Determination of competitive adsorption isotherms applying the nonlinear frequency response method: Part I. Theoretical analysis

In this work adsorption equilibria of binary mixtures are quantified analyzing the nonlinear frequency response of a chromatographic column. Local partial derivatives of an isotherm model can be estimated for certain steady-states from the low frequency asymptotes of the corresponding frequency response functions (FRFs). The required FRFs correspond to two different compounds and the type of the imposed inlet concentration changes, e.g. periodical inlet concentration changes of only one compound or of both of them. For an accurate determination of isotherm parameters, it is necessary to approach as close as possible the low frequency asymptotic behaviour of these functions. Based on principles valid for the FRFs corresponding to the adsorption of a single solute, frequencies needed to reach the low frequency asymptotes of the functions of interest for estimating competitive isotherms are defined in this paper. The relation between the accuracy of the isotherm parameters determined and numbers and types of periodical inlet concentration changes and steady-states analyzed is also evaluated.