Air Drying by Adsorption
In case compressed air with a pressure dew point of –40 °C / -40 F or lower is required, the application of a heatless adsorption dryer will be the best solution in terms of efficiency and costs in most of the cases. The adsorption drying process is very similar to the PSA process, but has less process steps and longer cycle times.
A heatless adsorption air dryer from NOXERIOR normally consists of two adsorption towers, both filled with a desiccant material, such as activated alumina or a specific molecular sieve. As moisture is adsorbed by the desiccant material while the compressed air travels from bottom to top in one of the towers, the desiccant bed slowly will become saturated. At the point of saturation the supply of compressed air to the first tower will be closed and the saturated desiccant bed will be purged by simply flowing some of the produced dry compressed air from top to bottom and to exhaust it with the released moisture from the bed to the ambient. While the desiccant bed of one tower will be regenerated, the other tower will be in production in order to guarantee a continuous flow of dried air. The term “heatless” refers to this particular regeneration procedure, as the alternative procedure would be a bed regeneration with heated ambient air.
The performance of heatless adsorption dryers depends on temperature as well as operating pressure and flow. With increasing operating temperatures, both efficiency and productivity will drop.
Air Drying by Dehydration Membranes
Dehydration hollow fibre membranes are an alternative technology for air drying, which become particularly attractive for dew point suppression or applications in environments with high ambient temperatures. Their functional principle is very similar to the hollow fibre membrane modules for nitrogen production. Dehydration membrane air dryers are quiet, reliable and require no electricity to operate.
Saturated compressed air (but free from liquids) passes through a bundle of hollow fibres packed inside a membrane module. At the same time, a small portion of the produced dry air is redirected along the outside surface of the fibres to sweep out the water vapour which has permeated the membrane wall. The moisture-laden sweep gas is then vented to the atmosphere. The sweep gas flow to the dehydration membrane module is automatically adjusted through an orifice installed in the sweep port of the membrane module.
Also the performance of dehydration membrane air dryers will depend on temperature as well as operating pressure and flow. Although efficiency will drop with increasing operating temperatures, productivity of dehydration membrane modules will increase at the same time. This particular behaviour makes dehydration membrane modules an interesting alternative at high ambient temperatures.
For some applications only a slight water vapour removal is required, for example to suppress the pressure dew point of air/gas upstream of an activated carbon vessels or air separation membranes. This dew point suppression has the same effect like a process heater which increases the air temperature to achieve a super-heated state.