Slide 1

Jangam, Mujumdar - Classification and Selection

Table 3.6. Conventional versus innovative drying techniques

Feed type Dryer type New techniques*

Liquid Suspension (^) • Drum

• Spray
  
  
  • Fluid/spouted beds of inert par-
    ticles
  
  
  • Spray/fluid bed combination
  
  
  • Vacuum belt dryer
  
  
  • Pulse combustion dryers
  
  
  • Spray freeze drying
  
  
  
  

Paste/sludge (^) • Spray

• Drum


• Paddle
  
  
  • Spouted bed of inert particles
  
  
  • Fluid bed (with solid backmixing)
  
  
  • Superheated steam dryers
  
  
  • Screw conveyor dryer
  
  
  
  

Particles (^) • Rotary

• Flash


• Fluidized bed (hot air
  or combustion gas)
  
  
  • Superheated steam FBD
  
  
  • Vibrated bed
  
  
  • Ring dryer
  
  
  • Pulsated fluid bed
  
  
  • Jet-zone dryer
  
  
  • Yamato rotary dryer
  
  
  • Screw conveyor dryer
  
  
  • Immerse heat exchanged dryer
  
  
  
  

Continuous sheets (^) • Multi-cylinder contact
dryers

• Impingement (air)
  
  
  • Combined impingement/radiation
    dryers
  
  
  • Combined impingement and
    through dryers (textiles, low basis
    weight paper)
  
  
  • Impingement and MW or RF
    *New dryers do not necessarily offer better techno-economic performance for all
    products
  
  
  
  

3.6.1. Classification of superheated steam dryers

Superheated steam drying technology was first proposed over 120 years ago but the
commercial applications started very recently. Superheated steam drying mainly in-
volves use of superheated steam as a drying medium instead of heated air or flue gases
to supply heat in a direct type of dryer. SSD is more complex and recovering heat from
the exhaust steam is a difficult task. However the net energy consumption can be lo-
wered if the exhaust steam is used elsewhere. Main advantages of superheated steam
drying to be highlighted are absence of any oxidation reaction during drying of food
products, improved drying rates (this has been proved for many food products), recov-
ery of toxic/expensive organic solvent removed during drying and pasteuriza-
tion/sterilization of food products. These types of dryers also have some limitations
such as complexity, initial condensation of steam, problems with drying of materials
which tend to melt or undergo glass transition. The superheated steam dryers can be
classified based on the operating pressure as shown in Figure 3. 5.