The mobile phase passes through several stationary columns which contain the stationary phase which may be silica gel, or a bonded phase, or, in GC a suitable coated support. There are, also, a number of different ports, one for the mobile phase and one for the return of the mobile phase. In addition there is a central feed port and two take-off ports. These ports can, by appropriate valve programming (usually by means of a computer) connect, sequentially, each column to its neighbor. In modern simulated moving bed systems, disc valves are no longer employed and have been replaced by sets of individual valves. It is clear that the apparent counter current movement of the stationary phase, relative to the mobile phase, is achieved by appropriate valve switching which simulates the rotation of the ports on the disc valve between each column.
Referring to figure 31, it is seen that part of the feed moves with the mobile phase and is collected by a small take-off flow in front of the feed port (B + solvent). The other, more retained portion of the sample, accumulates in a column on the other side of the feed port and is collected by a another small take-off flow behind the feed port (A + solvent). This particular system ideally, produces two products and thus could lend itself specifically to the separation of enantiomeric pairs. However, for effective separation with high purity yields, the stationary phase capacity for the two enantiomers must be fairly large and thus the phase system must be carefully selected. The technique has been successfully used to isolate single enantiomer drugs (16-18).
Radial Flow Chromatography
Another alternative chromatographic procedure for preparative separations is radial flow chromatography. The radial flow chromatography column consist of two concentric porous cylindrical frits between which the stationary phase is packed. basically, it is only effective when the separation ratios between the substance of interest and its neighbors is fairly high. This is because the column itself has a very limited length (equivalent to the radial thickness of the packing) and thus can produce relatively few theoretical plates. The efficiency can be significantly improved by using very small particles but the effect use of the radial column also requires a very homogeneous packing. A diagram of a radial chromatography column is shown in figure 32.. The mobile phase flows from the outer cylindrical frit, across the radius of the column, through the cylindrical bed of stationary phase, to the inner cylindrical frit. The radial gap between the frits is the effective bed 'height.' or 'length'.