ANALYSIS OF ISOTACHIC PATTERNS IN DISPLACEMENT CHROMATOGRAPHY

Most previous theoretical treatments of displacement chromatography have been confined to systems that follow the competitive Langmuir isotherm. With the assumption of such idealized multi-component adsorption behavior, in a sufficiently long column the final outcome of the process is always expected to be an isotachic displacement train having a series of adjacent separated bands of increasing concentration. Several recent reports, however, suggest that no such train forms when the single-component isotherms of the separands cross. In order to examine the possibility of separation under such conditions, the stability of the isotachic pattern is analyzed and criteria for displacement and for stability of the resulting boundaries are established. The approach requires the knowledge of the multi-component isotherm that governs the adsorption of the separands. As the competitive Langmuir isotherm completely fails to represent such behavior, the pertinent multi-component isotherms are generated from Langmuir single-component isotherms within the framework of the ideal adsorbed solution model. The results obtained with such a multi-component isotherm in binary separations show that three operating regions in displacement chromatography can be identified when the single-component isotherms of the separands cross. In one region at sufficiently low concentrations, the two components separate and appear in the same order as in linear chromatography. In a second region at sufficiently high concentrations, the bands are predicted to separate but appear in the reverse order. In the third region at intermediate concentrations, complete separation is not possible, and the resulting isotachic pattern contains a mixed zone. The stability analysis presented here facilitates the prediction of the outcome of displacement without the need for arduous computation; it is fairly general and may be applied to systems that follow other multi-component isotherms.