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The process of purifying the air with active charcoal is called adsorption. The technology of adsorption is based on the property of active charcoal to trap most Volatile Organic Compounds – V.O.C. Treatment takes place because active charcoal has a high degree of microporosity, which is difficult to imagine, that can reach over 1500 m2per gram of charcoal. Microscopic pores develop in depth and gradually become more narrow, which forms an extremely vast surface area. Active charcoal is mainly of vegetable origin and, when suitably processed and treated, is presented in the form of granules, chips or small cylinders. This last format is only a millimetres long.
Active charcoal filters/adsorption units are used in industrial sectors on process machinery and workplace environment purification units to protect air quality in the following sectors (for example):
Removal/recovery of solvents or mixtures of solvents Purifying air from production processes Removing oily mist from compressed air Deodorizing exhaust air and waste gases Removal of toxic substances in air conditioning systems Charcoal is contained in panels, pockets, and cartridges or simply inserted into containers of certain sizes that form the "bed" for the gaseous fluid containing the pollutant to adsorb. The function of active charcoal filters is based on the adsorption process, which is a molecular diffusion phenomenon that occurs between a gaseous component – a VOC
– and a solid substrate – charcoal. Charcoal's adsorption capacity is particularly indicated for the abatement of organic compounds with a molecular weight between 50 and 200. Organic compounds with a lower molecular weight are not adsorbed sufficiently because they are too small. Adsorption capacity is expressed in percentage weight, or rather in kg of organic contaminant adsorbed per 100 kg of active charcoal used. This capacity falls between minimum values of 1% and maximum values of 30%. The efficiency of active charcoal filters is influenced by a series of parameters such as the molecular weight and concentration of pollutants, temperature, humidity, pressure and presence of particulate matter in the flow to treat. Particulate matter reduces the charcoal's microporosity and also reduces the efficiency of adsorption. Therefore, this type of pollutant should be captured upstream with suitable prefilters. With moderate temperatures and humidity, active charcoal adsorption devices offer the best performance. To obtain the best results, it is advisableto work at temperatures lower than 50° with relative humidity not exceeding 70%, and obviously with the gaseous fluid passing over the charcoal bed at a carefully calculated flow rate.



It is very difficult to calculate the exact adsorption capacity of active charcoal with regard to a specific substance. It is more useful to do a spectrum classification. By defining four adsorption classes, it is possible to expect the results indicated in the table.