Diffusion Mass Transfer in Fluid Systems

In the previous chapters, we have discussed how diffusion involves physical factors. We calculated the gas diffusion through a polymer film, or sized a packed absorption column, or found how diffusion coefficients were related to mass transfer coefficients. In every case, we were concerned with physical factors like the film's thickness, the area per volume of the column's packing, or the fluid flow in the mass transfer. We were rarely concerned with chemical change, except when this change reached equilibrium, as in solvation.

In this chapter, we begin to focus on chemical changes and their interaction with diffusion. We are particularly interested in cases in which diffusion and chemical reaction occur at roughly the same speed. When diffusion is much faster than chemical reaction, then only chemical factors influence the reaction rate; these cases are detailed in books on chemical kinetics. When diffusion is not much faster than reaction, then diffusion and kinetics interact to produce very different effects.

The interaction between diffusion and reaction can be a large, dramatic effect. It is the reason for stratified charge in automobile engines, where imperfect mixing in the combustion chamber can reduce pollution. It is the reason for the size of a human sperm. It can reduce the size needed for an absorption tower by 100 times. The interaction between diffusion and reaction can even produce diffusion across membranes from a region of low concentration into a region of high concentration.