CHEMICAL ENGINEERING

MASS TRANSFER 267

Integrating for the steady state, then:

NADD

CA 1 CA 2

y 2 y 1

DDCT

xA 1 xA 2



L

i

Case (b):The net heat effect at the interface must be zero and hence:

N^0 A (^9) ACN^0 B (^9) BD 0
and: N^0 BD
(^9) A
(^9) B

N^0 AD

1

f

N^0 A

In this case:

Total flux ofADDiffusional fluxCBulk-flow

or: N^0 ADD

dCA

dy

CuFCADDCT

dxA

dy

CxA

(

N^0 A

1

f

N^0 A

)

Thus:N^0 A

[

1 xA

(

1 

1

f

)]

DDCT

dxA

dy

N^0 A

∫L

0

dyDDCT

∫CA

2

CA 1

1

1 xA

(

1 

1

f

)dxA

∴ N^0 ALD

DCT

(

1 

1

f

)

[

ln

[

1 xA

(

1 

1

f

)]]xA 2

xA 1

D

DCT

(

1 

1

f

)

L

ln

1 xA 2

(

1 

1

f

)

1 xA 1

(

1 

1

f

) ii

From equations (i) and (ii):

RD

N^0 A case b

NA case a

D

1

1 

1

f

1

xA 1 xA 2

ln

1 xA 2

(

1 

1

f

)

1 xA 1

(

1 

1

f

)

SubstitutingfD 1 .5 then:

1 

1

f

D

1

3

xA 1 D 0. 75 xA 2 D 0. 65

RD

1

1

3

Ð

1

0. 75  0. 65

ln

1  0. 65 ð^13

1  0. 75 ð^13

D 1. 303

ForB: RD 1. 303 / 1. 5
D 0. 869.