General case for gas-phase mass transfer

General case for gas-phase mass transfer in a binary mixture 

Whatever the physical constraints placed on the system, the diffusional process causes the two components to be transferred at equal and opposite rates and the values of the diffusional velocities uDA and uDB given in Section 10.2.5 are always applicable. It is the bulk How velocity uF which changes with imposed conditions and which gives rise to differences in overall mass transfer rates. In equimolecular counterdiffusion. uF is zero. In the absorption of a soluble gas A from a mixture the bulk velocity must be equal and opposite to the diffusional velocity of B as this latter component undergoes no net transfer. 

Total transfer = Transfer by diffusion + Transfer by bulk flow. 

Total moles transferred/area time Total molar concentration 

If in a distillation column, for example the molar latent heat of A is / times that of B, the condensation of 1 mole of A (taken as the less volatile component) will result in the 

---