

Mass Transfer Equations: Fick's LawSo far we have discussed the concept of mass transfer. We said that mass transfer is the movement of matter from a high concentration to a low concentration. This means that the matter wants to "get away" from the other similar matter and go to a place where there is open space. Let's relate this to an example. Everyone has sprayed air freshener before. When you spray the air freshener, it does not just smell where you sprayed it, but rather the smell spreads throughout the room. The air freshener (matter) moves from an area of high concentration (where you sprayed it) to an area of low concentration that is far from the place that it was sprayed. This movement of the material is called diffusion. Diffusion can be represented by a basic equation. The equation that we are about to study is often referred to as Fick's Law. Our task is to introduce Fick's Law and dissect it into understandable parts. J= D * D C/Dx Every symbol stands for a different quantity.
Note: area has units of length*length (or length^{2}) Example: mol/(h*ft^{2}), mol/(s*m^{2})
Example: ft^{2}/h, or cm^{2}/s
Note: volume is the representation of size in three dimensions. Therefore it has the units of length*length*length (or length^{3}) Example: mol / cm^{3}, mol/L
Example: m, cm, ft Therefore, returning to the equation, J_{ }= D_{ }* D C/D x It was described that the flux was equal to the negative diffusivity times the change in concentration divided by the change in distance.
How about doing an example using Fick's
Law? 
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