Page 50 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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boiling temperature evaporates, i.e. diffuses from the interface to the bulk of the
gas flow.
When one of the components is transferred towards the interface
without transfer in opposite direction of the other component, we speak of one
side diffusion. Examples for this process are absorption, desorption,
condensation and evaporation, including drying. The building up of a
conveetive flow in direction of the main diffusion process is specific for this
kind of processes and leads to increasing of the overall mass transfer. For better
understanding of the phenomena, let us consider, for example, absorption of a
component A mixed with the component B which is not soluble in the
absorbent. Let us accept that the equilibrium concentration at the interface is
zero. Because the sum of the partial concentration of the two components is
equal to 1, the decreasing of the concentration of the component A leads to
increasing of the concentration of the component B. This increasing, according
to Fick's law, leads to a diffusion flow opposite to the diffusion flow for A.
Both of these flows take away substance from the volume near the interface. To
compensate the substance taken away, a conveetive flow with opposite direction
is automatically induced. The explanation of these phenomena is offered by
Stefan [25] and the above mentioned conveetive flow is known by his name. It
must be mentioned, that there is no theoretical difference between Stefan's flow
in the liquid and in the gas phase. Usually the partial volume of the absorbed
component in the liquid phase is many times lower than in the gas phase. That
is why Stefan's flow in the liquid phase usually can be neglected. To take into
account the effect of Stefan's flow in the gas phase instead of diffusivity D the
expression

D—— (143)
P

is to be used, where P is the pressure, and p^ is the partial pressure of the
absorbed component.
In details the problems connected with Stefan's flow are discussed by
Brauer [34].
To describe the mass transfer rate from the bulk of the fluid to the
interface some physical models are presented. Later the more important of them
are considered.