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The effect is connected with reduction of the real way of the gas phase through
the packing and therefore with reduction of the real gas velocity.
It could be interesting to mention that at the same time when the above
described experiments were carried out in Bulgaria, Dr. Reichelt [10] in
Germany, who is the author of a good book [11] in the area of packed bed
columns, came to the same idea and obtained the same results for both mass
transfer coefficient and pressure drop. Since he obtained lower pressure drop for
shorter packing elements, contrary to his expectation, he was not quite sure in
his experimental data and delayed their publication [9]. His idea was that the
pressure drop increased because of increasing of the number of changes of the
flow direction per 1 m of the packing without taking into account that the real
way of the gas flow is shorter for the shorter packings.
The results of Kolev [8, 9] are confirmed by Ellis [340] in case of
rectification and also by Rao [190] and by James Jobling et al. cited by Ellis
[340] as a private communication.

3.2. Types of packings

Since 1827 when Gay-Lussac has offered the first packed bed column
filled with coke [12] till now, hundreds of packings of different types have been
proposed and patented. At the end of 19 century the first of the ring form
packings were implemented as arranged packing in sulphuric acid manufacture.
Some later, in 1914, Raschig patented the well known Raschig ring [13] which
th
was one of the most widespread packings to about the middle of 20 century,
used either as random or arranged. It is characterised by equal height and
diameter of the packing element. As already mentioned, a packing with a
smaller height is more effective. For a long time Berl [21, 96, p. 372] and
Intalox [15] saddles (Fig. 2) were also largely used. In 1953 the Pall rings (Fig.
3) were patented [16].
The existing now packings can be divided into two big groups: random
and structured packings. A group of packings operating in the inversion regime
over the flooding point and moving together with the liquid phase can be
consider as an additional third group. These packings have very high pressure
drop and are proper, first of all, for cases when the usual mass transfer processes
of absorption, desorption, or rectification are accompanied with crystallization
on the packing walls or with purification from dust. These types of packings are
not considered in this book.