Page 280 - Chiral Separation Techniques

10.3 SMB as a Development Tool 259

The affinity of A and B for the solid phase are different, B is more retained than
A. Thus, it is possible to choose flowrates in order to make A move upward and B
to move downward, leading to a spatial separation. This system requires inlet lines
for the feed and eluent and outlet lines for the raffinate A and extract B.
The classical moving bed is made of four different zones in which different con-
straints must be fulfilled [53].

Zone 1: between the eluent make-up and the extract points where the more
retained product (B) must be completely desorbed.
Zone II: between the extract and the feed points where the less-retained product
(A) must be completely desorbed.
Zone III: between the feed and the raffinate points where the more retained prod-
uct (B) must be completely adsorbed.
Zone IV: between the raffinate and the eluent make-up points where the less-
retained product (A) must be completely adsorbed.
All the internal flowrates are related to the inlet/outlet flowrates by simple mass
balances:
Q = Q − Q Q = Q + Q (1)
II I Ext III II Feed
Q = Q − Q Q = Q + Q
IV III Raff I IV El
In addition, the inlet/outlet flowrates are related by:

Q Ext + Q Raff = Q Feed + Q El (2)
In fact, it is extremely difficult to operate a TMB because it involves circulation
of a solid adsorbent. Thus, the concept must be implemented in a different way
where the benefit of a true countercurrent operation can be achieved by using sev-
eral fixed-bed columns in series with an appropriate shift of the injection and col-
lection points between the columns. This is the SMB implementation as presented in
Fig. 10.2.
In this mode, the solid is no longer moving. The shifting of the inlet and outlet
lines only simulates solid flow, and the solid flowrate downward is directly linked to
the shift period. Proper selection of flowrates is required to stabilize the different
fronts of species A and B in the proper zones. The adequate choice of the flowrates
requires a minimum knowledge of the physico-chemical properties of the system.
The influence of adsorption isotherms and plate numbers is simulated by the software.
A SMB, whatever the number of zones, consist of 4 to 24 columns with 3 to 5
pumps and a number of valves, which allow the columns to be connected to differ-
ent lines. Further discussions are restricted to the classical 4-zone SMB. There are
different ways to connect columns in order to build a SMB. An important option is
linked to the presence or absence of a recycling pump in order to build a SMB. The
examples are shown in Fig. 10.3.
As shown in Fig. 10.3a, the most classical way is to use a recycling pump that is
located between two columns (for instance 12 and 1). The recycling pump, which is