DIFFUSION

A. Fick’s Law

Molecular diffusion is defined as the transfer or the travelling in a random path of molecules of A through molecules of B due to concentration gradient.

8th Handbook, p:5-45, Eq. 5-189

C – the total concentration of A and B DAB – diffusivity of A through B xA – the concentration of A at some point z – direction of net transfer of A

 

B. Diffusivity

1. Diffusion coefficients of Binary mixture at 1 atm are listed in 8^th Handbook, p:2-454, T:2-324 .

2. Correlation of Diffusivity for Gases in 8^th Handbook, p:5-51, T:5-10 – correlates the diffusivity to parameters such as temperature and pressure.

 

C. General Cases for Gases

1. Equimolar Counter Diffusion

– a two-way diffusion where the total net flux is zero. This implies that the net flux of A is balanced by a counter net flux of B such that the total concentration will not change.

8th Handbook, p:5-49, Eq:5-197

C – the total concentration of A and B DAB – diffusivity of A through B xR – the concentration of A at Right Side xL – the concentration of A at Right Side – the length of the path

 

For gases, constant concentration is maintained if P and T remain constant.
By employing ideal gas equation for appreciably lower P, the equation will take the form of:

DAB – diffusivity of A through B yR – the concentration of A at Right Side yL – the concentration of A at Right Side P – absolute pressure of the gas – the length of the path