True moving bed

The easiest way to understand the SMB concept is to consider a true moving bed (TMB) as described in Eigure 10.1, in which a countercurrent contact is promoted between the solid and liquid phases. The solid phase moves down the column due to gravity and exits the system in Zone I. The liquid (eluent) stream follows exactly the opposite direction. It is recycled from Zone IV to Zone I. The feed, containing components A and B are injected at the middle of the column, and the fresh eluent is replenished in Zone I. 

FIG. 16-46 General scheme of a true moving bed (TMB) adsorption system for binary separations. A is less strongly retained than B. 

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The performance of HPLC separations can be increased not only by the application of two- or multidimensional techniques but also by the use of simulated moving bed (SMB) [100-102] or true moving bed (TMB) techniques [103,104], SMB is a multicolumn separation technique allowing the continuous separation of analyses with higher productivity and smaller eluent consumption than the traditional single-column procedures. TMB... 

Because of the difficulty in operating a true moving bed (TMB) (due to the solid circulation), this interesting idea must be implemented in a different way. As will be shown, most of the benefit of a true countercurrent operation can be achieved by using several fixed-bed columns in series and an appropriate shift of the injection and collection points this is the SMB concept (Fig. 2). The solid does not actually move, its flow is only simulated by shifting inlet and outlet lines. In fact, this simulated solid flow rate downward is directly linked to the shift period. 

If elution and frontal chromatography are still the main implementations used in preparative processes because of the simplicity of their development, processes like true moving bed (TMB) or simulated moving bed (SMB) have been used for about 40 years in large scale separations in petroleum or sugar industries [1,2]. In these processes, a countercurrent between solid and fluid phase is realized (or simulated) in order to improve process productivities and to decrease the eluent consumption. These implementations are now developed for laboratory and small productions and find a lot of applications in pharmaceutical and fine chemistry industries [3,4],... 

The basic principle of a true moving bed is to use a countercurrent contact between solid and fluid phases. The feed to be processed (A + B) is injected in the middle of the column. 

It can be shown that the performances of simulated moving beds are equivalent to those of true moving beds [11]. Also, SMB process has a lot of advantages compared to classical elution chromatography. With a continuous process, purified products are recovered at 100% with a low dilution and a low eluent consumption. Moreover, SMB process are not very sensitive to column efficiency and we can obtain very good product purities with even low column efficiencies [11],... 

A first model is used to compute the flowrates allowing to perform the separation with the greatest productivity. Then, the "mixed cell in series" model takes into account thermodynamic, hydrodynamic and kinetic properties of the system and compute the concentration profile inside the columns [14], In this model, we make the assumptions that the pressure drop inside the column is negligible compared to the pressure drop realized and controlled with the analogical valves, and we model the true moving bed assuming that the performance of SMB and TMB are equivalent. A mass balance equation is written for each stage and a classical Newton Raphson numerical method is used to solve the permanent state of the process [14],... 

In this system, the continuous injection of the feed and the continuous collection of A and B from the extract and the raffinate is an ideal system which provides the maximum efficiency of the adsorbent. A and B are recovered diluted in the desorbent but can be obtained pure after distillation. From a practical point of view, the true moving bed concept would be extremely difficult to implement. Although recycling the liquid would be feasible, the circulation of a solid packing from the bottom to the top of the fixed column... 

The True Moving Bed. The principle of a true moving bed is schematically illustrated in Figure 21-15 for the separation of a racemate on a chiral stationary phase, being the ideal problem for the separation of a binary mixture. There is countercurrent contact between the solid phase and the eluent which move in opposite directions. The racemate is injected in the middle of the column. Chiral discrimination provided for by the sorbent ... 

Figure 21-15. Schematic of a true moving bed for the enantioseparation of a racemate on a CSP.

In general a more efficient separation is achieved by counter-current operation, which is realised by the Simulated Moving Bed (SMB)-approach. Chromatographic counter-current operation requires the mobile and stationary phase to move in opposite directions. Figure 5.17 illustrates the ideal counter-current system, the True Moving Bed (TMB), which involves the actual circulation of the solid with a constant flow rate. 

Fig. 5.17 True moving bed (TMB) chromatography with internal concentration profile.

Counter-current Chromatographic Reactors 8.2.3.1 True Moving Bed Reactor... 

A counter-current flow of solid and fluid offers the advantage of continuous operation. The easiest concept of such a reactor is the true moving bed reactor (TMBR). It is a direct adoption of the true moving bed (TMB) process explained in Chapter 5.3.4. 

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