Solvents, volatile organic compounds (VOCs) and other contaminants in the liquid or gas phase can be adsorbed onto activated carbon.
Almost all materials containing a high fixed carbon content can potentially be activated. The most commonly used raw materials are coal (anthracite, bituminous and lignite), coconut shells, wood (both soft and hard), peat and petroleum based residues.
Most carbonaceous materials do have a certain degree of porosity and an internal surface area in the range of 10-15 m2/g. After activation, the carbon will have acquired a much increased internal surface area. The principle of adsorption is based on the highly porous structure of the granular carbon with each gram of activated carbon typically having a total surface area, including all its internal pore structure, of up to 1200 m2/g.
Contaminants are removed from the gas or liquid stream by the process of physical adsorption. Physical adsorption is dependent on the characteristics of the contaminant to be adsorbed, the temperature of the air stream to be processed, and the concentration of the contaminant in the air stream. The adsorption capacity for a particular contaminant represents the amount of the contaminant that can be adsorbed on a unit weight of activated carbon consumed at the conditions present in the application. Typical adsorption capacities for moderately adsorbed compounds range from 5 to 30 percentage of the weight of the carbon
When a carbon bed is ‘saturated’ with contaminants its efficiency of removal will reduce and would normally be removed and exchanged for new material with the exhausted carbon either disposed of or reactivated, in-situ by the carbon supplier.
Carbon adsorption is highly efficient and achieves high levels of removal, especially with VOCs and odours. However with large flows it may prove uneconomical. A useful procedure is to combine the economy of wet scrubbing or stripping with a subsequent ‘polish’ with activated carbon. The packed tower will remove the bulk of the contaminant while the final fraction – plus stray components not scrubbed or stripped – can then be adsorbed without prematurely exhausting the carbon bed.
Industrial applications for the control of odours and VOC emissions include exhaust air streams from textile heat-set stenters, printing, anaerobic digesters, waste transfer stations and food processing applications.