Preparative Chromatography - Alternative Preparative Techniques > The Simulated Moving Bed Preparative Chromatography System > Page 56
In this way, the bed moved continuously in one direction relative to the ports and the mobile phase moved in the opposite direction. As with the moving bed system, some solutes (those that eluted at a rate slower than the wheel velocity) moved in the direction of the wheel movement while others (those that migrated faster than the wheel velocity) moved in the opposite direction with the movement of the mobile phase.
The two fractions were collected from 'take-off' ports. This was the first form of the so called simulated moving bed system, and was used by Barker for continuous GC separations with moderate success. The main problems met in this system were leaks that occurred at the port seals on the wheel periphery, which seriously, limited 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 very heavy and rather cumbersome device.
The first practical simulated moving bed system was described by Hurrel in the late 1960s (15). The Hurrel system was basically an ingenious extension of the large circular column devised by Barker. He employed a number of short preparative columns connected in series and rotation was simulated by mean of a somewhat complex set of valves mounted in the form of a disc. This procedure had two main advantages over the actual moving bed system. In addition to the technical and constructional simplicity, there is considerable adsorbent conservation and, as the mobile phase is recirculated, there is substantial solvent economy. The system devised by Hurrel is shown diagramatically in figure 29.
The circular column consists of a number of sections, each section taking the form of a relatively short preparative column. The columns are formed in the shape of a slanting U to facilitate packing, and are packed individually employing standard packing techniques. The columns are all joined in series by means of the large rotary disc valve fitted with appropriate ports. The disc valve could be fabricated in a relatively simple manner, compared with the construction of the massive rotating wheel device and, by suitable surface grinding and lapping, could be made leak proof even at relatively high pressures.