Page 134 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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accuracy is higher when the process is controlled only by the side of one of the
phases.
The method gives also the possibility to calculate the mass transfer
coefficient for the diffusion model with the corresponding data from the piston
flow model.

2.1.2.5. Wetted and effective staface area of the packing
The effective surface area a e is the suffice area of the packing per 1 m 3
of its volume, effective for the mass transfer processes. Because it is related
with the wetted area of the packing a m both values are usually considered
3
1
together. Their dimension is m /m or m" .
Actually, there are some significant differences between these two
values, namely:
1. Not all the wetted surface area is effective for the mass transfer
processes. It is well known that a part of the wetted area is not movable first of
all because of capillary forces at the places of touching of neighbour packing
elements. After saturation the immovable area does not take part any more in
the mass transfer process and should not be taken into account in calculation of
the apparatuses.
2. The surface area of the drops and jets trickling in the free volume of
the packing is not a part of the wetted area, but in some cases it is a significant
part of the effective surface of the packing. For example the effective surface
area of Holpack, a packing of horizontal sheets of expanded metal, is up to three
times larger than its specific surface area [19]. The effective surface area of the
Raschig Super Rings No 3 is also more than two times larger then their specific
3
2
surface area, which is 74.9 m /m .
Both wetted and effective surface areas depend on the wettability of the
material of the packing, the liquid properties and especially the surface tension,
and on the liquid superficial velocity. Over the loading point, they depend also
on the gas velocity. Some of the methods used to determine the wetted and the
effective surface area of the packings are presented below.

2.1.2,5.1, Colour method
According to this method [13, 104, 113-115], the liquid phase is
coloured with a substance able to colour the packing elements. After every flow
rate the packing is taken out and the coloured surface area is measured. Because
the liquid streams in the packing change their direction, the coloured surface
area is some greater than the actual surface area in a given moment. Another
disadvantage of the method is the great labour consumption. That is why
nowadays the method is not of importance.