Activated Alumina Desiccant for Gas Drying

Activated alumina desiccant dries gases and liquids by adsorbing their moisture. It dries compressed air, natural gas, hydrogen, argon and other gases, as well as process liquids, taking water vapour out of the stream and holding it on its large internal surface so the gas leaves dry. It is one of the most common desiccants in industrial drying, and the same material is also used to remove fluoride and arsenic in water treatment.

In a normal twin-tower dryer, activated alumina brings a compressed-air or gas stream down to a pressure dew point of around minus 40 degrees, dry enough for most instrument air and process gas. It holds a heavy load of moisture before it has to be regenerated, and where an even lower dew point is required it is often paired with a molecular sieve that polishes the gas further. The dew point reached depends on the gas, the pressure and the drying cycle.

A desiccant sits in a bed and is cycled hard, so soft beads rub together, powder, and the dust both blocks the flow and lets the gas channel past the bed, which cuts the drying and fouls downstream filters. Activated alumina beads are hard and low in abrasion loss, so they hold their shape and keep the bed clean and even over a long service life. That toughness is a big part of why it is picked for continuous dryers.

Activated alumina is regenerated by driving the held water back off. In a heat-regenerated dryer the spent tower is warmed, commonly to around 175 to 250 degrees, while a purge gas carries the moisture away; in a pressure-swing dryer the bed is regenerated by dropping the pressure instead. Either way the desiccant returns to full capacity and the two towers swap over, so drying runs without a break. It regenerates over many cycles before it needs changing.

It comes down to how dry the gas must be and how much water there is to take out. Activated alumina is a strong general desiccant with a high moisture capacity and hard, reusable beads, and it suits most compressed-air and gas drying. Silica gel takes up more water at high humidity but is softer, while molecular sieve reaches a far lower dew point where very dry gas is needed. Many dryers layer activated alumina or silica gel first and finish with a molecular sieve.