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Adsorption principles

ADSORPTION DRYERS This method of drying produces compressed air with very low dew point (-20 °C to 70 °C). Drying by adsorption is a physical process. The water vapour in the compressed air is attracted to the surface of solid adsorption materials (desiccants) by molecular forces. Different types of desiccants are used in adsorption dryers like: activated alumina (standard), silicagel and molecular sieves. When the drying capacity of the desiccant has been used it must be regenerated, where after it is again active for drying. To enable continuous drying and regeneration to go on, the adsorption dryers have two drying towers that alternate functions: drying regeneration by means of an automatic valve system. Adsorption dryers can be classified depending on the type of regeneration used: Heatless, Heat reactivated, Rotating drum Adsorption dryers only handle water vapour. Since water and oil droplets can impair their efficiency, a filter is necessary to remove such droplets before they reach the dryer. Dust filters need to be installed down stream the adsorption dryers to stop any desiccant dust which might leave the dryer.
How it Works Water molecules are transported into the pores trough diffusion. Molecules are accumulated on the pore surface due to: physical binding chemical binding capillary-condensation
Water vapour transfers to the desiccant because. desiccant has a greater attraction for water than air. but The attraction slowly decreases as the desiccant gets wet meaning that the wet layer slowly rises up the tower, as dry desiccant is more attractive than wet desiccant. After 3 minutes (cycle time), the wet layer is almost at the top of the dryer. This means: That, the amount of water being taken out of the air is stable, and therefore a dew point -40°C is constant from the beginning to the end of the cycle. Also: If the dryer vessel continues to accept air, the PDP would slowly decay. After about 45 minutes it would be around -20°C. If the PDP becomes positive, the desiccant is ruined Finally: For a -70°C PDP, more water attractive desiccant is needed.
Things To Remember: The more desiccant used, the longer the vessel can keep the desire dew point. Dryer give the most optimum performance when the desiccant is suited to the PDP required In order that the dryer performance to reliable and stable, we add slightly more desiccant than the minimum required Crucial Parameters If the air temperature increases, the wet layer reaches the top more quickly. Pressure does NOT directly effect the dryer performance. BUT A desiccant bed is sized for a fixed volume of inlet gas, meaning the higher the pressure the more FAD can be dried The lower the nominal PDP, the less desiccant is needed for a fixed flow Why Do We Use Different Desiccant ? Different desiccant have different levels of water attraction Most desiccant cannot provide a -70°C PDP. Desiccant that can provide a -70°C PDP is more expensive What Desiccant Do We Use ? For PDPs of -20 to -40°C PDP Activated Alumina For PDPs of -70°C PDP Molecular Sieve