Page 144 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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Principally the results of Last and Stiehlmaier [102, 103] are in
contradiction with the investigations of Kolev [40, 50, 69, 77, 83, 108] and
Kolev et al. [106, 107] which show that it is possible to use the Danckwerts
method for calculation of the effectiYe surface area not only in a pilot plant but
also in industrial scale. Moreover, without the assumption that the effective area
is independent of the mass transfer process, it is impossible to use the principle
for additivity of the interface resistance. Otherwise, it has to be accepted that the
processes from the two sides of the interface pass at different effective areas.
That is why it is useful to discuss in details the reason for the results
obtained in the investigation of Last and Stiehlmair [102,103].
Undoubtedly, the difference in the rate of saturation of the stagnant
zones exists and has its influence on the experimentally determined effective
surface in a packed bed column. At the same time, without doubt, such an effect
can be important only in ease of extremely small ratio of the effective to the
total surface.
Last and Stiehlmair [102, 103] carried out their experiment with
Mellapak 500Y, i.e. a packing with a very large specific area, equal to 500
2
m /m . At the same time the liquid superficial velocity is very low, only 2.63
2
3
m /(m h), i.e. 5.26 liter/h per 1 m packing perimeter. Under such conditions
only a very small part of the packing specific area is wetted.
Of course, just under these conditions the investigated effect can be best
demonstrated. But just at this condition it is not possible to determine the
influence of the stagnant zones on the mass transfer in an industrial packed bed
column operating at usual liquid superficial velocity.
Perrin et al, [112] described a flexible pilot plant installation to evaluate
the performance of different packings using dynamic methods.

2.2. Investigations using hot stand

The investigation at cold stand gives the possibility to determine the
effective packing surface area, the mass transfer coefficients for gas phase and
liquid phase controlled process, and also such hydrodynamic parameters as
pressure drop, liquid holdup, loading and flooding points, and Peclet number for
the gas and for the liquid phase. Though it is possible when knowing all these
values to calculate an hidustrial column, the calculation needs knowing different
physical- and -chemical properties of the system, and first of all the diffusion
coefficients. That is why in many cases it is easier to obtain the necessary height
of a mass transfer unit or the height of a theoretical plate not by calculation but
by a direct experiment. As shown later in Chapter 8, with a proper liquid phase
distributor of the industrial apparatus and when avoiding the effect of the liquid
spreading to the column wall, it is possible to prevent the scale effect. That is, it
is possible to transfer the results from the laboratory (pilot plant) experiment