Modicon process

Another possible way to improve the separation performance of SMB units is to change the feed concentration during each successive period. This operation mode, called the Modicon process, was suggested by Schramm et al. [72]. These authors optimized the process using numerical solutions of the equilibrium-dispersive model and compared the performance of the Modicon process with that of conventional SMB. They concluded that a cyclic modulation of the feed concentration allows a significant improvement of the separation performance The productivity and the product concentration can be increased and the specific solvent consumption decreased compared to those achieved with conventional SMB. 

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).

Fig. 5.14. Simulated moving-bed chromatographic processes with periodic variation of the feed concentration (ModiCon).

Concentrations c]n are the inlet boundary conditions (Eq. 6.92 or 6.93) of the columns at the beginning of each section while cout are the outlet concentrations calculated at the end of each section. Intermediate node balances consist of setting equal volume flows and assigning the outlet concentration to the inlet concentration of the subsequent column. Since SMB is a periodic process, the boundary conditions for every individual column are changed after a switching period t,hm. If SMB modifications such as VariCol, ModiCon, etc. (Chapter 5) are used the boundary conditions are modified accordingly. 

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... 

Another modification of the classical SMB process is the so-called ModiCon approach by Schramm et al. (2003). In contrast to the partial-feed process, where the feed flow rate is varied during one switching interval, in ModiCon the feed concentration is altered instead of the feed flow rate. Figure 5.22 illustrates this variation of the feed concentration during the switching interval tshia- In the example shown, which is suitable for systems with favorable isotherms, at the beginning only pure solvent is fed to the plant while in the second part feed at a rather high concentration is used. The modulation of the feed concentration allows for improved process performance for systems with nonlinear adsorption isotherms. 

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. 

Based on theoretical simulations, Modicon is advantageous, but apart from additional hardware the practical realization is limited as the feed concentration is upper bounded by the solubility and in general the feed concentration of optimized SMB processes will already be dose to this limit. 

For the sake of completeness it has to be mentioned that further improvements of nonstandard simulated moving bed processes by combining Varicol, PowerFeed, and/or Modicon to hybrid processes have also been investigated (e.g., Rodrigues, Araujo, and Mota, 2007b Zhang, Mazzotti, and Morbidelli, 2004a). The presented results indicate further improvements, but these hybrids also increase the requirements on process control and robustness. 

For the same number of columns Varicol, PowerFeed, and Modicon can outperform standard SMB processes concerning product purities, productivity, and eluent consumption, respectively. 

Although theoretical optimizations indicate slight advantages of PowerFeed, at the stage of process design it should be considered equivalent to Varicol. The same is to say about Modicon and FF-SMB. 

Before Varicol, PowerFeed, or Modicon is taken into account for process design make sure that appropriate optimization tools are at your disposal. In contrast to SMB, no shortcut methods such as triangle theory are available for these processes. 

The final decision on processes design has to take into account process economics. These are mled primarily by savings in production costs, compared to SMB, and additional hardware and software for process control and additional pump systems in case of PowerFeed and Modicon. For Varicol only additional hardware and sofiware for process control is necessary. 

Zhang, Z., Mazzotti, M., and Morbidelli, M. (2004a) Continuous chromatographic processes with a small number of columns comparison of simulated moving bed with Varicol, PowerFeed and ModiCon. Korean J. Chem. Eng., 21, 454 64. 

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