FICKS LAW OF DIFFUSION

Fick's law of diffusion

• Mass transfer is the natural tendency to transfer a given component (species) in a mixture from a region of high concentration to a region of low concentration to bring about a uniform or equilibrium condition. 
• The mass transfer has three requirements: 

1. that transfer occurs only in a mixture,
2. that at least one substance within the mixture moves from region of high concentration to a region of low concentration,
3. that the rate of mass transfer—i.e., the “flux” of a given substance—be proportional to the concentration gradient of that substance. 

Fick's  law

• In 1855 FICK proposed a relation between the flux of the diffusing substance and concentration gradient as the first law of diffusion which named as Fick's law of diffusion.

Fick’s law states that the flux of a diffusing component A in the z-direction in a binary mixture of A and B is proportional to the molar concentration gradient.

• Hence Fick's law of diffusion for component A in a binary mixture of A and B for steady-state diffusion is

           Where

• The negative sign in the equation indicates that diffusion occurs in the direction of decrease in concentration. Hence the term dCA/dz is -ve and flux become positive
• JA = molar flux of A in the z-direction [kmol/(m2.s)]
• CA = molar concentration of A [ kmol/ m3]
• dCA/dz = concentration gradient in the z-direction
• DAB = proportionality constant, diffusion coefficient for component A diffusing through B [m2/s]
• Z = distance in the direction of diffusion