Theory of Diffusion

Abstract

In chemical engineering, diffusion is responsible for mass transfer. Three different patterns of diffusion are responsible in this process: ordinary diffusion, thermal diffusion, and pressure diffusion. Theory of diffusion primarily focuses on the mass-flux vector and its relation to concentration gradients and diffusion coefficients. These diffusion coefficients have to be calculated or estimated to make calculations of practical interest. There are numerous ways of expressing concentration in diffusion problems, the most important for the purposes being mass density, molar density, mass fraction, and mole fraction. It is necessary to know values, for chemical engineering calculations, of the coefficients of diffusion and thermal diffusion for a wide range of chemical systems. The theory that forms the basis for discussions of the transport phenomena in dense gases is Enskog's kinetic theory for a pure gas made up of rigid spheres. Enskog's kinetic theory for dense gases takes into account the fact that the diameter of the molecules is not small with respect to the mean free path. This chapter discusses theories and experiments of diffusion and measuring different diffusion coefficients. Solutions of the diffusion equations of interest in chemical engineering are illustrated.