TABLE OF CONTENTS Mass transfer involves movement of molecules under some driving force such as concentration or temperature difference. However, mass transfer discussed here will be primarily dealing with concentration difference. Mass transfer takes place by two mechanisms (1) Molecular diffusion (2) Convective diffusion
Fick's law: This basic relation for mass transfer by molecular diffusion in a binary mixture of A and B is given by 
Where
| J A | = | Molar flux of component A along axis z and moving at molar average velocity |
| D AB | = | Diffusivity of A in the solution containing A and B |
| C A | = | Concentration of A |
| z | = | Distance coordinate in the direction of molecular diffusion |
Another expression for the same case is J A = - c mD AB 
where y A is mol fraction of A and c m is average molar density of the mixture.
For gases, an empirical equation to calculate diffusivity of a component in a binary mixture is as follows
where v is the atomic diffusion volume of a simple molecule and other symbols have usual meaning e.g.
p = pressure. ? v's are obtained by summation of the atomic diffusion volumes.
Theoretical equation for calculation of diffusivity in binary gas mixtures at low-pressures is
Where T = temperature K, M A and M B = molecular weights, P = Pressure, atm,
? D = f( kT/ ? AB), collision integral, k...
