Page 159 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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3.2.1, Random packings

3.2.1.1. Description of the random packings
The random packings consist of elements with a given form dumped in
the column over its supporting grid. By form they can be divided into rings,
saddles and packings with other form. Another possibility is specification into
packings with continuous surface and packings with surface divided into
connected lamellas with small heights. The oldest type of elements, for example
Raschig rings, Berl saddles, Intalox saddles and others do not answer the
requirements following from the theory of the mass transfer processes.
The first packing construction that partially corresponds to these
requirements is the Pall ring [16]. In 1970 Eckert, comparing different packings
largely used at this time, wrote [293] that it exceeded all other packings (Intalox
and Berl saddles, and Raschig rings too). Now we can be sure that its advantage
is connected just with the fact that its constraction fulfils the requirements of
mass transfer theory better than that of any other packing at this time. For the
last thirty years of the last century practically there is no patented random
packing not corresponding more or less to these requirements.
Another requirement to the lamellas of the random packing elements is
that their width must not be greater than about 5 mm, at least for liquids with
properties similar to these of the water. This requirement follows from the
results of the experiments of Kolev and Razkazova-Velkova [242] who
measured the leakage of liquid flowing on a vertical well wetted plate. It was
found that at the bottom edge of the plate a long "drop" with a height of about 5
mm, in case of irrigation with water, is formed. It is able to lead away the liquid
in horizontal direction without leakage. I. e., if the width of the lamellas is not
greater than 5 mm, they can be completely wetted with moving liquid at
extremely low liquid superficial velocity. As shown later this phenomenon can
be used for creation of special structured packings able to operate at practically
fully wetted surface at extremely low liquid superficial velocity. At the same
time this phenomenon gives the possibility to create more effective random
packmgs too.
The packing elements are produced of ceramic metal and plastic. The
metal usually used is stainless steel. A mechanical or chemical treatment of the
plastic elements leads to increasing of their wettability. A small group of
packings are made of net which is wetted significantly better than the net
material itself. In some special cases of corrosive systems, carbon packings are
used.
Some of the most popular and widely used packing elements are
presented in Figs. 2 to 5. The first two of mem, figures 2 and 3, are taken from