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Over the last ten years, the classical concept of simulated moving bed (SMB) chromatography [1], using four distinct zones and constant operating conditions, has been modified to derive new, more sophisticated operating modes like VariCol, PowerFeed of ModiCon [2]. Recently, a new approach was presented that utilizes a combination of fractionation and internal feed-back to increase process performance [3,4].
In this Fractionation and Feed-back SMB process (FF-SMB), at least one outlet stream is fractionized into a product and a recycle fraction, thus dividing each switching time into a production and a recycle phase. During the production phase, the outlet stream fulfills a given purity requirement and is collected as "product". During the recycle phase, the stream is directed into a buffer vessel. The non-product fraction collected in the buffer vessel is periodically used as an alternative feed within one switching time, thus defining a distinct feeding scheme. This concept can be easily realized by adding two three-way valves to a standard SMB unit.
Two different aspects can be identified as essential for the performance improvement. Collecting the outlet stream for only a part of the cycle time allows for applying more productive operating conditions, as the purity requirements only have to be met within the production phase. Secondly, re-feeding the non-product fraction A) effectively circumvents additional product losses that would occur if only a fractionation was implemented and B) can lead to further improved purities within the production phase, as the non-product fraction has already passed the SMB unit and its composition is therefore typically shifted towards the desired component in the considered outlet. Thus, using the pre-purified mixture as feed at the right time can help to simplify separation.
In this presentation, the new concept will be introduced and its potential will be illustrated for both linear and non-linear isotherms based on a simulation study. For linear isotherms and the parameters considered, product concentrations and productivity could be almost doubled when compared to the conventional process, while eluent consumption could be reduced by 40%. For non-linear isotherms, productivities could be increased by 60% while keeping product losses on the same low level as in classical SMB chromatography. The differences between this process and another fractionation approach [5] will also be discussed.
[1] D.B. Broughton, C.G. Gerhold, US Patent 2 985 589 (1961)
[2] A. Seidel-Morgenstern, L.C. Keßler, M. Kaspereit, Chem. Eng. Technol. 31 (2008) 826
[3] L.C. Keßler, A. Seidel-Morgenstern, Lecture L-205, PREP 2007, Baltimore, MD, U.S.A., 2007
[4] L.C. Keßler, A. Seidel-Morgenstern, Journal of Chromatography A (accepted) (2008)
[5] Y.-S. Bae, C.-H. Lee, J. Chromatogr. A 1122 (2006) 161
