Overall mass-transfer coefficients in non-linear chromatography

In the case of mass transfer where concentration differences in both phases must be taken into account, one may define an overall mass-transfer coefficient based on the apparent overall concentration difference. If the equilibrium relationship is linear, i.e., in cases where a Henry's law relationship can be applied, the overall mass-transfer coefficient will be concentration independent. However, in mass-transfer operations, a linear equilibrium relationship is in most cases not a valid approximation wherefore the overall mass-transfer coefficient becomes strongly concentration dependent as shown in this paper. In this case one has to discard the use of overall mass-transfer coefficients and calculate the rate of mass transfer from the two-film theory using the appropriate non-linear relationship to calculate the equilibrium ratio at the interface between the two films.