Page 7 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

and exits the column through pipeline 6. In the packing the gas is treated with
liquid, mainly in a eountercurrent flow. The last enters trough pipe 7 in the
distributor 4 distributes over the packing and leaves the column through pipe 8.
The distillation (rectification) columns are more complicated. A
principle construction of these apparatuses is given in Fig, 1 of Chapter 7.

/. 1.2. Some terms largely used in the field of packed bed columns

A packing has three important geometrical characteristics: size dp,
specific surface area a, and void fraction (free volume) e. The specific surface
3
area of the packing is its area related to 1 m of the packing volume. The
packing void fraction is the volume of the free space of the packing related also
to 1 m of its volume. Obviously, the value of the packing specific area is equal
to the value of the sum of the perimeters of the channels formed in the packing
2
related to 1 m cross-section, and that value of e is equal to the cross-section of
2
these channels related also to 1 m of the cross-section of the packing.
Another important value is the equivalent (hydraulic) diameter of the
packing dh. There are two different possibilities to define this value. The first of
them comes from the consideration of the packing structure in terms of the so
called inner problem, i.e. when the packing is considered as a system of
connected channels with different dimensions. The equivalent diameter by this
model is equal to the average hydraulic diameter of these channels which is
equal to 4 times the hydraulic radius, or:

d h = 4efa. (1)

3
3
E is the packing void fraction in m /m , and a -the specific packing area in
2 3
m /m .
The second model for defining the equivalent diameter is the so called
outside problem. By this model the equivalent diameter is determined as a
diameter of a spherical packing element ensuring the same values of specific
surface and void fraction as those of the real packing. To distinguish the two
equivalent diameters, the second one is marked later as D a and is usually called
arithmetical. Simple calculations show that it is determined by the equation

D a=6(l-e)/a (2)

The great disadvantage of the second definition is that the hypothetical
spheres with the arithmetical diameter are quite smaller than the real packing