Page 46 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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concentrations of the transferred substance. At the conditions in packed bed
columns both types of driving forces give practically the same results, ka and k L
are the coefficients of proportionality, called partial mass transfer coefficient,
respectively for the gas and for the liquid phase in m/s.

1.5.1.2. Overall mass transfer coefficient
The usage of Eq.(112) needs information about the concentration of the
transferred substance at the interface which is usually unknown. So to calculate
the value of G without data for the interface concentration this concentration has
to be excluded. This is possible by presuming that the two phases at the
interface are in equilibrium. The results of this presumption correspond
perfectly to the experimental data for a great variety of processes and its
adequacy is doubtless. So when the mass transfer through the interface is in
direction from gas to liquid phase, it can be written:

N A=k c(C G-C* Ci), (126)

NA=*L(<Z,-C L). (127)

where C Gi and C Li are the equilibrium concentrations of the gas and liquid
3
3
phase at the interface in kg/m or kmol/m .
The concentration profiles on the two sides of the interface are
presented in Fig. 7. The values of C G and C £ are connected by the equilibrium
constant m c, Eq. (116).
From Eq. (126 and 127) it follows:

(128)

C (129)

According to Eq. (113):

<4 , (130)