A new type of composition front in nonlinear chromatography, which has been called delta-shock, has recently been discovered and added to the family of classical transitions, i.e. simple waves, shocks and semi-shocks. First, the occurrence of delta-shocks in the case of mixed competitive-cooperative isotherms, where the weak retained component is Anti-Langmuirian whilst the strong retained component is Langmuirian, was predicted theoretically. Then their behaviour was analyzed in the frame of equilibrium theory of chromatography . The delta-shock can be viewed as a growing travelling spike superimposed to the discontinuity separating the initial and the feed state, which propagates along the column at constant speed and constant rate of growth.
Within a co-operation with the group of Prof. Guiochon, these findings have been complemented from an experimental point of view, by proving that the delta-shock indeed occurs also in the laboratory . The binary system consisting of phenetole (component 1) and 4-tert-butylphenol (component 2) in methanol-water (63/37 v/v) on a Zorbax 300StableBond-C18 column from Agilent has been shown to be subject to the competitive-cooperative isotherm of the type above. This system has also been shown to exhibit a delta-shock when the operating conditions are chosen according to theory, namely when Phenetole initially saturating the column is displaced by 4-tert-butylphenol, both at high feed concentrations (the minimum concentrations exhibiting a fully developed delta-shock in this series of experiments were c1 = 24 g/L and c2 = 90 g/L).
In this work, the focus is on an accurate measurement of the adsorption isotherms characterizing the two species. The goal is that of showing that the agreement between theory and experiments is not only qualitative but also quantitative. Among the effects that have been analysed there are that of initial and feed concentration and that of the column length. An important question is still open, namely whether the phenomenon of the delta-shock is bound to play any role in preparative chromatography.
 M. Mazzotti, Ind. Eng. Chem. Res. 48 (2009) 7733-7752.
 M. Mazzotti, A. Tarafder, J. Cornel, F. Gritti, G. Guiochon, J. Chrom. A. 1217 (2010) 2002-2012.
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