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Chemical Engineering Operations
DryingDr. Anand V. Patwardhan
Professor of Chemical Engineering
Institute of Chemical TechnologyNathalal M. Parikh RoadMatunga
(East), Mumbai-400019
[email protected]; [email protected];
[email protected]
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Drying
Wet bulb, dry bulb and adiabatic saturation temperatures
Humidity
Drying mechanisms
Drying rate curves
s ma on o ry ng meProcess design of dryers such as spray,
rotary, tunnel, tray, fluid
bed and thin film dryers
Performance evaluation of dryers
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Final product is a solid .
Definition : A unit operation in which the liquid
(generallywater), present in a wet solid (or a suspension or a
solution) is
removed by vaporisation to get a relatively liquid-free solid
pro uc .
Solid-liquid operation such as filtration, settling,
centrifugation
Difference : In filtration, settling, and centrifugation, liquid
isremoved by mechanical means. The residual liquid may be
removed by drying . Also, in case of some liquids,
evaporation
is followed by drying . Drying
Milk CondenseEva
d Milk Milk porati
Poon
wder
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Physical Mechanism of Drying
Transport of Heat and Mass : When a wet solid is heated,moisture
vaporises at or near the solid surface (gas-solidn er ace . ea s
supp e y a o gas excep ons :
microwave drying, freeze drying).
.
Moisture movement: various mechanisms (depending on solid,,
Transport by capillary forces
qu us on
Pressure induced transportVapour diffusion
Different mechanisms may be predominant at different stages
4o ry ng.
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In case of granular and porous solids, moisture transport by cap
ary orces as ong as ere s enoug mo s ure n e uof the solid.
Larger capillaries are emptied first, but the smaller
onescontinue to supply liquid to the surface.
Also, some drying gas enters the solid through free passagesand
cracks Gas-filled spaces increase as drying proceeds.
.
High drying rate fast drying of outer layer compressive.
This is called pressure-induced transport of liquid within
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Colloidal, gelatinous, materials having great affinity for
water:.
As drying proceeds, the water concentration near the surfacev w
u
within the wet solid to exposed surface.
apor sat on may occur even e ow t e sur ace, an t e vapour may
have to diffuse out Vapour Diffusion .
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Transport Resistances in the Drying Process
Resistance to liquid / vapour transport inside the solid
.Resistance to convective mass trans ort of a our fromsurface to
surrounding gas bulk .
gas bulk to the solid surface .
important at low moisture content on solid)
Sometimes, if drying is very fast solid surface loses
moisturevery quickly surface becomes very hard and impervious
tomoisture drying stops (although sufficient moisture is still
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Drying Equilibriumqu r um re a on e ween e mo s ure con en o a
so anthe humidity of the surrounding .
As long as the moisture content of the solid >
equilibriumvalue (at given T and surrounding humidity) drying
occurs.
The pressure exerted by moisture in solid on surrounding .
The exerted pressure = f (nature of solid, moisture content of
solid T .
For nonhygroscopic solid: the equilibrium moisture content of
solid is almost zero at all T and surroundin humidit values.
For hygroscopic solid: the equilibrium moisture content of solid
have different values depending on T and surrounding
8humidity values.
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Below a certain moisture in solid and at a particular T,
partial
pressure of moisture < vapour pressure at that T.All moisture
below this threshold value: bound moisture.All moisture above above
threshold value: unbound moisture.
g
1.0
14 5 6
r o u n
d i
0.82
7M
y o f s u
0.6 T = room temperature1: asbestos fibre2: PVC
h u m
i d i 0.4
0.2
3: wood
4: Kraft paper5: ute
e l a t
i v e
0.0
6: wheat7: potato
9. . . . M
Equilibrium moisture content in solid
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Consider point M (wheat curve): relative humidity ( R HM ) =
1,and moisture content of solid = X
M.
On the right side of XM , the curve becomes horizontal, that
is,the relative humidity ( R HM ) = 1.
,equilibrium pressure exerted by wet solid = vapour pressure
atthe prevailing T.
This also means for a moisture content of solid < XM ,
the
the prevailing T.
mo s ure a ove M : un oun .
All moisture below XM : bound.
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Hygroscopic solids have high equilibrium moisture content.,
because a liquid filled in a capillary exerts a pressure
