Page 8 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

elements and are not touching each other. To make the equations with the
arithmetical diameter useful, in the presented equations the arithmetical
£
diameter is multiplied additionally by . Thus, an equation with equivalent
1-e
and with arithmetical diameter gives practically the same results by
compensating the difference between them with a constant.

D
1-e a 1-e a

The effective surface area of the packing a e, through which the mass
transfer process takes place, is the wetted area of the packing a w and also the
surface of the liquid drops and jets trickling through the free space of the
packing. In case of small packing elements, especially at very high liquid
loading, it is possible that the area between the gas and the liquid phase is
smaller than the wetted area of the packing, i.e. it is possible that

2
3
The dimension of a e and a** is m /m .
3
2
The liquid superficial velocity L in m /(m s) is the liquid flow rate for
the whole column divided by its whole cross-section.
The liquid holdup Hf, is the volume of the liquid in the packing related
to the whole packing volume. Its value is important for strength calculation of
the support grid of the column, and especially for determination of the residence
time for the liquid phase, a value very important for thermo-unstable liquids.
The total liquid holdup Hi, consists of two components: static and
dynamic liquid holdups. The static holdup H s is mis part of the holdup which
remains in the packing because of the capillary forces when the irrigation is
stopped. It depends on the wettability of the packing, the surface tension and
density of the liquid and increases with the packing specific surface.
The dynamic holdup Ha is the difference between total and static
holdup of the packing. It is the liquid in the packing held by the resistance
forces. In case of large industrial packings, and they are more important, the
value of the static holdup is to be neglected.
The dynamic holdup increases with the liquid superficial velocity and
packing surface, and decreases with increasing of the liquid density.