Drying compressed airs through adsorption represents a purely physical process in the course of which water vapour (adsorbate) is bound to the drying medium (adsorbent) through binding forces of molecular adhesion. Adsorbents are solids in spherical or granular form which are permeated by a multiplicity of pores. The water vapour is deposited onto the internal and external surface of the adsorption medium, without the formation of a chemical compound taking place, therefore the adsorption medium does not have to be replenished but only regenerated periodically. The adsorption process can achieve compressed air pressure dewpoints of down to -110°C.

For drying compressed air, the adsorption medium is packed into a container. The size of the adsorber depends on the required quantity of drying agent which has to store the introduced moisture from the compressed air to be dried.

Adsorptive drying takes place in a two (or more) chamber system and is made up from adsorption1 and desorption².

Adsorption makes use of the ability of porous solids with large surfaces such as

Silica gel SiO₂
Activated alumina Al₂O3₃
Molecular sieve Na AlO₂ SiO₂

to selectively accumulate gases and vapours contained in low concentration from gas mixtures and thus separate them from the mixture.

¹ deposition of a material onto the surface of a solid
² release of the deposited material to the surrounding medium

For adsorption to take place, moist air is directed through the adsorber at operating pressure. In order to achieve effective drying, there must be a sufficiently long contact time between the compressed air and the bed of drying medium. The contact time depends on the flow velocity and the filling height. A typical twin tower adsorber is dimensioned in the proportion of about 1:2 of container diameter to filling height (The modular designed PNEUDRI compressed air dryer (Section 5.7) operates on a length/diameter ratio of 1:13).

Loading up the drying medium with moisture from the compressed air system takes place from bottom to top. Correct dimensioning of the drying chamber prevents swirling or lifting-off of the drying material in the adsorber through the upwardly directed movement of flow. In this, it is assumed that the prescribed flow velocity in relation to the operating pressure is adhered to. This direction of flow has the advantage that, should the operation of the installation be interrupted, the entering moisture does not overload the bed of drying medium but is caught, through the force of gravity, in the lower zone, free from drying material (fig. 5.1.2 item. 4), of the adsorber. A dust filter (item. 5) at the outlet of the adsorber, protects the equipment installed downstream from abraded particles of drying medium.

Too low a flow velocity causes an undesirable channel formation within the desiccant bed. Such a formation of channels is caused, when the effective speed of flow velocity in the adsorber is less than 10-15% of the nominal flow velocity

Two adsorbers are required for the continuous operation of an adsorption dryer, each filled with adsorption medium. Drying the air under pressure takes place in the first adsorber whereas, in the second adsorber connected in parallel, desorption of the drying material is effected in the unpressurised state. Each of the two adsorbers is connected to the other at the inlet and outlet by piping. The valves required for the switching over of the adsorption dryer from adsorption to desorption are integrated into the piping.

During desorption, the direction of flow takes place from top to bottom, i.e. in the opposite direction to that used during adsorption. At the beginning of desorption the adsorber, which is pressurised, is discharged to atmospheric pressure. This discharge momentarily causes a "blow-like" high flow velocity in the adsorber. However, with direction of flow from top to bottom, the drying medium is not swirled but pressed against the lower sieve (item. 4) in the adsorber.

On the other hand, a pressure release directed from bottom to top gives the drying material an extremely strong swirl in the upper drying medium zone. A consequence of this would be an excessive mechanical stressing of the drying medium, shortening the service life. As with adsorption, the flow velocity of the regeneration air must not lead to the formation of channels in the adsorber. If channels are formed, pockets of moisture stay behind in "dead corners" and these exert a negative influence on the number of cycles of which the drying medium is capable. Desorption is carried out by means of differing processes.

In one case, desorption is achieved by purging the adsorption medium using a branched off current of dried and depressurised air with an appropriately low water vapour pressure and without adding heat (Heat Regeneration).

Alternatively, the drying medium is regenerated by being subjected to heat, whereupon the vapour pressure of the water related to the adsorption medium rises correspondingly (Heat Regeneration).

The regeneration air removes moisture from the adsorber. These processes, heatless and heat regeneration, form the basic types of regeneration with adsorption drying.

Adsorbers are designed in order to remove the humidity contained in the compressed air in the form of vapour. Additional condensed moisture forms a supplementary load on the drying medium and, therefore, amounts to an overloading of the adsorption dryer.

Adsorption drying must always be applied in conjunction with filtration. A filter should be installed upstream of the dryer in order to eliminate condensate, oil droplets and solid particles, a filter downstream from the dryer to remove any abraded matter from the adsorber.