Page 142 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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equilibrium concentration, small enough to be sure that the process is gas-film
controlled, and great enough to be precisely analyzed in the gas phase.
5. The mass transfer coefficient obtained in point 3 is recalculated for
the different value of the diffusivity of the evaporated liquid in the gas phase,
taking into account that the gas-film controlled mass transfer coefficient is
proportional to the diffusivity to the power of 2/3.
6. Knowing the volumetric mass transfer coefficient obtained in point 4
and the mass transfer coefficient obtained in point 5, the effective surface area
can be obtained.

2.1.2.5.10. End effect of effective surface area
Similarly to the liquid holdup, the effective surface area has also its end
effect. In this case it is the surface of the drops and jets over and under the
packing. Calculating the value of a e using for example Eq. (69), we take into
account also the end effect. Practically, at a given value of the packing height
Hi the real end effect can be obtained by the equation:

(70)

where a e/ is the effective surface area calculated using Eq. (69) at a height H t
without correction for the end effect, a e is the real effective surface area and A e-
the end effect.
In the same way for experiments carried out with another packing
height Hi, quite smaller than Hi, it can be written

(71)

a e2 is also the effective surface area calculated using Eq. (69) at a height Hi
without correction for the end effect.
From the two equations the real value of a e can be obtained:

H a H a
a = ' "- ' * (72 )
e
H,-H 2

From Eq. (70) it follows: