Drying Compressed Air Using Absorption and Adsorption Drying

All atmospheric air contains some amount of water vapor. When air is compressed, the water concentration increases. To avoid future problems in the compressor installation, the compressed air needs to be treated. In this article we are talking about absorption drying and adsorption drying.

The general working principle of adsorption dryers is simple: moist air flows over hygroscopic material (typical materials used are silica gel, molecular sieves, activated alumina) and is thereby dried. The exchange of water vapor from the moist compressed air into the hygroscopic material or "desiccant", causes the desiccant to gradually be saturated with adsorbed water. Therefore, the desiccant needs to be regenerated regularly to regain its drying capacity, and adsorption dryers are typically built with two drying vessels for that purpose: The first vessel will dry the incoming compressed air while the second one is being regenerated (Similar to the workings of a nitrogen generator).

Each vessel ("tower") switches tasks when the other tower is completely regenerated. Typical PDP that can be achieved is -40°C, which makes these dryers suitable for providing very dry air for more critical applications. There are 4 different ways to regenerate the desiccant, and the method used determines the type of adsorption dryer. More energy-efficient types are usually more complex and, consequently, more expensive.

1. Purge regenerated adsorption dryers ("heatless-type dryers"). These dryers are best suited for smaller air flow rates. The regeneration process takes place with the help of expanded compressed air ("purged") and requires approx. 15–20% of the dryer's nominal capacity at 7 bar(e) working pressure.
2. Heated purge regenerated dryers. These dryers heat up the expanded purge air by means of an electric air heater and hence limit the required purge flow to around 8%. This type uses 25% less energy than heatless-type dryers. 
3. Blower regenerated dryers. Ambient air is blown over an electric heater and brought into contact with the wet desiccant in order to regenerate it. With this type of dryer, no compressed air is used to regenerate the desiccant material, thus the energy consumption is 40% lower than for heatless-type dryers. 
4. Heat of compression dryers ("HOC" dryers). In HOC dryers the desiccant is regenerated by using the available heat of the compressor. Instead of evacuating the compressed air heat in an after-cooler, the hot air is used to regenerate the desiccant. This type of dryer can provide a typical PDP of -20°C without any energy being added. A lower PDP can also be obtained by adding extra heaters.

Guaranteed separation and drainage of the condensation water must always be arranged before adsorption drying. If the compressed air has been produced using oil-lubricated compressors, an oil separating filter must also be fitted upstream of the drying equipment. In most cases a particle filter is required after adsorption drying. HOC dryers can only be used with oil-free compressors since they produce heat at sufficiently high temperatures for dryer regeneration A special type of HOC dryer is the rotary drum adsorption dryer. This type of dryer has a rotating drum filled with desiccant of which one sector (a quarter) is regenerated by means of a partial flow of hot compressed air (at 130–200˚C) from the compressor.

Regenerated air is subsequently cooled, the condensation is drained and the air is returned via an ejector device into the main compressed air flow. The rest of the drum surface (three-quarters) is used to dry the compressed air coming from the compressor after-cooler. A HOC dryer avoids compressed air loss, and the power requirement is limited to that required for rotating the drum. For example, a dryer with a capacity of 1000 l/s only consumes 120 W of electrical power. In addition, no compressed air is lost and neither oil filters nor particle filters are required.