Page 132 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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(38)

2.1.2.4.1.2. Gas-side controlled mass transfer coefficient
The best type of processes which can be used for determination of the
gas-side controlled mass transfer coefficient is nonequilibrium absorption
accompanied with instantaneous chemical reaction. Such processes are, for
example, the absorption of NH3 in water solution of strong acids, or absorption
of SO 2 in NaOH or KOH. To be sure that the liquid-side mass transfer is
completely eliminated, the concentrations in the gas phase have to be as low as
possible and the concentration of the reagent in the liquid phase has to be
higher. Taking into account the above mentioned and for reduction of the
inaccuracy from analytical point of view, the following concentrations can be
recommended:
- for acid or for alkali hydroxide in the liquid 0.3N;
- for NH 3 and SO 2 in the gas phase, about 0.2- 0.3%.
For carrying out the experiments, the installation presented in Fig. 2 can
be used.
Under the loading point 3-5 minutes are enough to reach steady state.
Because the concentrations in the gas phase are very low the mass
transfer equation can be written as follows:

G = w o(C G1 -C G2) = K GaH Cai CG2 , (39)

where w§ is the gas velocity related to the whole cross-section of the apparatus
3 2
in m /(m s);
3
and Car the initial and end concentrations of the absorbed gas in mol/m .
Because the values of CG are very low it is easy to see that instead of
3
kmol/m , without reducing the accuracy of the experiment, we can use the
concentrations in %. Eq. (39) does not take into account the influence of the
mass transfer on the surface of the drops and jets flowing before and after the
packing. To obtain this end effect, investigations at two different packing
heights, Hi and H 2, should be carried out. Based on the results, an equation
similar to that for liquid-side controlled mass transfer can be written: