A. Introduction

Diffusion refers here to transport of a material within one phase due to molecular characteristics alone: there is no external force or turbulence causing the transport. Diffusion can be caused by gradients within the phase created by a number of variables. The only case considered here is diffusion caused by a concentration gradient. In a binary mixture, if component A diffuses from position 1 to position 2 at a given rate, then to maintain equal molal flow, component B must diffuse at the same rate in the opposite direction. If F^m is the molal flux, the diffusion coefficient is defined as

\[\label{eq1} F^{m}_{A} = -CD_{AB}Vx_{A}\tag{1}\]

\[\label{eq2} F^{m}_{B} = -CD_{BA}Vx_{B} \tag{2}\]

where C = total molar concentration