Page 82 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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The angle of wettability varied from zero (fully wetted) to 90° (not
wetted material).
The difficult measuring of this angle, dependent on the packing material
and its preliminary treating, and also on the gas and liquid phase properties, is
the reason why, in many equations in literature for calculating the effective
packing area, this angle is not taken into account. The other reason is that this
angle is partially related to the liquid surface tension, Eq. (267).
As already mentioned the wettability is connected with the
intermolecular forces. A simple rule of chemistry says that the similar dissolves
in a similar. The rule can be expanded for wettability. I.e., if the packing
material is a not polar one, it is better wetted by not polar liquids, and vice
versa. For example, good results are achieved in improving the wettability of a
plastic packing by treating with oxidants. Moreover, it is well known that after
some time of operation the wettability of a plastic packing improves. The reason
is the chemical reaction on its surface.
The investigation of Kolev [47], carried out with a very wettable
ceramic packing and PVC Raschig and Pall rings which have been under the
influence of atmospheric air for a long time, has achieved a good coincidence
with the data for all packings without using the angle of wettability in the
equations.
All 16 different packings (Raschig rings, Pall rings, Intalox saddles and
shorten rings) with sizes from 15 to 50 mm, made of ceramic and PVC, obtained
by 4 authors, have good agreement with the obtained correlation which does not
include the angle of wettability. Only the data of Danckwerts and Rizvi [48],
obtained for polypropylene newly produced Intalox saddles not preliminary
treated, show quite smaller effective area than the predicted one. The reason is
that the difference in the angle of wettability is not taken into account.
Besides the contact angle, defined by the equation of Young which is
not depending on the surface geometry, there is another contact angle which is
depending on it and which can be measured in the same way. This angle is
responsible for the wettability of the surface important for the effective area of
the packing. The existing of this second angle of wettability is connected with
the following.
Let us assume that the surface is covered with small sharp channels,
Fig. 13, with width and distance between them quite less than the dimension of
the liquid drop. When the channels are crossed, the liquid phase, fallen on the
surface, spreads in the channels thus wetting the whole surface. As it is easy to
see from Fig. 13 (a), the real value of the angle of wettability, in relation to the
horizontal plane, is less than zero, i.e., the material is fully wetted. There are
different ways to obtain small crossing channel structure on the packing surface,