It is a common question we are often asked. Luckily the answer isn’t as complex as it seems. In order to understand how carbon filters work we first need to know how the activated carbon is produced.
How is Activated Carbon Made?
Activated carbon is most commonly made from coconut husks (shells). There are some carbon types that are made from other products such as bamboo, willow peat wood, coal and pitch but the vast majority are coconut husks.
There are two main ways to produce activated carbon.
- Physical Activation: The source material (coconut husk) is either Thermally Decomposed in an inert atmosphere with gases like Argon or Nitrogen, which irreversibly changes the chemical composition of the husk (Pyrolysis) at temperatures around 600 – 900 degrees Celsius. Oxidation can also be achieved by exposure to very hot (above 250 degrees C) oxidising atmospheres (oxygen or steam)
- Chemical Activation: Prior to carbonisation the raw material is impregnated with certain chemicals such as an Acid, strong Alkali or a Chemical Salt (such as Phosphoric Acid, Potassium Hydroxide, Sodium Hydroxide, Calcium Chloride or Zinc Chloride) These chemicals also change the properties of the carbon, making it more effective at adsorbing different chemical vapours more effectively. The raw materials can now also be carbonised at a lower temperature.
Why Carbon Filtration?
Activated carbon filters have several advances. Not only are they easily to produce and environmentally friendly (thus making them cost effective) but activated carbon filters are highly effective at removing vapours and gases from air streams. Particulate filters (such as HEPA filters) will only remove solid matter (dust) in the air stream, they won’t remove the gases and vapours at all.
It is worth noting at this point a Fume is a combination of solid matter, gases and vapours so both HEPA and Carbon filters are required to control them.
How does Activated Carbon Work?
Activated Carbon filters work in two ways;
Physical Adsorption
Physical adsorption is non-specific and adsorption of the molecule is by diffusion (Brownian Motion – The random movement of microscopic particles suspended in a liquid or gas, caused by collisions with molecules of the surrounding medium.) or adsorption/ condensing by Van Der Waals Forces (distance dependant interactions, attractions, repulsion’s between atoms or molecules)
The gas molecules move into and empty area and diffuse into the pore, Attracted and captured in the space by Van Der Waal’s force the molecules penetrate into the pores, impact on the carbon surface and become trapped.
Activated carbon particles have a vast surface area compared to the unit weight and therefore can adsorb a vast amount of molecules. Depending on the type of carbon used, aggregate surface area is in the region of 2000 square meters per gram. This means approximately 5 grams of activated carbon has an equivalent surface area of a football pitch.
The physical process of adsorption is followed by Chemical adsorption. This is the chemical reaction between the surface of the carbon and the captured molecule. The two react to each other and “bond”, trapping the molecule to the carbon.
Carbon Efficiency
How effective the carbon filters are is down to several factors. These include:
- Temperature – of the gases/ vapours being extracted
- Humidity – the carbon will naturally adsorb moisture in the air
- Dwell time – how long it takes for the air to pass through the carbon. More carbon and longer dwell times increase adsorption rate.
- Filter age
- Evaporation rate – how much and how fast the vapour or gas is produced
- Vapour/ Gas concentration
All of these factors combine to make carbon filters a highly effective control for most gases, vapours and fumes (along side HEPA filters).
