Powerfeed

As reported by Zhang, Mazzotti, and Morbidelli (2003), the performance of the PowerFeed process is similar to VariCol and significantly improved with respect to classical SMB processes. Furthermore, the extent of improvement is particularly increasing for more difficult separations. However, this improved performance is paid for by the increased complexity of operation and design for more details see Section 7.5. 

3 Partial-Feed, Partial-Discard, and Fractionation-Feedback Concepts 

An extension of the above is the fractionation-feedback approach (FF-SMB, see section 5.2.5.6) suggested by Kefiler and Seidel-Morgenstern (2008), wherein collected fractions with insufficient purity are collected, recycled, and refed into the SMB unit, which overcomes yield limitations. 

Besides the concepts discussed above there are ongoing activities in the development of further refined operating modes for SMB systems. A complete review is out of scope here. 

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Figure 1. (a) Operating diagram of a four section SMB unit (b) Fluid flow rates schemes in the SMB and PowerFeed operations during one switching period, U... 

In this p er, we present and compare the optimal performances of the SMB, Varicol and PowerFeed operations, using a multiobjective optimization technique, on two chiral separation systems from the literature. The aim is to provide a clear picture, although inevitably confined to the cases examined, of the relative potentials of these three operation modes. 

The separation problem examined in this case requires the simultaneous maximization of the raf ate (Pb) and the extract purity (Pe.) for a given feed flow rate, F, eluent flow rate, D and fixed configuration of the unit, for the same chiral separation considered in section 3.2. This optimization problem in the case of PowerFeed operation can be represented mathematically as follows ... 

Figure 5. Comparison of (a) the average mj and m3 among the 5-colunm SMB, the Varicol and the PowerFeed processes and (b) Values of m3 in the 4 subintervals of the PowerFeed process corresponding to the points in...

Our results show that the Varicol and the PowerFeed operations improve the performance of the SMB process, particularly when the total number of columns is small and the separations are difficult. The optimal design of the Varicol and PowerFeed processes can be replaced by the optimal design of the corresponding SMB process given by the equilibrium theory, followed by the search of the optimal ports switching and flow rates variation schemes within the switching period, respectively. 

Recently developed improved SMB processes, e.g. VariCol (Ludemann-Hombourger et al., 2000b), PowerFeed (Zhang et al., 2003) and ModiCon (Schramm et al., 2003), can also be used in reactive chromatography. Application of the VariCol... 

These results show that both the Powerfeed and the Varicol processes provide a performance that is significantly improved compared to that of the conventional SMB process, and that the extent of the improvement achieved is larger for more difficult separations. A rigorous comparison between the Varicol and the Power-feed processes in general terms is not possible, but it seems fair to say that they are equivalent in terms of potential performance, although the implementation of the Varicol process may be simpler than that of the Powerfeed process. 

Powerfeed An implementation of the simulated moving bed process in which the feed flow rate varies during the cycle. 

Kawajiri, Y. and Biegler, L. (2006b). Optimization strategies for simulated moving bed and powerfeed processes, AIChE Journal 52, 4, pp. 1343-1350. 

Figure 5.19 Flow rates during PowerFeed operation (illustrative example).

Zhang, Z., Mazzotti, M., and Morbidelli, M. (2003) PowerFeed operation of simulated moving bed units changing flow-rates during the switching interval. /. Chromatogr. A, 1006, 87-99. 

Figure 7.29 depicts the optimization results for the four- and five-column processes. Varicol, PowerFeed, and Modicon clearly outperform SMB while the performance of PowerFeed and Modicon is slightly better than that of Varicol for five-column processes. 

Figure 7.29 Comparison of the optimal separation performances of the four- and five-column SMB, Varicol, PowerFeed, and Modicon processes (reproduced from Zhang, Mazzotti, and Morbidelli, 2004a).

Columns Productivity SMB Varicol PowerFeed Extract purity (%) Modicon... 

The lower continuous curve represents optimal PowerFeed-F separation with feed rate as the only variable, while the upper curve (PowerFeed-F-Q-D) stands for an optimization where the flow rates of feed, section II, and eluent are variable in each subinterval. For PowerFeed-F and Varicol the results are nearly the same while the more sophisticated optimization with three variables is slightly better for the sake of a more difficult operation because three flow rates have to be controlled in each subinterval instead of only one. All in all, the investigations of Zhang, Mazzotti, and Morbidelli (2003a, 2003b) show that for the same productivity, eluent consumption and stationary phase standard SMB processes can be substantially improved by Varicol or PowerFeed... 

Figure 7.32 Nominal and robust Pareto curves for four-column SMB, Varicol, and PowerFeed processes ( ) the robust Pareto curves (reproduced from Mota, Araujo, and Rodrigues, 2007).

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