8. Diffusion and Mass Transfer

Diffusion is the transport of material due to concentration gradients or more precisely due to gradients in the chemical potential. Mass transfer is commonly described as diffusional phenomena in the presence of convective motion. Diffusion and mass transfer play a significant role in many materials processing operations. In metal extraction and refining, the chemical changes are usually associated with the transport of the reactants to the reaction sites and the transport of products away from the reaction site. The overall rates of the processes are often mass transfer-controlled, because the chemical kinetics tend to be fast at elevated temperatures. Diffusion and mass transfer are also of crucial importance in affecting the structure of solid products. Typical examples include segregation phenomena in castings and impurity distribution in crystal growth. This chapter introduces the mathematical formulation of diffusion in homogeneous and porous media and discusses associated phenomena including mass diffusion and chemical reaction. The boundary conditions required to solve mass transfer problems are presented. The chapter concludes with diagnostic examples on the basic principles presented.