Page 116 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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The difference between the second and the third method is that by the
third one the level of the liquid in the bottom of the column is measured without
stopping the inlet and the outlet liquid flow at each flow rate. Of course, in
every experiment it must be waited until the level of the liquid in the bottom
stops changing. The third method is used for the first time in [17], where no
detailed description is given.
The first measurement of the level in the column bottom is made before
feeding the liquid. In both the second and the third method the liquid phase
distributor should be constructed so that after stopping the liquid it is
completely full. One of the possibilities to fulfil this requirement is to make the
orifice for the liquid phase small enough, for example 1.5-2 mm. At this
diameter, when stopping the liquid phase by the corresponding tap, the liquid is
not dripping from the orifices.
In the measurement of the liquid holdup as a function of the gas
velocity, the evaporation of the liquid in the gas phase should be taken into
account. This effect leads to increasing of the measured holdup in comparison
to the real one. To eliminate, or at least, to reduce the effect of the evaporation,
there are two possibilities:
1. to saturate the air, for example in a preliminary column, before
entering the experimental column;
2. to make a correction for the initial liquid level at the bottom of the
column, after each one or several experiments.
The impulse tracer method is proposed by van Swaaij et al. [88] for
simultaneous measurement of the holdup of the liquid phase and the residence
time distribution. The investigations show a good coincidence between the
experimental data, obtained by this method, and the draining one. Several
researchers [89-91] use step decrease timcer approach to study static and
operating holdup. In these investigations it is found that the static holdup
determined by the draining method is greater than that determined by the tracer
method.
The fifth method for determination of the liquid holdup [18] measures
the intensity of x- or y-rays passing through not irrigated and irrigated packing.
The method is based on the Beer-Lambert law for absorption of rays in
substance. The advantage of the method is that it gives the possibility to
determine the liquid holdup at different heights of the packing. Its disadvantages
are that the absorption of rays is not directly proportional to the liquid holdup
and to obtain trustworthy results the method must be corrected using data
obtained by another experimental method.
The considered methods allow determination of the average value of the
liquid holdup in tire packing. It means that the effect of the liquid distributor
design cannot be studied in this way. To obtain the distribution of the liquid