Page 120 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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industrial packed bed columns, is the so called double closed model. In
principle, these types of models can take into account not only the residence
time distribution, but also the stagnant zones of the packing. The investigations
of Vilermaux and Swaaij [72] show that the effect of these zones is smaller than
the experimental error, while its description leads to significant complication of
the model. That is why, determination of the Peelet, respectively Bodenstein,
number only for the simplest dispersion model, presented in Chapter 1, Eqs.
(250) to (255), is considered here.

2.1.2.3.1. Axial mixing in the liquid phase
The mixing in a column apparatus, respectively the axial mixing
coefficient, can be determined by measuring before and after the packing the
distribution of a tracer fed in the flow. There are different types of tracers and
different types of tracer methods. [71 pp. 241-277, 84, pp 36-61]. The
substances usually used as tracers are electrolytes, radioactive substances, and
more seldom colouring agents. The methods used can be stationary, quasi-
stationary and nonstationary. The tracer inlet signal can be delta (unit) impulse,
steplike impulse, or can have an arbitrary form, for example sinusoidal. It is
especially proper in the investigation of the axial mixing of the liquid phase to
use an electrolyte solution as a tracer and to feed it in the form of a delta
function in the liquid flow. The initial concentration of the electrolyte should be
determined so that in the measured range of concentrations the conductivity of
the solution is proportional to the concentration. In this case, it is possible to use
the conductivity of the liquid instead of the concentration.
Good results are also obtained by using of radioactive tracers [111], for
example water with 10 mCi brome - 82 isotope as NELjBr,
The axial mixing coefficient D L is determined from the experimentally
obtained value of the Bodenstein number defined as follows:

(10)
H dD L

where JJHa is the average axial velocity of the liquid phase.
Practically in the mathematical model not D L, but the Bodenstein
number is necessary.
For determination of Bo L the following equation is used: