Page 58 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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mixing in every phase. The chaotic movement of the micro volumes in
comparison to their average velocity gives the possibility to express the axial
mixing by equations analogous to these for the diffusion processes. The
coefficient corresponding to the diflusivity in this case is called axial mixing
coefficient. Practically the axial mixing leads to reduction of the driving force
of the heat and mass transfer processes, and thus to reduction of their efficiency.
In the piston flow model this reduction is not taken into account or, more
precisely, it is taken only partially trough the mass transfer coefficients. More
details about the possibility to use the model, regardless of this disadvantage,
are given in Chapter 8 (8.5 and 8.6).

1,5,4.2, Piston flow model
According to the way of explaining of the driving force and the kinetics
of the mass transfer processes, the piston flow model has three versions
considered below.

1.5.4.2.1. Piston flow model with mass trmsfer coefficient
For description of multi-component absorption accompanied with
chemical reaction the piston flow model can be written as follows:

_ in Gi L dC lt _ * (165)
K&ae(Ca CGt)
~ I T ~ ^r~ " '

dt r dt, ^ ,dG,

(167)
Pa Potfi dh

ft, (168)

3
where p G and p L are the densities of the gas and the liquid in kg/m ;
w in is the inert gas velocity related to the whole cross-section of the apparatus in
3
z
nm /(m s);
Wo- the gas velocity related to the whole cross-section of the apparatus in
3 2
m /(m s);