Page 128 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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The problems with the measurement cell can be eliminated using a
radioactive substance as tracer. The difficulties connected with the radioactivity
are usually greater.

2,1.2.3.2. Axial mixing in the gas phase
As already mentioned, the gas phase flow in a packed bed column is
closer to the piston flow conditions than the liquid flow. That is why the effect
of the axial mixing in the gas is many times smaller and taking it into account is
not so important. Nevertheless, the information about this phenomenon is
substantial.
Since the gas velocity is significantly higher than the liquid one, the
influence of the residence time in the measurement cell in this case is more
important. That is why the usual analytical methods for measuring the tracer
concentration are not appropriate, and the best possibility is to use as a tracer a
radioactive substence, fed in the gas phase before entering the column. The data
of the end concentration are processed in the same way as in the measurement
of the axial mixing coefficient in the liquid phase.
The radioactive method and its application are described in details by
Blet et al. [110 s 111]. For example in [111] the gas phase is marked with 2 mCi
argon 41 isotope. Due to the fast phenomena occurring in the column, the
sampling period is reduced to 10 ms for the first 30 s of the experiment. It must
be mentioned that the quantity of radioactive argon injected in the air flow is
very small and does not change the total partial pressure of argon and its
quantity initially dissolved in water.

2.1.2.4. Determination of the mass transfer coefficients
As already mentioned the mass transfer coefficients used for
calculations in chemical engineering are rather parameters in the calculation
model than physical values. That is why speaking of methods for determination
of these coefficients we should distinguish methods for determination of the
mass transfer coefficients for the piston flow model and for the diffusion model.
Since for the latter model these coefficients are connected with the axial mixing
coefficients in gas and liquid phase, the methods for their determination are
discussed after the methods for determination of the axial mixing coefficients.
All methods for determination of the mass transfer coefficient give the
possibility to obtain directly only its product with the effective surface area.
In principle, there are methods for determination of more than one
performance characteristics of the packing by one and the same type of
experiment, using optimization procedures. These are, for example, the gas- and
liquid-side mass transfer coefficients, or these coefficients and the axial mixing
coefficients for gas and liquid phases. To increase the accuracy of the