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Drying
We are primarily concerned with drying wet, porous solids
(granules).
Important to ensure proper moisture content in the granules:
 low enough to prevent product degradation and ensure good
powder flow, but high enough to provide good tablet
properties
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CHEE 440
Definitions
psychrometry
 determination of the vapor concentration and carrying
capacity of drying gas
wet bulb T
dry bulb T
relative humidity (RH)
Pwate r in ai r
%RH 
 100
Pw ,sa t T 
moisture content
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CHEE 440
Psychrometric
Chart
Determine the relative
humidity and moisture
content in the air if the
wet bulb T is 35 ºC and
the dry bulb T is 50 ºC.
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CHEE 440
Types of Dryers
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CHEE 440
Static Dryers
Tray/truck driers
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CHEE 440
Drying behavior for static bed
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CHEE 440
Fluidized Bed
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CHEE 440
Spray Dryer
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CHEE 440
Drying Equations
Tray/Truck Dryers
 constant rate drying stage only
dm h c AT
=
dt

• parallel flow over a static bed
8.8G 0.8
hc 
D 0.2
• perpendicular flow through a bed
Nu  1.17Re 0.585 Pr 0.333
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CHEE 440
Drying Equations
Fluidized beds and spray driers
 use correlation for flow past an individual spherical
particle
Nu  2.0  0.61Re 0.5 Pr 0.333
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CHEE 440
Example
Wet granules (density = 1.5 g/cm3) are spread onto a
screen at an amount of 10 kg/m2 of screen. The bed
porosity is 45% and the wet granules have an average
diameter of 300 mm. Air at 60ºC(dry bulb) and a wet
bulb temp of 25ºC is passed through the bed at a
velocity of 0.15 m/s. How long will it take to dry the
granules from 20 to 10 % moisture? The critical
moisture content of the granules is 9 %.
B. Amsden
CHEE 440