Page 45 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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column. Nevertheless, the existing purely theoretical equations based on a
simple model could be very important for the development of these apparatuses.
Therefore, only the basic theoretical models which could be used for
development and calculations of industrial packed bed columns are presented
below. These models could be divided into two types: models describing the
elementary act of the mass transfer process through the interface, and models
describing the mass transfer in the apparatus as a whole.
The mass transfer is connected with transfer of molecules or elements
of fluid flow caused by difference in concentrations, or to be precise in chemical
potentials, which is a driving force of this process [10], It can be divided into
four large and important phenomena [10]: molecular diffusion in immovable
medium, diffusion in liquid in case or laminar flow, mixing in free turbulent
flow, and mass transfer between the phases. Speaking for mass transfer we
mean further first of all the last of these processes.
The rate of all mass transfer processes can be present as a product of
two main values which take into account the statics and the kinetics of the
processes. These values are the driving force (difference between the real and
equilibrium concentration) and the mass transfer coefficient.

1.5.1. Mass transfer coefficients

1.5.1.1. Partial mass transfer coefficients
The equation of mass transfer between the bulk of the fluid and the
interface can be written as follows:

for the gas phase

N d=k G,A l3 (124)

for the liquid phase

N A=k L.A L, (125)

where NA is the mass flow of substance G transferred for unit of time trough
2 2
unit of interface in kg/(m s) or kmol/(m s); A Gl and A Li are driving forces of
3 3
the process in kg/m or kmol/m . More precisely it is the difference between the
chemical potentials of the transferred substance in the fluid bulk and at the
interface with the gas, respectively for the liquid phase. Because of the
difficulty to use the chemical potential it is expressed by the corresponding