Introduction
In a system consisting of one or more components whose concentrations vary from point to point, there is a natural tendency for mass to be transferred, minimizing the concentration differences within a system. The transport of one constituent from a region of higher concentration to that of a lower concentration is called mass transfer.
Examples of mass transfer in industry :
- Humidification of air in cooling tower.
- Evaporation of petrol in the carburettor of an IC Engine.
- Refrigeration by the evaporation of liquid ammonia in the atmosphere of H2 in electrolux refrigerator.
Modes of mass transfer :
The mass transfer mechanism depends upon the dynamics of the system. The different modes of mass transfer are as follows :
- Mass transfer by diffusion.
- Mass transfer by convection.
- Mass transfer by change of phase.
Mass transfer by diffusion:
Mass transfer by diffusion is the spontaneous movement of molecules from a high concentration area to low concentration area, driven by random thermal motion, aiming for uniform distribution. It occurs when a substance diffuses through a layer of stagnant fluid and may be due to concentration, temperature or pressure gradient. In a gaseous mixture, molecular diffusion occurs due to random motion of the molecules.
Mass transfer is more rapid by eddy diffusion than by molecular diffusion.
Mass transfer by convection:
Mass transfer by change of phase: