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Such type of equation can be easy obtained for an empty column filled
only with air, and also for the ease when the column is filled only with water.
At a fixed radiation source head and detector position, the radiation
intensity transmitted trough the medium is recorded by the detector. A series of
such measurements are obtained along a series of chords in one horizontal
plane. This data comprise a projection for the tomographic reconstruction. To
obtain the special holdup distribution, the cross-section area is divided into a
given number of small cells, assuming that within each cell the value of the
local holdup is constant.
The obtained results are calculated by means of a special computer
program to obtain the distribution of the holdup over the cross-section of the
column. Thus, its distribution in the column volume can be also determined.
It is noted in [101] that x-ray computed tomography is a nonintrusive
method which provides quantitative information about both the small- scale and
the large-scale liquid maldistribution in any section of a bed consisting of
structured packing, essential for development and validation of rigorous
predictive models. Very thin flowing liquid structures may be evident and
visualized. The possibility to locate high liquid holdup zones might lead to
modifications of present commercial structured packings.

2.1,2.3. Determination of the axial mixing coefficients
There are two possibilities for determination of the axial mixing
coefficients, respectively the Peclet numbers. The first is to obtain them from
mass transfer experiments and the dispersion mass transfer model using
optimization procedure. The second is to determine them separately using tracer
methods. By the first method the Bodenstein, respectively Peclet, numbers for
both phases are calculated together with the volumetric mass transfer
coefficients for the gas and the liquid phases. This leads to enormously great
influence of the primary experimental error on the obtained results. The error is
especially great for the cases when the influence of the respective obtained
values on the mass transfer in the packing is comparatively slight. That is why
the so determined values are reliable only for the cases they are obtained in, for
example for automation purpose. They are not the best solution for calculation
of new apparatuses and new processes. That is why in this book only the tracer
method is considered.
The theory of mixing in industrial apparatuses is described in details in
the monograph of Levenspiel [71]. The best of the models presented mere, for