In order to prepare the drying medium bed loaded with moisture for renewed adsorption, the stored humidity must be extracted from the drying material by means of regeneration. In heat regenerated adsorption dryers, the regeneration of the drying medium is carried out by countercurrent. Through regeneration by countercurrent, the high concentration of moisture at entry is not carried right through the total bed, thus at the same time preventing energy wasting double adsorption. With countercurrent regeneration, the pressure dewpoint is qualitatively set by the layer of drying medium located at the outlet from the adsorber during the adsorption phase. Only in the course of countercurrent regeneration is this layer exposed to an accurately specified temperature and moisture for a longer period, the pressure dewpoint thus being determined during adsorption.
The total sum of regeneration passes successively through two separate phases:Heating and cooling.
At the beginning of regeneration, the integrated electric heating is switched on and the drying medium bed slowly but steadily heated to the final regeneration temperature. The heating elements distributed within the adsorber radiate a ring of even heat (Fig. 5.4.3.1). The geometrical arrangement of the heating elements within the bed is decisive for an even distribution of heat within the adsorber. An ideal arrangement aims at preventing heated zone overlaps as well as cold zones. During the heating phase, the high temperature expels the moisture stored within the adsorption media. At a certain temperature, moisture evaporates and the increasing surface energy overcomes the adsorbtive force.
Figure 5.4.3.1
There is a by-pass duct with fixed orifice (item 5) at the outlet side of the adsorption dryer (Fig. 5.4.2.1). When the exhaust valve (item 6) is open, a fraction of dried compressed air flows towards the atmosphere in the depressurised state in a downward direction through the drying medium bed to be regenerated. Assisted by gravity, this flow drives the moisture out of the system. During the heating phase, the quantity of regeneration air amounts to about 5% on average. After the final temperature of 140-220°C is reached, a signal from the thermostat switches off the heating. Temperatures above 250°C should be avoided in order to prevent thermal damage to the drying medium. Such heating time amounts to 2-4 hours, depending on the loading level. The heat introduced into the drying medium bed in the course of the heating phase must be removed from the adsorber by the end of the regeneration period. Remaining residual heat at the end of a cooling phase, caused by too low a quantity of cooling air, leads to a peak in the pressure dewpoint level difficult to quantify as to time and extent. The required quantity of cooling air depends on the amount of heat which must be removed from the adsorber by the end of a cooling phase. Under favourable cooling conditions, the quantity of cooling air can amount to 4-8%. The cooling period with this system of dryer lasts 1-2 hours on average. The adsorption dryer using internal heat generation cannot be switched off during the heating phase and should not be switched off during the subsequent cooling phase as, otherwise, it is not possible to achieve an optimum time cycle of the build-up and run down of heat required for regeneration. Failing this, the danger of a large percentage of the moisture remaining in the drying medium bed, and being recondensed during cooling, arises. A safe point in time for switching off occurs only during the holding and pressure build-up period.
