Lesson 4 of4
In Progress

Principles of Diffusion

Abdulaziz July 6, 2020
  • 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:

J_A=-D_AB\ \ \begin dc〗_A/dz<img width="175" height="19" src="">

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.