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Experimental implementation of the Cycle to cycle optimizing control of Simulated Moving Bed (SMB) processes using online HPLC measurements


C. Grossmann, M. Mazzotti, M. Amanullah, M. Morbidelli, M. Morari, C. Langel

American Institute of Chemical Engineers Annual Meeting, (AIChE). Philadelphia, PA, USA.

Recently, our group has validated experimentally the new ‘cycle to cycle’ optimizing control scheme for simulated moving beds (SMB) for the separation of achiral and chrial components. The feedback information used for control purposes was measured by online optical detectors, i.e. UV and polarized light detectors, positioned at the extract and raffinate streams. The accuracy of such devices is highly sensitive to experimental factors like impurities in the mixture to be separated or pressure fluctuation in the measuring cell; and has a direct impact on the performance of the controller. This can become a limiting factor when extending the control scheme to multicomponent systems, chiral separations or other operating modes of the SMB unit, e.g. VARICOL. In the spirit of widening the spectrum of applicability of the controller and improving the accuracy of the measurements, we have extended our control scheme to make use of an existing and reliable monitoring technique, namely HPLC. This approach has, on one hand, the advantage that the HPLC measurements are straightforward, more accurate than online optical detectors, can handle multicomponent systems and are not greatly affected by impurities. On the other hand, these measurements are less frequent and provide only information about the average concentration of the two species at the outlet streams. Moreover, the analysis time may be in order of the cycle time, which would introduce a significant time delay in the measurements. Hence, the controller relies on less but more accurate information that is available with a lower frequency, i.e. once every cycle, and can be delayed. This work presents the design and implementation of the required online HPLC system that measures the average concentration over one cycle of the extract and raffinate streams for control purposes. Furthermore, it reports the theoretical modifications and extensions that had to be done to the control scheme to cope with the implemented monitoring system. The performance of the 'cycle to cycle' control scheme is also demonstrated by several experiments in an SMB unit. The results illustrate that the 'cycle to cycle' controller is able to meet the products' purity specifications and operate the process optimally using minimal information about the system regardless of the disturbances that might take place during the operation.


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M. Morari

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