Simulation moving bed

Fig. 8. UOP Parex simulated moving bed for adsorptive separation. AC = adsorbent chamber RV = rotary valve EC = extract column ...

Since the 1960s the commercial development of continuous countercurrent processes has been almost entirely accompHshed by using a flow scheme that simulates the continuous countercurrent flow of adsorbent and process Hquid without the actual movement of the adsorbent. The idea of a simulated moving bed (SMB) can be traced back to the Shanks system for leaching soda ash (58). 

To complete the simulation, the Hquid-flow rate relative to the soHd must be the same in both the moving-bed and simulated moving-bed operations. Because the soHd is physically stationary in the simulated moving-bed operation, the Hquid velocity relative to the vessel wall must be higher than in an actual moving-bed operation. 

The theoretical performance of the commercial simulated moving-bed operation is practically identical to that of a system ia which soHds dow continuously as a dense bed countercurrent to Hquid. A model ia which the dows of soHd and Hquid are continuous, as shown ia Figure 7, is therefore adequate. 

Aromatic and Nonaromatic Hydrocarbon Separation. Aromatics are partially removed from kerosines and jet fuels to improve smoke point and burning characteristics. This removal is commonly accompHshed by hydroprocessing, but can also be achieved by Hquid-Hquid extraction with solvents, such as furfural, or by adsorptive separation. Table 7 shows the results of a simulated moving-bed pilot-plant test using siHca gel adsorbent and feedstock components mainly in the C q—range. The extent of extraction does not vary gready for each of the various species of aromatics present. SiHca gel tends to extract all aromatics from nonaromatics (89). 

Chromatography may also be advantageous when it is required to separate several pure products from a single feed stream. A simulated moving-bed system can yield only two weU-separated fractions from a single feed stream. 

The question of whether adsorption should be done ia the gas or Hquid phase is an interesting one. Often the choice is clear. Eor example, ia the separation of nitrogen from oxygen, Hquid-phase separation is not practical because of low temperature requirements. In C q—olefin separation, a gas-phase operation is not feasible because of reactivity of feed components at high temperatures. Also, ia the case of substituted aromatics separation, such as xylene from other Cg aromatics, the inherent selectivities of iadividual components are so close to one another that a simulated moving-bed operation ia hquid phase is the only practical choice. 

However, ia some cases, the answer is not clear. A variety of factors need to be taken iato consideration before a clear choice emerges. Eor example, UOP s Molex and IsoSiv processes are used to separate normal paraffins from non-normals and aromatics ia feedstocks containing C —C2Q hydrocarbons, and both processes use molecular sieve adsorbents. However, Molex operates ia simulated moving-bed mode ia Hquid phase, and IsoSiv operates ia gas phase, with temperature swiag desorption by a displacement fluid. The foUowiag comparison of UOP s Molex and IsoSiv processes iadicates some of the primary factors that are often used ia decision making ... 

Njlene Separation. -Xylene is separated from mixed xylenes and ethylbenzene by means of the Parex process (Universal Oil Products Company). A proprietary adsorbent and process cycle are employed in a simulated moving-bed system. High purity -xylene is produced. 

Another approach is the simulated moving-bed system, which has large-volume appHcations in normal-paraffin separation andpara- s.yXen.e separation. Since its introduction in 1970, the simulated moving-bed system has largely displaced crystallisation ia xylene separations. The unique feature of the system is that, although the bed is fixed, the feed point shifts to simulate a moving bed (see Adsorption,liquid separation). 

A. Gentilini, C. Migliorini, M. Mazzotti and M. Morbidelli, Optimal operation of simulated moving-bed units for non-linear cliromatograpliic separ ations. II. Bi-Langmuir isotherm , 7. Chromatogr. 805 37-44 (1998). 

R.-M. Nicoud, G. Fuchs, P Adam, M. Bailly, E. Kusters, F. D. Antia, R. Reuille and E. Sclimid, Prepar ative scale enantiosepar ation of a cliiral epoxide compar ison of liquid cliromatography and simulated moving bed adsorption technology . Chirality 5 267-271 (1993). 

D. W. Guest, Evaluation of simulated moving bed cliromatography for pharmaceutical process development , J. Chromatogr. 760 159-162 (1997). 

B. Pynnonen, Simulated moving bed processing escape from the liigh-cost box , ]. Chromatogr. 827 143-160(1998). 

S. Nagamatsu, K. Murazumi and S. Makino, Cliiral separation of a pharmaceutical intermediate by a simulated moving bed process , ]. Chromatogr. 832 55-65 (1999). 

M. Juza, Development of a high performance liquid cliromatographic simulated moving bed separation from an industrial perspective , J. Chromatogr. 865 35-49 (1999). 

E. R. Francotte and R Richeit, Applications of simulated moving-bed cliromatography to the separation of the enantiomers of chiral drugs , ]. Chromatogr. 769 101-107 (1997). 

On that basis, crystallization is often used in combination with other enantiose-lective techniques, such as enantioselective synthesis, enzymatic kinetic resolution or simulated moving bed (SMB) chromatography [10, 11]. In general, when referring to crystallization techniques, the aim is to obtain an enantiomeric enrichment in the crystallized solid. However, the possibility of producing an enrichment in the mother liquors [12, 13], even if this is not a general phenomenon [14], must be taken into account. 

Gattuso M. J., McCulloch B., House D. W., Baumann W. M., Gottschall K. (1996) Simulated Moving Bed Technology - The Preparation of Single Enantiomer Drugs, Pharm. Tech. Europe 8 20-25. 

Cavoy E., Deltent M. E, Lehoucq S., Miggiano D. (1997) Laboratory-Developed Simulated Moving Bed for Chiral Drug Separations. Design of the System and Separation of Tramadol Enantiomers, J. Chrotnatogr. A 769 49-57. 

Erancotte E., Richert P. (1997) Applications of Simulated Moving-Bed Chromatography to the Separation of the Enantiomers of Chiral Drugs, J. Chrotnatogr. A 769 101-107. 

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