Drying

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• Drying

R. Gary Hollenbeck
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• Thermal Dehydration

y Solid. x H2O y Solid x
H2O
x amount of water y amount of solid WD x/y
moisture content (dry weight) WW x/(xy)
moisture content (wet weight)
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• Comparison of Moisture Content Expressions

WW
1
0.8
0.6
0.4
0.2
0
0
1.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
WD
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• Comparison of Moisture Content Expressions

WW
1
0.8
0.6
0.4
0.2
0
0
10
1
2
3
4
5
6
7
8
9
WD
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• Comparison of Moisture Content Expressions

WW
WD
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• Heat Transfer Mechanisms

• Convection
• Conduction
• Radiation

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• Mass Transfer Mechanisms

• Diffusion
• vapor
• liquid
• Capillary Flow
• Shrinkage/Pressure Gradients
• Gravity
• Vaporization/Condensation

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• Tray Drying

Hot Air
Qc Convective heat transfer rate Qc
energy/time cal/hr
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• Tray Drying

Hot Air
A
L
Qc Convective heat transfer rate Qc hc A
?T hc convective heat transfer coefficient A
Area ? T Temperature differential
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• Drying of Wet Solid

Hot Air
A
L
-dx/dt rate of moisture loss -dx/dt Qc/ ?Hvap
(moles water/hr) ?Hvap heat of vaporization
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• Tray Drying

Hot Air
A
L
-dx/dt rate of moisture loss -dx/dt hc A ?T/
?Hvap
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• Constant Rate Period

y weight of dry solid y r V r A L r bulk
density V Volume of bed -dx /y -dWD
hc ?T dt dt r ?Hvap
L -dWD/dt Kc


Note terms on rhs are constant as long as free
moisture exists at the bed surface.
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• Drying Curve

Wo
Constant Rate Period slope Kc
Wc
WD(t)
time
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• Falling Rate Period

When dry areas begin to appear on the surface of
the bed, and mass transfer of water to the
surface becomes rate-limiting.
-dWD/dt KF (WD(t) - We) KF Falling Rate
Constant We Equilibrium Moisture Content
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• Drying Curve

Wo
Wc
WD(t)
Falling Rate Period
We
time
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• Drying Curve

Wo
Constant Rate Period
KC KF (WC - We)
Wc
WD(t)
Falling Rate Period
We
time
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• Classical Bed Dryer

Adjustable Louvers
Exhaust Outlet
Fresh Air Inlet
Fan Screen
Heater
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• Vertical Fluid Bed Dryer

• Fresh Air Inlet Filter
• Steam or Electric Heater
• Air Circulating Fan
• Air Volume Damper
• Air Diffuser
• Vessel Containing Material
• Nylon Filter Bags
• Air Exhaust

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• Conveyor Fluidized Bed Dryer

Exhaust Blower
Surge Hopper
Dust Collector
Exhaust Air Duct
Dry Material
Vibrating Conveyor
Conveying and air distributing surface
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Spray Dryer

• Feed Tank
• Centrifugal Atomizer
• Drying Chamber
• Inlet Air Filter
• Air Supply Fan
• Air Heater
• Triple Inlet Duct
• Adjustable Air Disperser
• Cooling Air Fan
• Chamber Product Collector
• Cyclone Product Collector
• Exhaust Fan

Hot Air Liquid Feed Dry Powder Hot Air
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3
8
12
1
7
9
11
6
10
5
4
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• Activity 1

In a continuous dryer, should the air flow be
con-current or counter-current?
Con-current
Counter-current
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• Activity 2

How will the drying rate curve change if the wet
solid is dried in a fluidized bed dryer instead
of a tray dryer?
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• Activity 3

Analyze the stages in drying for a solution being
dried in a spray dryer.
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• Activity 4

Predict how the distribution of a water-soluble
drug in a wet granulation would be influenced by
the drying process in a tray dryer.
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• Activity 5

Predict how the distribution of a water soluble
drug in a granulation would be influenced by the
drying process in a fluid bed dryer.