The element carbon has been in existence since the dawn of time. It has a diverse presence in our world, with coal being the most familiar example. Diamonds, with their very ordered structure, are the purest form of carbon. Chains of carbon atoms also make up the backbone of all molecules in the human body and all living material around us. It is from these sources, particularly the botanical materials, that another form of carbon – activated carbon – has been derived.
Activated carbon is defined as a microporous adsorbent made from a carbonaceous material. The commercial raw materials are coal, wood, peat and coconut shells. The term “microporous” means that activated carbon, despite its solid appearance, is actually composed of a network of inter-linked fissures and pores which are present throughout the material. The pores are extremely small, ranging in size from <2>1000 nanometres (10-9m), and can only be seen with a powerful scanning electron microscope. The extent of the pores, and the range of their sizes, reflect to a large degree the structure of the original material. Volatile materials, such as water, tars and resins, are burnt off during the manufacturing process. Subsequently, the remaining matrix of carbon atoms is selectively oxidised to open up the original botanical pore network. The result is a material with an extremely high internal surface area (~1000 m2/g).
Due to its complex internal pore structure, activated carbon is used in the purification and filtration of gases. Contaminant gas molecules in an air stream can enter the large pores at the surface of the carbon through a process called diffusion and move towards the internal surface of the smallest pores.When a gas molecule collides with the carbon surface at a suitable site, an attraction is formed and it will be retained. Gas molecules are extremely small and are most strongly adsorbed in the smallest diameter pores. Commercial carbons that traditionally satisfy these criteria are manufactured from coconut shell and coal.
The first major application of activated carbon in air filtration was during World War I, when it was used in respirators to combat war agents such as chlorine, mustard gas and phosgene. Eighty years later, activated carbon is extensively used to remove odours and toxic or corrosive gases. The following table gives examples of well-established applications.
Camfil is at the forefront of carbon filter technology with the unique test facility in Sweden. This facility has the capability to test life-scale ventilation filter systems against a wide range of inorganic gases and organic vapours. Clearly this facility is unique in the filtration industry and demonstrates Camfil’s commitment to providing world-class products for the control of gas phase pollutants in any environment.
Carbon is indeed an old material but, in light of the above developments, it has a fresh and vital future in the field of air filtration.