These dryers use a desiccant, which absorbs the water vapor in the air stream. A distinction needs to be made between adsorbing and absorb. Adsorb means that the moisture adheres to the desiccant, collecting in the thousands of small pores within each desiccant bead. The composition of the desiccant is not changed, and the moisture can be driven off in a regeneration process by applying dry purge air, by the application of heat, or a combination of both. Absorb means the material that attracts the moisture is dissolved in and used up by the moisture. Absorption takes place in a deliquescent desiccant type dryer.

Regenerative desiccant dryers normally are of twin tower construction. One tower dries the air from the compressor while the desiccant in the other tower is being regenerated after the pressure in the tower has been reduced to atmospheric pressure. Regeneration can be accomplished using a time cycle or on-demand by measuring the temperature or humidity in the desiccant towers or by measuring the dew point of the air leaving the on-line tower.

In the heated regenerative desiccant type, internal or external heaters are used. Purge air requirement can range up to 18% of the total air-flow. The typical regenerative desiccant dryer at 100 psig has a pressure dew point rating of -40°F but dew points down to -100°F can be obtained.