Page 267 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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Theoretically, as already noted, the best structure of the packing is this
which ensures equal channels uniformly distributed over the cross-section of the
apparatus. This condition is fulfilled in the best way with the honeycomb
packing (Fig. 45-c). For a long time this packing has been considered as low
effective [143] and, because of its large free cross-section, proper only for
processes controlled by the chemical reaction in the gas phase. The
investigations showing this low efficiency were carried out with a packing with
a diameter of the circle inscribed in the packing channel equal to 50 mm [143],
without comparison of the obtained mass transfer data with those for other
packings with the same specific surface area. The influence of the packing size
was not investigated. Assuming that theoretically packings with equal channels
have to be better and that the construction of the honeycomb packing ensures
high mechanical stability at small wall thickness, new investigations with this
type of packings were carried out [144]. The gas-side controlled mass transfer
and the pressure drop of 5 different packings were studied. The geometrical
characteristics of the packings are presented in Table 24.
The comparison of the pressure drop per mass transfer unit in case of
gas-side controlled process, presented in Fig. 46 shows that at one and the same
volumetric mass transfer coefficient the pressure drop of the rings with a cross
is about 3.7 times higher than that of Raschig rings with the same diameter. The
pressure drop of the other packings is lower than that of Raschig rings,
respectively: Honeycomb packing No 1- 8.3 times, Honeycomb packing No 2-
2.16 times, and Honeycomb packing No 3- 6.4 times.
It is easy to see from this comparison that the packing building equal
channels for the gas phase flow is better. The increasing of the void fraction of
the packing leads to strong increasing of the pressure drop per mass transfer unit
at equal volumetric mass transfer coefficients. The reduction of the element
height leads to pressure drop reduction [144].
The volumetric mass transfer coefficient is used by this comparison
because it is reverse proportional to the packing volume, and thus it can be used
as a measure for the capital investments. The pressure drop per mass transfer
unit can be used as a comparable measure for the operating cost, or more
precisely, for the operating cost for energy consumption of the movement of the
gas flow trough the packing.
In Table 25, taken from Billet [177], the geometrical characteristics of
different other structured packings are presented, including stacked Pall rings
and stacked Hiflow rings.