The SMB concept was introduced to solve practical difficulties arising from mechanical problem: solid movement in a transient TMB unit. It is a technical realization of a countercurrent adsorption process in which the counter-currency between mobile and stationary phases is accomplished without physical movement of the solid. The essential element in SMB operation is rotary valves (Broughton and Gerhold, 1968) or a number of conventional two-way valves (Marteau et al., 1994), which allows periodic shifting the position of feed, desorbent, extract and raffinate lines along the bed, and a pumparound pump to circulate liquid through the adsorbent chamber. The specific location of inlet (feed and desorbent) and outlet ports (extract and raffinate) divide the entire system into four zones, each assuming a certain role. In general, most of the benefits of a countercurrent operation can be achieved by subdividing the adsorbent into a number of static beds and regularly moving all inlet and outlet ports simultaneously one column forward in the direction of liquid flow at fixed time interval.
An SMB-plant consists of a number of chromatographic columns (usually 8 - 24) the input streams to which are switched periodically in order to approximate the countercurrency in true moving bed chromatography. In the limit of an infinite number of columns and short switching periods, the operating mode comprises a real countercurrent process. The operation of an SMB process is complicated and requires a careful choice of many parameters, in particular of the various flow rates and of the switching times. The process has nonlinear dynamic characteristics and it may drift away slowly such that after some hours, a breakthrough of a substance in the wrong stream will occur. As the SMB process is using little energy and can separate difficult mixtures, e.g. substances which
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only differ in the geometric orientation (left and right hand molecules), it is becoming popular as a separation technique for low volume high price chemicals.
In the latest development, chemical industry has developed numerous processes based on the use of adsorption and some of the most recent ones are based on the use of chromatographic principles. This is the case of separation process based on the simulated moving bed principle. Initially developed for the extraction of a few specific compound from complex mixtures, such as paraxylene from reforming streams or fructose from corn syrup, these processes are beginning to compete with the simple processes evolved from direct scaling up of the lab procedures. Currently, overloaded elution for production rates below 500 ton/year and simulated moving bed for production rates above 10.000 ton/year are dominating the field, apparently leaving a serious gap in between.
The schematic diagram of SMB operation loop can be visualized in Figure 2.3 below:
Feed, F Extract, E
Raffinate, R Desorbent, D
Zone I, Q1
Zone III, Q3
Zone IV, Q4
Zone II, Q2
A
B B
A S + B
S + A + B S + A
S
D + A + B
D + A + B D + A
D + B D
B A D D + B
D + A
1
3 2
4
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The general case of simulated counter-current process is that the movement of solid is simulated by successive switching of the feed and product positions at timed interval. The hypothetical velocity of solid phase is the ratio of length of column of each section and the switching time. The process shows a cyclic behavior, in the absence of disturbances, in which the profile at the end of the interval is equal to that at the beginning of the interval shifted one column in forward direction. In general, the simulated system does not reach a steady state in the time interval between successive switching due to the disturbances introduced each time a switching is made. The stationary regime of this process is a cyclic steady state, attained after several switching, in which an identical transient during each period between two valve switches takes place in each section. The system exists at a new transient behavior regardless of the state of the system had been in just before the switching.
The installation allows a continuous production by chromatographic separation by simulating the displacement of the counter-current bed of the eluent phase. The simulation is done by sequenced displacement of the injection points, from one column to another, upstream to the eluent phase. During this lapse of time, the chromatographic profile migrates in the same direction as the fluid inside the separator. A pseudo steady state mode allows continuous collection of raffinate and extract at specified yield and purities.
The desorbent liquid is selected so as to possess a boiling point significantly different from those of the feed components. The desorbent must also be capable of displacing the feed components from the pores of the adsorbent and conversely, the feed components must be able to displace the desorbent from the pores of the adsorbent. Therefore the desorbent must be chosen in such a way to be able to compete with the feed components
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for any available active pore space in the adsorbent, solely on the basis of concentration gradients.
The actual liquid flow rate within each of the four zones is different, due to injection and withdrawal of the net streams. Both the concentration profiles as well as the zones position moves toward the adsorption chamber. The overall liquid circulation rate is controlled by the pumparound pump, which should operate at four different flow rates, depending on which zone is passing through the pump.
Even though SMB lends itself to production scale better than elution chromatography due to its continuous operating modes, it requires more development time and is sensitive to dead volume effects which must be accurately quantified and modeled (Schulte and Strube, 2001). Despite these disadvantages, several contract manufacturers and pharmaceutical companies such as Novasep, UPT and Merck have invested heavily in SMB technology, treating it as cost effective production scale enantioseparation technique (Juza et al., 2000; Francotte, 2001; Schulte and Strube, 2001).