drying

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 B. Amsden

CHEE 440

Definitions psychrometry  determination of the vapor concentration and carrying capacity of drying gas

wet bulb T dry bulb T relative humidity (RH)

Pwater in air
%RH =
×100
Pw,sat(T )

moisture content
B. Amsden

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 hcAΔT
=
dt λ • parallel flow over a static bed
8.8G0.8
hc = 0.2
D

• perpendicular flow through a bed
0.585

Nu =1.17Re

B. Amsden

0.333

Pr

CHEE 440

Drying Equations
Fluidized beds and spray driers
 use correlation for flow past an individual spherical particle 0.5

Nu=2.0+0.61Re Pr

B. Amsden

0.333

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 m. 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