Carbon adsorbs H+W releases

An activated carbon adsorption system is helping Hickson & Welch reduce the emissions from its Castleford plant to unprecedentedly low levels. Aimed particularly at reducing solvent emissions, the system was supplied by Waterlink Sutcliffe Carbons, which also supplies a carbon regenerating and recycling service.

The system was necessary because of the type of chemistry the company carries out at Castleford. Specialising in `difficult chemistry', the complex includes 16 separate plants running complex reactions such as nitration, chlorination, hydrogenation and phosgenation. All of these are essentially based on toluene, and use organic solvents. This, of course, increases the risk of fire, so H&W uses nitrogen to form an inert blanket over the reactions and exclude oxygen. However, this inert gas then has to be treated to remove the solvent vapours it adsorbs in the reactor.

The first stage of the removal process is a condensation step using cold calcium brine, which removes 90 per cent of the solvents. The gases then pass into one of two cylindrical static beds packed with 750kg of carbon. This reduces the solvent concentration to 300mg/m3. A subsequent pass through the other bed cuts it even further, to less than 1mg/m3.

H&W considered several methods for solvent removal. Two options, low-temperature condensation and thermal oxidation, were rejected on cost grounds. `In selecting activated carbon there is great flexibility,' comments SHEQ manager Jack Poppleton. `Adsorber units need relatively low capital expenditure and have reasonable operational costs.'

Copyright: Centaur Communications Ltd. and licensors