be able to handle this volume. If you have any questions we will be able to better advise you about this.
- At higher temperatures (80-100 degrees C) the concentration in the air will decrease and thicker carbon heads will be required to obtain the same removal efficiency. If the temperature becomes too high, the carbon might even release the absorbed odour.
A relative humidity of over 75% will have a negative influence on the performance. The carbon will preferentially absorb water and the odour molecules will be transported out of the filter without being absorbed.
When will the carbon be exhausted?
Because of varying circumstances, calculations can hardly be accurate. Experience, however, learns that a correct installation will enable the carbon to perform for at least one year. You are well advised to change the filter on a regular basis and not wait until the damage is done. The cost of a new filter will be nothing compared to the possible consequences of a late change!!