Types of Diffusion

(a) Concentration diffusion: Concentration diffusion is due to concentration gradient. Fick’s law gives a linear relation between the rate of diffusion of chemical species and the concentration gradient of that species.
(b) Thermal diffusion: Diffusion due to a temperature gradient. Usually negligible unless the temperature gradient is very large.
(c) Pressure diffusion: Diffusion due to a pressure gradient. Usually negligible unless the pressure gradient is very large.

Diffusion in solids
Knudsen diffusion: Diffusion phenomena in porous solids is called Knudsen diffusion.

Cause Of Diffusion
Concentration difference—Whenever there is concentration difference in a medium, there is mass transfer by natural flow from the high concentration to the low concentration region.

Practical Examples Of Mass Transfer (Diffusion)

1. There is a tank which is divided into two equal parts by a partition.
Initially, the left half of the tank contains oxygen O2 gas while the right half contains N2 gas at the same temperature and pressure. As soon as the partition is removed, the O2 molecules will start diffusing into the N2 while the N2 molecules will diffuse into O2. Over the passage of time, there will have a homogeneous mixture of N2 and O2 in the tank. It is mass transfer due to concentration gradients either way.

2. A drop of red liquid dye when added to a cup of water, the dye molecules will diffuse slowly by molecular diffusion in all directions in the remaining water till a homogeneous color is established.
3. Water evaporates from an open pan into air because of the difference in concentration of water vapor at the water surface and the surrounding air.
4. Evaporation of water from an open tray
5. Mass transfer is there in any open thermodynamic system i.e. suction, compression and discharge by a compressor is mass transfer.

6. Mass transfer in extraction

7. Mass transfer in distillation

8. Mass transfer in absorption

9. Mass transfer in separation

10. Mass transfer in purification