Page 114 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

109

0,03

rO.02

0.01 .
0.2 04 0,6 0J 1 1.2 1.4 1.6 1.8

Fig. 10. Hysteresis of the pressure drop of Fig. 11. Hysteresis curve for the liquid-
the packing of horizontal sheets of side controlled volumetric mass transfer
2
3
3
expanded metal [7]: 1- 1= UxlO" 3 coefficient at!=5.7xlO' m /(m s).
3 2 2 3 2
m /(m s); 2-1= 1.2xlO" m /(m s).
2.1.2.2. Liquid holdup
The liquid holdup is the volume of the liquid held in the packing per
unit of packing volume. The knowing of this value could be especially
important for calculating mass transfer accompanied with slow chemical
reaction. But the packed bed columns are not proper for such kind of processes.
The data for the liquid holdup are necessary for the following;
1. Strength calculations of the supporting grid of the column;
2. To understand better the phenomena in the column, because the
holdup is connected with the pressure drop and with the effective surface area.
The liquid holdup can be divided in two types: static and dynamic
holdup [14,15].
The static holdup gives the liquid remaining in the packing volume
because of capillary forces. That is why it is depending only on the type, the
dimension, and the material of the packing and on the type of the liquid. This
value is significant only for very small packing elements which are not of
importance for industrial packed bed columns. Nevertheless, it is investigated in
the literature first of all in connection to some catalytic processes.
The dynamic holdup represents the liquid flowing in the packing in the
form of films, drops and jets.
To determine the liquid holdup, the following methods can be used:
1. Weigh method;
2. Method of cutting the liquid flow;
3. Volumetric method of measurement of the liquid in the bottom of the
column;
4. Tracer method;