Multicomponent Diffusion | Mass Transfer - Chemical Engineering PDF Download

2.6 Multicomponent Diffusion

It is very complicated process and it is generally non-ideal and non-uniform. With the help of Maxwell-Stefan approach, analysis can be done.
(a) The approach is applied to binary mixture and then it is extended to multicomponent mixture.
(b) Basic assumptions at steady state: Difference in partial pressure of A over a small distance ΔZ is proportional to molar concentrations of A and B; diffusional path length and difference in velocities of A and B.
Therefore,


Equation (2.66) is identical to Equation (1.21) of module 1 which was obtained from Fick’s law. Now consider a mixture of n components; (n-1) binary mixtures are there.

where,
                                                                            (2.72)

Special case:
If all the components except i are non-diffusing or inert, i.e. N_j = 0 and j ≠ i, then
                                                                                               (2.73)
Integrating Equation (2.71) we have,
                                                                            (2.74) 

                                                                                   (2.75)
Kraaijeveld and Wesselingh (1993) proved that in some cases the diffusivity values of ions in a mixture of electrolytes or in ion-exchange can be negative.