Drying: General Methods of Drying
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bluestardiverDRYING
General Methods of Drying
• Batch:
o Material is inserted into the drying equipment
and drying proceed for a given period of time
• Continuous:
o The material is continuously added to the dryer
and dried material continuously removed
• Based on the physical conditions used to add heat
and remove water vapor:
o Heat is added by direct contact with the heated
air at atm pressure, an the water vapor formed is
removed by the air
o Vacuum drying: the evaporation of water via low
pressure, heat by contact with a metal wall or by
radiation
o Freeze drying: water is sublimed from the frozen
material
Rate of Drying Curves
• To estimate the size of dryer needed, various
operating conditions of humidity and temperature of
the air used, and the time needed
• Experimental determination of rate of drying
Drying 1
• For constant drying condition:
W − WS
Xt =
WS
Xt = free moisture content (kg H2O / kg dry solid) at
different times t hours in the drying period
W = weight of the wet solid (kg)
WS = weight of the dry solid (kg)
X = Xt – X*
X = free moisture content (kg free H2O / kg dry
solid)
X* = the equilibrium moisture content (kg
equilibrium H2O / kg dry solid)
• Method 1: plot X vs t
L S dX
R=−
A dt
R = the drying rate (kg H2O / h.m2
LS = dry solid used (kg)
A = exposed surface area (m2)
• Method 2: calculate the weight loss ∆X for a ∆t time
L S ∆X
R=−
A ∆t
The R is the average over the period ∆t and plotted at
the average concentration X
Drying 2
• Plot of rate-of-drying curve:
t=0 A or A’
A to B evaporation rate increases
B to C constant-rate-of-drying period
C to D falling-rate period (often linear)
C critical free moisture content XC
D to E falls more rapidly
E the equilibrium moisture content X*
Drying in the Constant-Rate Period
• The surface of the solid is initially very wet
• A continuous film of water on the drying surface
• The rate of evaporation is independent of the solid –
the same as the rate from a free liquid surface
• Increased roughness – higher rates
Drying in the Falling-Rate Period
• At critical free moisture content XC – insufficient
water on the surface to maintain a continuous film of
water
• Surface is no longer wetted
• Wetted area – continuous decreases until complete
dryness at D
• After D – the plane of evaporation recedes from the
surface
• Heat is transferred through the solid to the
vaporization zone
• The amount of moisture removed – relatively small
• The time required – long
Drying 3
Moisture Movement in Solids
• Liquid diffusion – if the wet solid is at T below Tb of
the liquid
• Vapour diffusion – if the liquid vaporizes within
material
• Knudsen diffusion – drying at very low T & p, eg.:
freeze drying
• Surface diffusion (possible but not proven)
• Hydrostatic pressure differences – when internal
vaporization rates exceed the rate of vapour transport
through the solid to the surroundings
• Capillary movement in porous solids
o Granular & porous solids (e.g.: clays, sand, soil
etc)
o Capillary action (not by diffusion) – provides
driving force for moving water through the pores
to the surface
• Effect of shrinkage
o Colloidal and fibrous materials (e.g. vegetables)
o Development of a hard layer – blaockage &
slows the drying rate (case hardening)
Drying 4
Calculation Methods for Constant-Rate Drying
Period
• Method 1: Use the experimental drying curve
Example:
A solid whose drying curve is represented by
(Figure) is to be dried from a free moisture content
X1 = 0.38 kg H2O/kg dry solid to X2 = 0.25 kg
H2O/kg dry solid. Estimate the time required.
From the Figure,
X1 = 0.38 t1 = 1.28 h
X2 = 0.25 t2 = 3.08 h
The time required: t = t2 – t1 = 1.80 h
• Method 2: use the rate-of-drying curve for constant-
rate period
L S dX
R=−
A dt
t2 =t X
L S 1 dX
t = ∫ dt =
t =0 A X∫ R
1 2
R = constant = RC
LS
t= (X1 − X 2 )
AR C
• Example: LS/A = 21.5, RC = 1.51 kg H2O/h.m2
Drying 5
Calculation Methods for Falling-Rate Drying Period
• Method using graphical integration
X
L S 1 dX
A X∫ R
t=
2
Material & Heat Balances for Continuous Dryers
• Simple heat and material balances
Entrance: solid enters at LS kg dry solid/h, having
free moisture content X1 and TS1
Exit: X2, TS2
Gas enters at rate G kg dry air/h, having a humidity
H2 kg H2O/kg dry air, TG2
• Material balance on the moisture:
GH2 + LSX1 = GH1 + LSX2
• Select a heat datum T0oC for the heat balance → 0oC
• The enthalpy of gas H’G in kJ/kg dry air.K
HG’ = cS(TG – T0) + Hλ0
Drying 6
λ0 = latent heat of water at T0oC (2501 kJ/kg at 0oC)
cS = humid heat (kJ/kg dry air.K)
cS = 1.005 + 1.88H
• The enthalpy of wet solid
HS’ = cpS(TS – T0) + XcpA(TS – T0)
cpS = heat capacity of the dry solid (kJ/kg dry
solid.K)
cpA = heat capacity of the liquid moisture (kJ/kg
H2O.K)
• The heat of wetting is neglected
• A heat balance on the dryer:
GH’G2 + LSH’S1 = GH’G1 + LSH’S2 + Q
Q = heat loss in the dryer (kJ/h)
For adiabatic process: Q = 0 and if heat is added, Q is
negative
• Example
Drying 7