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Simulated moving bed (SMB) chromatography has been successfully applied to a broad spectrum of separation tasks, ranging from the separation of enantiomers to the purification of pharmaceutical proteins. In most cases these separations are realized in a conventional SMB system, using four distinct zones and constant operating conditions for internal factors (e.g. switching time, liquid velocities) as well as for external factors (e.g. solvent strength). In this classical implementation the process acts as a binary fractionator, providing two exit streams. This is sufficient if the desired compound in a multicomponent mixture is either least or most strongly retained. In the majority of cases however this requirement cannot be met. That led to various suggestions focusing on the continuous separation of multicomponent mixtures.
A sensible next step is to concentrate on ternary separations, as many more complex separation problems can be considered pseudo-ternary [1].
A number of different approaches have focused on the extension of zone numbers to tackle the problem, ranging from 5-Zone open loop up to 14-Zone closed loop setups [2]. Recently a 8-Zone closed loop separation unit was presented [3], consisting of two interconnected 4-zone sub-units. The feed mixture containing at least three components enters the first sub-unit, e.g. between zones II and III. Either the two most or least retained components are then directed to either the extract or raffinate outlet. The stream withdrawn at this port is then fed back into the unit between zones VI and VII. As shown previously [3] additional treatment of this stream is necessary to counteract limitations due to the unavoidable fixation of solid flow rate in all zones. Besides the introduction of a purge stream the use of an extra-column enrichment step is an interesting option as it can provide performance enhancing effects.
In this presentation the previously described 8-zone process for ternary separations will be used to illustrate the possibilities of integrated-zone set-ups to perform multicomponent separations. Also the possible beneficial effects of using concentration steps in these types of processes will be discussed.
[1] Y. Shan, A. Seidel-Morgenstern, Chem. Eng. Technol. 28,No.6 (2005) 695.
[2] P.C. Wankat, Ind. Eng. Chem. Res. 40 (2001) 6185.
[3] L.C. Keßler, A. Seidel-Morgenstern, J Chromatogr A 1126 (2006) 323.
