SIMULATED COUNTERCURRENT TECHNIQUES

The SMB process was invented by Broughton in 1961 and developed by Universal Oil Products under the general name Sorbex . Initially used for separating n-paraffins in bulk, it is now used for a variety of individual-isomer separations and class separations, and is currently attracting considerable interest for separating pharmaceutical enantiomers. The SMB process is described in Section 17.9.4 and in a growing literaturel-21 22 11 - 74), 

Compared with elution chromatography, the advantage of both the SMB and SCCR forms of simulated countercurrent operation is that each product is taken off as soon as it is separated. The disadvantages are mechanical complexity and the fact that the number of pure products readily obtainable from one column is limited to two at most. Elution chromatography allows many components to be separated on one column. If no components are taken off before reaching the column exit, however, any components that are much more easily separated than the key components occupy space in 

Several types of reaction may be carried out in a chromatographic reactor. The reaction can be chemical or biochemical, taking place on the stationary phase, in the mobile phase, or both. The stationary phase must be chosen to have a good retention (affinity) for at least one component of the reaction system, and in some cases it has to act as a catalyst or catalyst support. Chromatographic reactors are particularly suited to enzyme-catalysed reactions such as the inversion of sucrose and biosynthesis of dextran, to various 

In general, chromatography is a powerful but relatively expensive separation technique whose advantages and disadvantages need to be evaluated carefully in selecting a separation system. The expense arises chiefly from the packings and the need in many cases to recycle the mobile phase. GLC and some LC and SFC packings need occasional replacement. Published cost data are as yet limited. 

The same advantages are exhibited by LC in comparison with techniques such as fractional crystallisation, liquid extraction, ultrafiltration and adsorption. It has already been pointed out (Section 19.6) that LC now plays a major part in bioseparations, where the technique needs to be integrated into the process train as part of a systems approach. 

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Eluxyl A process for separating /7-xylene from its isomers, using an adsorbent-solvent technique. The process is based on simulated countercurrent adsorption where the selective adsorbent is held stationary in the adsorption column. The feed mixture to be separated is introduced at various levels in the middle of the column, as in the Sorbex process. The /r-xylene product can be more than 99.9 percent pure. Developed by IFP and Chevron Chemical. A large pilot plant was built in Chevron s site at Pascacougla, MS, in 1994 and a commercial plant on the site was announced in 1996, Since then, the process has been widely licensed. 

Both GC and LC may be operated in one of several modes. The principal modes currently used for large-scale separations are elution, selective adsorption or desorption, and simulated countercurrent chromatography. In addition, reaction and separation can be combined in a single column with unique advantages. Elution is the most used and best developed form of the technique and is described first. 

A modification of this technique, the simulated countercurrent moving-bed chromatographic reactor [35], comprises several catalyst beds (Fig. 4(b)). The locations of inlet and outlet ports between the catalyst beds are changed sequentially, thus the countercurrent movement of solid and gaseous phases is simulated in a discrete manner. Such an operation avoids the technical difficulties (catalyst attrition, nonuniformity of solid flow, etc.) associated with solid-phase movement. Part of the reactor sections can be purged by the carrier gas. To increase the separation effect, a bed of adsorbent can be added in each section. 

This technique (Fig. 3.8), which is examined in greater detail in connecdon with the upgrading of aromatic Cs cuts, is based on the use of a pseudo-countercurrent between the liquid feedstock and the adsorbent bed. The displacement of the solid is in fact simulated by means of a rotary valve with multiple inlets and outlets, which causes a gradual change ih the injection and collection points of the liquid streams in the molecular sieve placed in a rould-stage column. 

The simulated moving bed adsorption technique is based on the movement of the stationary phase. The front and back ends of a series of columns are connected to form a circle, and during rotation of the columns a countercurrent movement of the phase relative to the liquid stream in the system is developed [158]. Injections of the fresh chiral analyte and the solvent are made at various coimecting points, and the separated enantiomers are withdrawn simultaneously at certain time intervals. This is a continuous process that provides certain advantages for enantiodiscrimination. The chiral selectors used in this technique are the same as those utilized in liquid chromatography and capillary electrophoresis. 

Liquid solutions, where the solute to be removed is adsorbed relatively strongly compared with the remainder of the solution, are treated in batch, semicontinuous, or continuous operations in a maimer analogous to the mixer-settler operations of liquid extraction (contact filtration). Continuous countercurrent cascades can be simulated or actually realized by use of such techniques as fluidized beds. 

Controversies about nomenclature have accompanied CCC since the early development of the first apparatus. The very name of the technique has been a matter of controversy as in most cases there is absolutely no countercurrent circulation only one of the liquids flows, while the other is stationary. Moreover, a significant number of reports uses the countercurrent chromatography terms when referring to simulated moving bed (SMB), a liquid-solid continuous modality of liquid chromatography [22,23]. 

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