- Molecular diffusion of A in the direction of decreasing concentration (i.e. from left to right)
- Net diffusion of B in the opposite direction
- Diffusion continues until the concentration is uniform throughout
// Fick’s Law
The molecular diffusion flux of component A in the z-direction in a mixture of A and B, is given by Fick’s law:
Where,
JA = rate of molecule diffusion, kmol/(m2.s)
DAB = diffusion coefficient (or diffusivity) of A in a mixture of A and B, m2/s
dcA/dz = concentration gradient of A, kmol.m-3/m
The concentration gradient is the driving force, as temperature gradient in Fourier’s law for heat flux by conduction
For gases , Fick‟s law is expressed in terms of partial pressure of component A , pA .
The ideal gas law gives:
Corresponding to a concentration of A,
With the derivative:
// Diffusion with bulk of mass in motion
- With uniform concentration of A, the molar flux of A past a stationary observer is cAV.
•A concentration gradient dcA/dz gives an additional flux due to molecular diffusion, JA, or total molar flux of A.
// Diffusion of A through a stagnant B
Also known as Stefan diffusion
At point 1, pA1is the vapour pressure of A at the temperature of the bath, and at point 2, pA2 may be close to zero.