The prediction of mass transfer rates when equilibrium does not prevail at the phase interface

It is shown that departures from thermodynamic equilibrium at the interface call for only small modifications in standard procedures for calculating mass transfer rates. If enthalpy-composition diagrams are used for representing thermodynamic properties and evaluating mass transfer driving forces, the modifications comprise the replacement of the single curve, valid for equilibrium mixtures in contact with the neighbouring phase, by a family of curves; the parameter of this family is the ratio of the mass transfer conductance to a molecular flux.

These and other aspects of mass transfer theory are illustrated by reference to: the vaporization of water into air; the combustion of carbon in air; the pyrolysis of a solid or liquid without chemical reaction; and the combustion of a pellet of ammonium perchlorate in a stream of fuel gas. The examples discussed numerically relate to axi-symmetrical stagnation point flows with laminar boundary layers.