As we saw earlier, mass transfer refers to mass in transit due to a species concentration gradient in a mixture, and mass convection is one of the mechanisms for this transit.
Mass transfer by convection involves the transport of material between a boundary surface (such as solid or liquid surface) and a moving fluid or between two relatively immiscible, moving fluids.
Convective mass transfer is really diffusion (the random movement of molecules) in combination with advection (molecules being carried along with the motion of the fluid).
Boundary layers
To better understand mass transfer via convection, it is important to consider boundary layers in fluids flowing over surfaces.
Newton’s law of cooling
To quantify the rate of mass transfer owing to convection, we can make use of Newton’s law of cooling (which, as we will see, also applies to convective heat transfer).
Newton’s law of cooling for mass transfer
�˙�=ℎ��(��,�−��,∞)
Hence, the driving potential for convective mass transfer is ��,�−��,∞
In the above equations,
| Variable | Definition | Typical units |
|---|---|---|
| �˙�,� | moles of species � transferred per unit time from the surface to the bulk fluid far from the surface | mol/s |
| ℎ� | the mass transfer coefficient | m/s |
| � | the area (cross section) through which transfer occurs | m2 |
| ��,�−��,∞ | the difference in concentration between the surface and bulk fluid | mol/m3 |
Questions:
- In what ways is Newton’s law of cooling a major simplification.
- What factors influence the mass transfer coefficient?
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