CH351 Mass Transfer 21Sep07

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CH351 Mass Transfer

Sitaraman Krishnan

9/26/07

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Fick¡¯s First Law: Steady State

Molecular Diffusion

J A,z

J A,z

?c A

= ?DAB

?z

= flux of component A along z - axis, mol/(cm 2 ? s)

DAB = diffusion coefficient, cm 2 /s

?c A

= concentrat ion gradient along z - axis

?z

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From Kinetic Theory or Low Density Gases :

DAA *

DAA *

1

2

?¦Ê T ?

?

?

= 3

? m ?

2

2

?

3¦Ð d P ?

= self diffusion coefficient

2

3

3

d = diameter of (spherical) molecules

m = mass of a molecule

Fuller, Schetler and Giddings equation :

DAB =

0.00143T 1.75

P (M AB )

1

2?

??

(¦²v )A

1

3

+ (¦²v )B

1

3?

??

2

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Wilke - Chang Equation for

Diffusion in Liquids :

DAB =

7.4 ¡Á 10

?8

(¦µBMB )

¦ÇBVA

12

T

0.6

Species A is diffusing in solvent B

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Partial Pressures, Molar

Concetrations, Total Pressure

nA

pAV = n A RT ? pA =

RT

V

pA = c A RT

pB = cB RT

(pA + pB ) = (c A + cB )RT

P = cRT

? pi = partial pressure of

component i

? ci = molar conc. of

component i

? P = total pressure

(atm, bar, N/m2, mmHg,

Torr)

? c = total molar

concentration of gas

(mol/cm3, mol/m3, ¡­)

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