Systems - Simulated Moving Bed

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. 

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). 

Proll T, Kusters E. (1998) Optimization Strategy for Simulated Moving Bed Systems, J. Chromatogr. A 800 135-150. 

A classical Simulated Moving Bed system consists of 4 to 24 columns distributed between 4 zones, in addition to 3 to 5 pumps and valves which connect the different streams between the columns. In general a 4 column SMB should be sufficient to test and optimize the conditions for any given separation problem. The optimal number of columns per zone must be determined in the simulation of the SMB process. The rule is more columns per zone result in a better separation, while too many columns per zone make the system too complex. If an infinite number of columns per zone are used the SMB approaches a TMB. 

A simulated moving bed system has been proposed for the production of p-cresol from mixtures of cresol isomers even derived from coal tar [52]. Neuzil et al. give details of the development of the adsorbent and desorbent system reviewing balancing mass transfer issues with selechvity [53]. The desorbent for the cresol system is 1-pentanol. For these Hquid adsorptive systems where highly polar molecules are adsorbed and desorbed with polar desorbents, the tolerance of the system for trace polar contaminants is higher because the feed and desorbent can more easily exchange with them on the surface of the zeolites. 

The enzymatic ring expansion is neither complete nor selective, necessitating product isolation via chromatography in a simulated moving bed system. Because adipyl-7-ADCA rather than G-7-ADCA is produced, a new enzyme, adipyl-7-ADCA acylase, was developed to remove the side chain from adipyl-7-ADCA [132, 133], as the latter is not a substrate for penicillin G acylase. 

Zhang Z., Hidajat K., Ray A.K. and Morbidelli M., Multiobjective optimization of simulated moving bed system and Varicol process for chiral separation, AIChE J. (2002) in press. 

Proll, T., Kiisters, E. Optimization strategy for simulated moving bed systems, J. Chromatogr. A, 1998, 800, 135-150. 

Kurup, A. S., Hidajat, K. and Ray, A. K. (2006a). Comparative study of modified simulated moving bed systems at optimal conditions for the separation of ternary mixtures under nonideal conditions, Ind. Eng. Chem. Res., 45, pp. 3902-3915. 

The two fractions were collected from take off ports and the mobile phase returned for recirculation. This was the first form of the so called simulated moving bed system, and was used by Barker for continuous LC separations with moderate success. Leaks at the port seals on the wheel periphery were a source of difficulty, limiting the pressure that could be applied to the system to produce the mobile phase flow. The wheel was about 5 ft in diameter and thus was a somewhat heavy and rather cumbersome device. 

FIGURE 12,12, Schematic diagram of the UOP. Sorbex process showing simulated moving-bed system. (From ref. 2, reprinted by permission of UOP.)... 

Hirota et al. (1981) of Mitsubishi Kasei Co. (MKC) developed a similar simulated moving bed system by employing eight sets of columns and pumps as shown in Fig. 10.10. This process does not require a sophisticated rotary valve such as the one adopted in the Sorbex process. An example of concentration profile for fructose-dextrose separation is shown in Fig. 10.11 (Shioda, 1987). 

Another successful strategy used to achieve the performance of a countercurrent solid-fluid adsorber is to employ a simulated moving bed system. We have briefly described it in Figure 8.1.6 in Section 8.1.1. We will provide an illustrative treatment of it later in this section. 

Numerical computation of the simulated moving bed system provides a second alternative description to the continuous countercurrent and equilibrium stage models (Barker et al. 1983, Ching 1983, Hashimito et at. 1983, Carta and Pigford 1986, and Ching etal. 1987). Each bed of total volume V is considered to be equivalent to a number, n, of ideal mixing cells in which the fluid and adsorbed phases, volumes Vl and Vs, respectively, are distributed according to... 

Isothermal operation is not necessarily suitable for all simulated moving bed systems. The concentration of extract and raffinate streams can never exceed the concentration of feed components for a system with a linear isotherm. Furthermore, when the operating and equilibrium lines are close an excessive number of theoretical plates (to use the parlance of distillation and absorption technology) or height of each bed would be required for separation. Constraints such as these may be circumvented by non- isothermal operation of a simulated moving bed (Ching and Ruthven 1986). By maintaining bed section 1 at an elevated temperature with sections 2,3 and 4 at a lower temperature. 

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