Page 113 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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the packing and the sampling device. A distance about 100 mm is a good
choice.

2.1.2. Determination of the performance characteristics of the packings

2.1.2.1. Pressure drop, loading and flooding point
The packing pressure drop is measured by a differential manometer (5),
Fig. 2. When the pressure drop is less than 200 Pa, a special construction of a
differential manometer with a precision of 0.1 Pa can be used. The height of the
packing is determined, so that the measurement accuracy is high enough. The
real accuracy of determination of the pressure drop is usually connected not
with the accuracy of the measurement of the flow rates and pressure drop, but
rather with the difficulty in reproduction of the same structure of the packing in
the column at refilling. It is especially true for random packings. According to
the investigations of Kolar [6] in refilling of the column with the same packing,
the difference in the obtained results is up to ± 10%. Our not published
investigations confirm these results. It is to be expected, that the difference in
case of structured packings is smaller.
The investigations of the irrigated packing are carried out at constant
liquid superficial velocity with changing of the gas velocity. It is convenient to
plot the obtained results in double logarithmic coordinates. As mentioned in
Chapter 1 the loading and the flooding points are turn points of the lines for the
pressure drop in these coordinates. That is why by measuring the pressure drop,
the data for the loading and flooding points are also obtained.
In some cases there is a difference in the results obtained at increasing
of the liquid superficial velocities and at its reduction to a given value. This
must be always taken into account when repeating the experiments. There are
also some packings at which in the area of the loading and flooding point, a
hysteresis exists. This is easy seen from the experiments taken from [7]
obtained for the packing Holpack, consisting of horizontal sheets of expanded
metal. The data are presented in Fig. 10.
It must be mentioned that a hysteresis in the same range of gas and
liquid velocities for the same packings is to be expected also for the effective
surface area and for the liquid- and gas-side controlled mass transfer
coefficients. In Fig. 11 the hysteresis curves for the liquid-side controlled
volumetric mass transfer coefficient are presented [7].
Such type of a hysteresis is well known [8-10] in case of some types of
perforated plates. It means that the presence of this phenomenon is to be
expected also for other packings of horizontal perforated plates.