Drying

Reference: Unit operations, McCabe Smith

Drying Equipment

Fluid-bed dryers

• Solids are fluidized by drying gas

• High rate of mixing and heat transfer

– low residence times: 30-120 s

• Dry products taken out from side near bottom

• Substantial entrainment of solids

• Continuous or batch-mode of operation

Hot inlet air

Distributor plate

Feed

Dried Product

Exhaust gas

Fluidized-bed Dryer

Spray Dryers

• Slurry or liquid solution is dispersed in a stream of hot gas in form of tiny droplets

• Moisture is evaporated and fine particles of dry solid settle down in the tunnel

• Gas flow may be co-current, cross-current, or counter-current

• Droplets can be created using pressure nozzles, twofluid noozzles, high speed spray discs, etc

• Droplets/solids should not hit the walls of the tunnel

• Not very efficient, as large amount of heat is lost in discharge gases

• Product particles are hollow and very porous

Instant Drying Chamber

Drying Equipment

Hot Air inlet

Atomizer

Dried Product

Feed Inlet

Spray Dryer

Freeze drying

• Drying of heat sensitive materials

– Drying foods, vitamins, etc

• Carried out below 0 deg C under vacuum

• Instead of water evaporating, ice formed sublimes

• Drying time proportional to square of sample thickness

• Expensive

• Ice formation may damage small capillaries in the product

Design Considerations

• Batch dryers used for lower throughout: 150-200 kg/h

• higher product rates generally use continuous dryers

• Adiabatic dryers are less efficient than non-adiabatic ones but cheaper

• Dust removal systems may be need to remove particulates from drying gases, especially in adiabatic dryers

• Non-adiabatic dryers used for fine particulates, chemically reactive solids, solvent removal/recovery