3. Simulated Moving Bed and Varicol Process
3.1. Schematic Diagram of SMB and Varicol Process
SMB system consists of cascade of columns arranged in a circular way. Each column is connected by a flexible valve injection and withdrawal ports. Columns are classified into four zones by two inlets (feed and desorbent) and two outlets (raffinate and extract) and loaded with resin that could be used both as catalyst and adsorbent. The discrete movements of the inlet and the outlet terminals of columns mimics solids flow in the opposite direction of liquid flow.
Figure 3.1 Schematic diagram of 4-zone 8-column SMB with 2 columns per zone
The key elements in SMB operations include the selection of an appropriate adsorbent and adjustment of internal flow rates. This is to ensure that the more retained species,
Feed (F) Extract (E)
Raffinate (R) Desorbent (D)
Zone IV
Zone II
A
B C
D
Zone I Zone III
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collected at the extract node, migrates with the stationary phase and the less retained species, withdrawn at the raffinate node, travels with the mobile phase. Comprehensive studies focusing the importance of adsorbent selection in order to maximize throughput (Schulte et al., 1997) and flow rates setting to ensure that each zone perform its specific role (Kawase et al., 1996) have been reported.
Figure 3.2 Switching profile of 4-zone 5-column SMB and Varicol processes
Varicol process (Adam et al., 2000) adopts similar apparatus in which the four terminal points are not shifted concurrently as in SMB operation. The overall switching time is divided into several subintervals and this number of subinterval depends on the magnitude of switching time due to operation feasibility. In fact, one column can be moved more than once during a switching period but the column configuration at the end of the switching interval returns to that attained by the SMB before the switching
D
E R
F 1st switching 2nd switching
D R
E F
R
E D
F
ts ẵ ~ ts ắ
E D R
F
ts ắ ~ ts D
E R
F
0 ~ ts ẳ 1st switching
R
D F
0 ~ tsE ẳ 2nd switching ts ẳ ~ ts ẵ
E R
F
D
SMB Process
Varicol Process
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operation. Figure 3.2 demonstrate switching development of 4-zone SMB in comparison with 4-subinterval Varicol process.
Table 3.1 Possible column configurations for SMB and Varicol processes
0/1/1/2 1/0/1/2 1/1/1/1 1/2/1/0 2/1/1/0 0/1/2/1 1/0/2/1 1/1/2/0 2/0/1/1
Ncol = 4 (with 3-zone
configuration) 0/2/1/1 1/1/0/2 1/2/0/1 2/1/0/1 Ncol= 5 1/1/1/2 1/1/2/1 1/2/1/1 2/1/1/1 1/1/1/3 1/2/1/2 1/3/2/1 2/2/1/1 1/1/2/2 1/2/2/1 2/1/1/2 3/1/1/1 Ncol = 6
1/1/3/1 1/3/1/2 2/1/2/1
1/1/1/4 1/2/1/3 1/3/2/1 2/1/3/1 3/1/1/2 1/1/2/3 1/2/2/2 1/4/1/1 2/2/1/2 3/1/2/1 1/1/3/2 1/2/3/1 2/1/1/3 2/2/2/1 3/2/1/1 Ncol = 7
1/1/4/1 1/3/1/2 2/1/2/2 2/3/1/1 4/1/1/1 1/1/1/5 1/2/3/2 1/5/1/1 2/2/3/1 3/2/1/2 1/1/2/4 1/2/4/1 2/1/1/4 2/3/1/2 3/2/2/1 1/1/3/3 1/3/1/3 2/1/2/3 2/3/2/1 3/3/1/1 1/1/4/2 1/3/2/2 2/1/3/2 2/4/1/1 4/1/1/2 1/1/5/1 1/3/3/1 2/1/4/1 3/1/1/3 4/1/2/1 1/2/1/4 1/4/1/2 2/2/1/3 3/1/2/2 4/2/1/1 Ncol = 8
1/2/2/3 1/4/2/1 2/2/2/2 3/1/3/1 5/1/1/1
Each zone in SMB process accounts for specific function. Zone I, between the desorbent and the extract stream, allows solid regeneration by ensuring complete desorption of the more retained component from the solid phase. This helps to reduce contamination of the raffinate product as zone I becomes zone IV in subsequent switching. Zone II, the section between the extract and the feed injection point, is responsible for desorption of the less retained component off the adsorbent in order for the feed section has a head start. Zone III, the section between the feed and the raffinate nodes, is the adsorption zone for the more retained component to prevent this component from being conveyed by the mobile phase to the raffinate collection outlet. Solvent
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regeneration takes place in zone IV, the section between the raffinate and the desorbent port, whose role is to ensure adsorption of the less retained component.
Subdivision of bed (or columns) results in numerous possible configuration of SMB and Varicol process. High number of total columns of a SMB, or Varicol, unit is preferred
Figure 3.3 Schematic diagram of 3-zone 4-column SMB/Varicol process
as it allows more flexibility in column configuration thus introduces more degree of freedom in defining the right combinations of columns in respective zones. This effect is largely exercised by Varicol process, because it overcomes the rigidity of SMB in acquiring countercurrent motion between the mobile and the stationary phase. There are
NI = 0
NII = 0
E
R
D F R
D E F R
Zone II Zone III Zone IV
Zone I Zone III Zone IV
D Zone I E Zone II F Zone IV
NIII = 0
R
D Zone I E Zone II F Zone III
NIV = 0
liquid direction
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configurations for 6-column setup. Table 3.1 spells out the detail of possible configu- rations for 4-zone setup, except for 4-column unit.
The principle of 3-zone SMB (Bjorklund et al., 2001) is applied to 4-column Varicol process as can be seen in Figure 3.3. The number of columns could take zero value for certain configurations. For illustration, configuration 2/1/1/0 results in the absence of zone IV while there are 2 columns in zone I and 1 column each for zone II and III to constitute 3-zone 4-column Varicol unit. This is particularly true for process in which the desired product has absolutely no or little affinity toward the adsorbent. Thus zone IV, whose main task is to prevent the less retained component from entering zone I, is no longer needed.