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8/12/2019 Class Problem Solution
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SOLUTION OF CLASS PROBLEM-18 , ChE422 (Ad. Sepro)
On
Drying
Prob-1
A batch of solid for which the following table of data applies is to be dried from 25%
to 6% moisture under conditions identical to those for which the data were tabulated.
The initial weight of the wet solid is 300 kg and the drying surface is 1 m2/8 kg dry
weight. Determine the time for drying.
X 0.35 0.25 0.20 0.18 0.16 0.14 0.12 0.10 0.09 0.08 0.064
N 0.3 0.3 0.3 0.266 0.239 0.208 0.180 0.150 0.097 0.07 0.025
Where X = kg moisture / kg dry solid
N = kg moisture evaporated / hr m2
Rate of batch drying,d
dX
A
LN S=
Drying time from above equation, = 1
2
X
X
S
N
dX
A
L
Drying time consists of a constant rate (C) and a falling rate period (f)
Thus
c
cS
cNA
XXL )( 1 = and =
1
2
X
X
S
fN
dX
A
L
If the rate (N) vs moisture content (X) plot during falling rate is a linear one, then
*
2
** ln)(
XX
XXXX
AN
L cc
c
s
f
=
X* = equilibrium moisture content, X2= final moisture content
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Solution of CLASS PROBLEM-19 , ChE422 (Ad. Sepro)
On
Drying
Prob-1
Where X = kg moisture / kg dry solid
N = kg moisture evaporated / hr m2
Rate of batch drying,d
dX
A
LN S=
Drying time from above equation, = 1
2
X
X
S
N
dX
A
L
Drying time consists of a constant rate (C) and a falling rate period (f)
Thus
c
cS
cNA
XXL )( 1 = and =
1
2
X
X
S
fN
dX
A
L
If the rate (N) vs moisture content (X) plot during falling rate is a linear one, then
*
2
** ln)(
XX
XXXX
AN
L cc
c
s
f
=
X* = equilibrium moisture content, X2= final moisture content
SOLUTIONSince the falling rate of drying is linear in the moisture concentration, X, the drying
time can be calculated directly from equation
GIVEN:
Ls/A = 35 kg/m2dry basis, Xi= 0.30, Xc= 0.1, X
*= 0.002
Xf(X2) = 0.01, Nc= 4.5
c= 1.55 hr
f= 1.91 h
total time = 3.46 hr (i)ANS
(ii) The drying rate in the falling rate period say N = pX + qAt X = Xc= 0.1, N = Nc= 4.5 and X = X
*= 0.002, Nc= 0
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Which gives p = 45.92 and q = -0.092
Therefore when X = 0.05, N = (45.92 x 0.05 ) 0.092 = 2.2 kg/m2hr
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Prob 2
Selective permeation CO2from a mixture of 10% CO2 (A) and 90% CH
4(B) occurs
at 35 C and 10 atm total pressure in a small apparatus with a well mixed feed
compartment. An asymmetric polysulphone membrane of 1 micron skin layer
thickness is used. The permeate side is continuously swept with nitrogen gas.
Calculate the flux of CO2and average diffusivity of CO
2in polysulphone
Given SA= 2.1 cm3
(STP)/(cm3
)(atm), permeability of CO2=5.6*10
-10
cm3 (STP)
cm/(m2
)(s)(cmHg)
Solution
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PROBLEM 3
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THIS PROBLEM 3 OF CLASS PROBLEM 22 IS FOR KNOWLEDGE ONLY,
NOT FOR EXAM