What are the three main mechanisms of Diffusion in mass transfer processes? Why

Question

What are the three main mechanisms of Diffusion in mass transfer processes? Why is the effective diffusivity in porous solids less than it would be in free liquid? Give a simple laboratory experiment that could be used to illustrate the following processes: Steady state molecular diffusion Unsteady state molecular diffusion Fick's Law for mass transfer by molecular diffusion can be expressed as S_A = -D dC_A/dx Define each of the terms in the above equation, stating typical units, and explain the significance of the negative sign. How is Fick's law modified to apply in the following situations Eddy Diffusion Diffusion through a porous solid

Explanation / Answer

a) Three main mechanisms of diffusion in mass transfer processes are:

1) Gas Absorption

2) Distillation

3) Liquid Extraction

b) The effective diffusivity is less in porous solids because the cross section available for diffusion is less than for free liquid.

d) JA = molar flux of component A

Dv- volumetric diffusivity

cA=concentration

x= distance in the direction of diffusion

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