Page 49 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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G = K Ga BV f^ = K La,V p\ , (139)

where G is the quantity of the transferred substance per unit of time.

1.5.2. Physical models for calculation of the mass transfer coefficient

1.5.2.1. Molecular diffusion at interface
The first step in calculating the mass of the transferred substances G per
unit of time, using equation (132), (133), (136) and (137), is to obtain the partial
mass transfer coefficients ki and fe. There are two possibilities to transfer the
substances from the bulk of the fluid to the interface, diffusion and convective
mass transfer. Independently of the convection effect the great diffusion effect
on the mass transfer processes always exists.
According to the first Fick's law of diffusion the molar diffusion flux
2
NA (mol/m s) of the component A in a two-component system (A and B) is
proportional to the concentration gradient of this component dC u/dx .

(140)

where the diffusivity DAB, depends on both substances A and B, their
concentration, and the temperature.
In the same way for the component B it can be written:

^ (141)
dx

In case of equimolecular diffusion from Eq. (140) and (141) it follows;

dx dx

The equimolecular diffusion takes place usually when the substance is
transferred through the interface in two directions. For example in case of
rectification, the component with higher boiling temperature condenses, i.e.
diffuses in the gas phase towards the interface. The component with lower