Shaped charcoals are activated only after having been formed. This makes the particles very hard and endows them with a high level of activity in contrast to granulates from powdered coal. Shaped charcoal is predominantly used in the gaseous phase. The adsorption processes then occurring, function on the basis of 4 - 5 seconds of contact time, the linear flow velocity of the gaseous phase lies within 0.1 - 0.5 m/s. To achieve the optimum effect of activated charcoal used for compressed air purification, turbulent flow has to be maintained.
Diagram 6.5.3.1
The favourable kinetics of activated charcoal are based on the fact that most substances to be adsorbed from the gaseous phase are present as relatively small molecules, the diffusion of which (within the network of pores) meets with few obstacles. Activates with fine pores are mainly utilised in the gaseous phase. The pore radius distribution and the level of activity of shaped charcoal up to 4 mm diameter is varied, thus offering optimum adsorption and desorption characteristics for solving any particular task. According to the course of the water vapour isothermal (Diagram 6.5.3.1), activated charcoal is hydrophobic (water repellant). Only from relative humidity above about 30%, water vapour is adsorbed to a significant extent. At even higher water contents in the air, the load capacity even for well adsorbable vapours is impaired through co-adsorption of water.
Diagram 6.5.3.2
When designing activated charcoal fillings for gaseous phase utilisation,.flow resistance must be taken into account. Diagram 6.5.3.2 shows pressure loss as a function of gas velocity and grain size.
