Page 51 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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1.5.2.2. Models for determining the partial mass transfer coefficients

1.5.2.2.1. Model of immovable film
Lewis and Whitman [16,17] based on the data for solution of solid in
liquid phase accept that on bom sides of the interface between gas and liquid, an
immovable or laminar film is build up, which separates the interface from the
fluid bulk. The concentration in the bulk is constant and the mass transfer to or
from the interface takes place only due to molecular diffusion. The value of N4
according to this model can be obtained by integrating of Eq. (140). Therefrom,
for the film at the gas phase side it follows:

— (C G-C* Gi), (144)

where DQ is the diffusivity in the gas phase, and zo is the corresponding film
thickness.
Respectively at the liquid phase side it can be obtained:

-C, ) fHSI
jy\ =ziL.fc* t

Di is the diffusivity in the liquid phase and z L is the corresponding film
thickness.
The comparison of the Eq. (144) end (145) with equations (126) and
(127) shows that according to the film model

B
k G=- - (146)

and

047)
z
i

The disadvantages of the film model are that the value of z (% and z£)
cannot be predicted, and especially that the power at D is equal to 1 for both
liquid and gas phase, which is in contradiction with the experimental results.