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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