Page 83 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

78

for example treating with sand or using of sintered porous materials. Especially
proper is the sintered PVC [51-53].

(b)

Fig. 13. Filling up with liquid of the channels on a solid surface; (a) single channel; (b) many
channels.

In details the problem of wettability of materials with rough (textured)
surfaces is investigated by Palzer et al. [50].

1.6.3.2. Influence of the swfaee tension
The liquid surface tension influences the effective area of the packing
through its effect on the contact angle, Eq. (267), and also directly on the
surface of drops and jets trickling in the free volume of the packing. It is clear
that the reduction of the surface tension leads to increasing of the effective
packing area.

1.6.3.3. Influence of the viscosity
The forces which are responsible for the holding of the liquid phase on
the surface of the packing are the intermolecular adhesion forces. That is why
there is no physical reason to expect any direct effect of the liquid viscosity, at
least under the loading point. Nevertheless, there are a great number of
equations in which the effective surface is presented as a function only of Re L,
assuming that the effect of viscosity is equal to that of the liquid superficial
velocity. Moreover, according to this type of equations the increasing of the
viscosity leads to reduction of the effective surface.
By simple physical considerations, it is to be expected that the
increasing of the dynamic holdup can lead to some increasing of the wetted and
also of the effective surface area. Because the two values are not proportional
and the liquid holdup depends weakly on the viscosity, the expected effect is
very small being of the experimental error magnitude for this type of
investigations. In case of small packing elements, there is a reason to expect
also some reduction of the effective surface area with increasing of the
viscosity. It is because of some reduction of the ratio of wetted packing area to