The principles on which adsorption in heat regenerated adsorption dryers are based were explained in Section 5.4.2. For this reason, the materially important facts are only mentioned briefly and supplemented with important system related peculiarities. Under normal conditions, the dynamic drying capacity of adsorption dryers regenerated externally by means of blower air, amounts to about 16-18%. For this, the internal and external surface of the drying medium is used for the storage of moisture. Taking up water vapour does not alter the form of the adsorbents. Water vapour adsorption by these substances depends on the temperature and on the water vapour concentration of the gas to be dried. Of practical importance for this type of drying is the so-called break-through load. After this is reached, complete drying is no longer possible and the adsorption medium must be regenerated. The regeneration temperatures usual in practice are around:
120-180°C for silica gel
150-200°C for activated alumina
180-320°C for molecular sieves
In most cases, silica gel is preferred as a drying medium, whereas the molecular sieve is considered when a pressure dewpoint of -40°C is not sufficient for the application in question. Activated alumina is used in special cases only then when the compressed air contains ammonia or hydrogen fluoride, as silica gel would be impaired in its adsorptive properties by the these substances over a period of time. Internal fittings in the adsorber depend on the system, and the entire content of adsorbent can be utilised for drying purposes. Loading the drying material with humidity from the compressed air flow takes place with the current flowing from bottom to top. The Mass Transfer Zone migrates with increasing saturation from the point of entry to that of outlet of the adsorber. Before the break-through point is reached, the system switches over to comply with the periods of the regeneration cycle. The cycles from adsorption to regeneration and the reverse are set with a view to full capacity utilisation, an economical loading up period of 4 - 6 hours meets most practical requirements. In practice, adsorption dryers must be able to cope with the most varied states of the ambient air. Fluctuations are caused by the seasonally conditioned meteorological changes. These different changes of state of the air exert a direct influence on the heat of adsorption and, as a secondary effect, may also cause significant changes in:
- the dynamic capacity of the adsorption material
- the running time of the drying installation between cycles
- the temperature of the dried air and thus also of
- the residual moisture in the dried air.
Taking into account these phenomena, up-to-date installations make use of two-layer adsorbers. The lower, waterproof, charge with large pores makes the initial contact with the compressed air entering in the moist or oversaturated state. This layer is endowed with a high loading capacity and offers the additional advantage not to be disintegrated by the impingement of high air moisture, water mist or water droplets, thus avoiding grain fracture, abrasion and increased pressure loss which would diminish the capacity or service life of the drying medium.
Figure 5.5.2.1
The first layer is arranged to be of sufficient height that the essential drying operation, accompanied by significant heat of adsorption, is concluded in this zone. Immediately afterwards, the predried air enters the second layer situated above. This layer is filled with hyperactive, small-pored drying medium. Depending on operating conditions, pressure dewpoints down to -55°C can be achieved in this layer under continuous operating conditions. The rise in temperature caused by loading the drying medium bed of adsorption dryers with heat regeneration always depends on the moisture content at entry and the operating pressure of the air to be dried. The outlet temperature of the compressed air lies between 12 - 20°C above the level of the inlet temperature under normal conditions of utilisation.
