Theoretical study of closed-loop recycling liquid-liquid chromatography and experimental verification of the theory

• Analysis of processes with the detector located after the column.
• Separation is controlled by two counteracting mechanisms.
• Equations for three types of resolution.
• An increase in the recirculation tube length can improve separation.
• Good agreement between theory and experiment.

The non-ideal recycling equilibrium-cell model including the effects of extra-column dispersion is used to simulate and analyze closed-loop recycling counter-current chromatography (CLR CCC). Previously, the operating scheme with the detector located before the column was considered. In this study, analysis of the process is carried out for a more realistic and practical scheme with the detector located immediately after the column. Peak equation for individual cycles and equations describing the transport of single peaks and complex chromatograms inside the recycling closed-loop, as well as equations for the resolution between single solute peaks of the neighboring cycles, for the resolution of peaks in the recycling chromatogram and for the resolution between the chromatograms of the neighboring cycles are presented.It is shown that, unlike conventional chromatography, increasing of the extra-column volume (the recycling line length) may allow a better separation of the components in CLR chromatography.For the experimental verification of the theory, aspirin, caffeine, coumarin and the solvent system hexane/ethyl acetate/ethanol/water (1:1:1:1) were used. Comparison of experimental and simulated processes of recycling and distribution of the solutes in the closed-loop demonstrated a good agreement between theory and experiment.