Column response curves of nonlinear chromatography for equilibrium isotherms with inflection points

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Column response curves of nonlinear chromatography for equilibrium isotherms with inflection points

Column response curves of step and pulse inputs in nonlinear chromatography were analysed with the concept of concentration wave velocity and validated with detailed numerical simulations. The characteristics of concentration waves of typical favourable, unfavourable, S shaped and L4 type equilibrium isotherms were illustrated for various feed concentrations. For favourable isotherms, a step up input in concentration generates a sharp concentration wave and a step down input generates a diffuse wave. A pulse response has a sharp front and a diffuse end. For unfavourable isotherms, the column response curves exhibit the opposite behaviours. However, for equilibrium isotherms with inflection points, when the feed concentration is higher than the concentrations at the inflection points, the column response curves contain composite concentration waves with both sharp and diffuse segments. The numbers and the characteristics of the wave segments were illustrated. The maximum number of segments in a composite wave is equal to the number of the inflection points plus one. The relationship between the isotherm curvature and column response curves may provide useful explanation for some chromatography behaviours, such as the slow saturation and long tailing phenomena, and it could be helpful in the studies of solute adsorbent interaction mechanisms in chromatography.