Page 155 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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Because the overall mass transfer coefficients increase with the partial
mass transfer coefficients, the increasing of the efficiency of the packing is
connected with the increasing of its partial coefficients.
For each industrial apparatus the flow rates of the gas and liquid phases
are preliminary fixed. In case of uniform distribution of the phases over the
cross-section of the column, these values determine completely the average
concentration difference. That is why the driving force cannot be used for
intensification of a given process in a good column with uniform distribution of
the phases. The problems of the uniform distribution are discussed in Chapter 8.
The values of the partial mass transfer coefficients ka and feand of the effective
surface area a e depend on the physico-chemical properties of the system, the
velocities of the flows, and the form and dimensions of the packing. Because
the physico-chemical properties are connected with the system for which the
apparatus is designed, their change cannot be used for intensification of the
processes in this apparatus either. The increasing of the gas and liquid velocities
leads to strong increasing of the respective partial mass transfer coefficients and
of a g9> and this increasing is not only limited by the loading and flooding points
but also leads to strong increasing of the pressure drop, namely of the energy
loss. That is why for every technological process and type and dimension of the
packing, the gas and liquid velocities optimal from economical point of view
are firmly determined. Therefore, the only possibility for significant
intensification of the processes in the packed bed column is the changing of the
oldest, not so effective packings, with new more effective constructions. Thus
the problem for intensification of the packed bed columns is usually
transformed to a problem of creating of new more effective packings and their
proper selection for every concrete technological process.

The models, discussed in Chapter 1, of the elementary act of the mass
transfer processes are able to predict the mass transfer coefficient only for a
limited circle of hydrodynamic regimes and types of interface, which are
usually far away from the conditions of the industrial packed bed columns.
Nevertheless, they allow tracing the ways for creation of new, more effective
packings.
According to the models of Higbie [1], Danckwerts [2], and
Kischinevsky [3], the increasing of the mass transfer coefficient is connected
with the reduction of the average contact time on the interface. This time
decreases with increasing of the flow velocity and with reduction of the length
of the liquid film, respectively the length of the packing element on which the
film is flowing down.
The model of the diffusion boundary layer [4-7] also leads to the same
conclusion. According to this model, both the increasing of the flow rate and the