The basic principles of adsorption, related to velocity, dwell time and direction of flow inside the adsorber, also apply to adsorption dryers with heat regeneration. The following section, therefore, puts the stress on characteristics particular to systems with heat regeneration thereby, omitting the generally valid properties of adsorption drying. Adsorption dryers with internally applied heat regeneration, utilise the dynamic receiving capacity of the adsorbents up to maximum 16-18%. Dynamic capacity makes use of the internal and external surface of the drying medium in order to store moisture. In line with the maximum capacity utilisation, the cycles from adsorption to regeneration and back are considerably longer as with a heatless regeneration system. An adsorption period from 4-8 hours has proved its worth. Longer adsorption times call for a larger adsorber with a correspondingly larger quantity of drying medium. Shorter periods present problems, particularly in the case of unfavourable load patterns, because of the regeneration time running in parallel, during which heating and cooling must take place while respecting the overall heat requirement. From the air or gas mixture, adsorption media preferentially adsorb components with high boiling points. In general, the differences between the boiling temperatures of the adsorbate and of the carrying gas are large, so that the carrying gas has no effect on the course of adsorption. However, adsorbates with closely adjacent boiling temperatures make separation into components difficult or even impossible. The partial pressure gradient from the dry adsorbent to the moist compressed air causes the deposition of the moisture from the compressed air onto the receptive drying material. In the case of dynamic adsorption in systems with heat regeneration, the compressed air to be dried flows around the drying medium. This causes the drying material to be slowly charged with moisture in the direction of mass flow. A so-called loading or mass transfer zone is formed. Once this zone reaches the adsorber outlet, intensity of drying diminishes and the dewpoint rises steadily.
Figure 5.4.2.1
In contrast to heatless regeneration, heat regeneration also makes use of the internal surfaces of the adsorbents for storing moisture. To have a better understanding of what happens, the storage of moisture from the compressed air by the drying medium is explained in more detail here. Bonding forces are effective between the individual molecules of the adsorbents. Within the substance, each molecule is orientated in line with the adjacent molecules and thus subjected to the same forces. This state is not achieved at the outside surfaces because the bonding forces are unsaturated here. This free energy exerts an attraction for the water molecules, as soon as the latter reach the tension range of the surface. If this attracting force of the drying medium molecules is sufficiently large in order to overcome the inherent energy of the water molecule, it adheres to the surface. The adsorptive forces in the micropores of the drying medium are particularly strong because of the adjacent surfaces with overlapping of the potential fields. As the pore diameters can be as low as the size range of molecule dimensions, these pores can become filled although the surface itself is covered by a monomolecular layer only. Capillary condensation is a further action effective in the boundary pores on the strength of monomolecular deposition. The phase capable of adsorption has a high surface energy through its large specific surface. Both materials, moisture and drying medium, aim to achieve the state with the lowest energy level, so that the moisture content of the compressed air condenses, supported by the capillary and adsorption forces, while surface and surface energy diminish at the same time. This process releases a quantity of heat, adsorption heat, in the adsorber bed. Latent heat of adsorption depends on the amount of moisture already stored in the drying medium under conditions of operating pressure. The higher the percentage of moisture at the point of entry into the adsorber, the higher will be the heat of adsorption. Under unfavourable operating conditions, adsorption may lead to temperatures in the drying medium bed which approach the lower temperature range of regeneration and thus prevent effective drying. At the outlet of the adsorption dryer, the temperature can be 12-20°C higher than at the point of entry, given normal operating conditions.
