ACTIVATED CARBON SYSTEMS

Description

Activated carbon is a solid material that is able to adsorb and remove gaseous organic pollutants from gas streams. Organic pollutants are adsorbed on the surfaces of the carbon and as the surface area is greatly increased by the extensive internal pore structure its ability to trap pollutants is very great. Up to 20 % of its own weight of organic material can be adsorbed. The activated carbon can then be regenerated by desorbing the pollutant material with steam. If the pollutant materials are clean or valuable then there is opportunity for recycling or recovery.

Contaminated gas may be blown through beds of activated carbon or through carbon-covered filters. Single-bed systems, cease operation when the activated carbon approaches saturation point. The carbon is then either desorbed or replaced. Desorption can take place in-situ, on-site or off-site. Double bed systems can operate continuously. In rotary systems adsorption and desorption occur simultaneously in different segments of the wheel.

Activated carbon can be made from a variety of raw materials such as coal, wood or coconut shell. These are roasted and treated with steam or chemical agents to create the vast pore structure. Wood-based activated carbon has larger pores and is suitable for the reduction of high loadings of contaminants. Coconut-shell-based activated carbon is rich in micropores and is appropriate for the removal of low concentrations of contaminants. Blends of different activated carbons confer specific properties. To increase the efficiency of removal of specific gases, such as hydrogen sulphide, activated carbon can be impregnated with other chemicals.

Activated carbon is available in pellets, granules or powder forms. Pellets, being larger, exert less backpressure on a system but are generally less efficient at removing contamination.

Application

Activated carbon systems have applications in removing VOCs from relatively dry air streams. A heater may be required to decrease the relative humidity of the gaseous emission, in order to avoid condensation in the bed. Gas temperatures should be below 30 °C for optimum operation. Typical applications are found in the printing, paper, textiles, and paint industries for the removal of alcohols, ketones, aromatics, aldehydes, esters and chlorinated hydrocarbons as well as sulphur dioxide, nitrogen dioxide, hydrochloric acid, hydrogen fluoride and chlorine.

Sizing

For the removal of a single type of compound, for example chlorinated hydrocarbons, small units tend to be sufficient. More-complex installations are required for the recovery of blends of alcohols, esters and hydrocarbons. A typical activated carbon system treating 2500 m3/hour of gas would require a 5 kW fan and a 10 kW heater. A system capable of on-site regeneration would tend to have a higher capital cost but lower running costs.