Drying Solids

A.  Spray dryer have drying times of a few seconds.  Rotary dryers have drying times ranging from a few minutes to up to an hour.
B.  Continuous tray and belt dryers have drying times of 10-200 minutes for granular materials or 3-15 mm pellets.
C.  Drum dryers used for highly viscous fluids use contact times of 3-12 seconds and produce flakes 1-3 mm thick.  Diameters are generally 1.5-5 ft (0.5 - 1.5 m).  Rotation speeds are 2-10 rpm and the maximum evaporation capacity is around 3000 lb/h (1363 kg/h).
D.  Rotary cylindrical dryers operate with air velocities of 5-10 ft/s (1.5-3 m/s), up to 35 ft/s (10.5 m/s).  Residence times range from 5-90 min.  For initial design purposes, an 85% free cross sectional area is used.  Countercurrent design should yield an exit gas temperature that is 18-35 °F (10-20 °C) above the solids temperature.  Parallel flow should yield an exiting solids temperature of 212 °F (100 °C).  Rotation speeds of 4-5 rpm are common.  The product of rpm and diameter (in feet) should be 15-25.
E.  Pneumatic conveying dryers are appropriate for particles 1-3 mm in diameter and in some cases up to 10 mm.    Air velocities are usually 33-100 ft/s (10-30 m/s).  Single pass residence time is typically near one minute.  Size range from 0.6-1.0 ft (0.2-0.3 m) in diameter by 3.3-125 ft (1-38 m) in length.
F.  Fluidized bed dryers work well with particles up to 4.0 mm in diameter.  Designing for a gas velocity that is 1.7-2 times the minimum fluidization velocity is good practice.   Normally, drying times of 1-2 minutes are sufficient in continuous operation.