2.3 Chemical dryers

When absolute dry air is needed chemical dryers are used. These dryers are of two types namely adsorption dryer and absorption dryer.

2.3.1 Adsorption dryers

Fig. 6.3.4 Adsorption dryer

In Adsorption dryers, the moisture collects on the sharp edges of the granular material. The adsorbing materials can be silicon dioxide (silica gel) or other materials which exist in hydrated and dehydrated state (copper sulphate, activated alumina). Moisture from the adsorbing material can be released by heating in the column as shown in Fig. 6.3.4. At a given time, one column will dry the air while the other column will regenerate the adsorption material by heating and passing low purge air. The column B dries the air and column C regenerates. The rotary valves are opened using time clock at regular interval to reverse the process. These driers are also called regenerative dryers.

2.3.2 Absorption dryers

These are also called as deliquescent dryers. Figure 6.3.5 shows a schematic of the same. It uses chemical agents like phosphoric pentoxide or calcium chloride as drying agents.  The moisture in the compressed air chemically reacts with the drying agent. The agent dissolves to form a liquid compound which collects at the bottom of the dryer where it can be drained out. The deliquescent agent has to be replenished regularly as it gets consumed during the drying process.

Fig. 6.3.5 Absorption dryer