Figure 2 is a standard SMB plot showing concentrations of bovine serum albumin (BSA) and equine heart myoglobin (EHM) solutes (sampled at a specific time into each step) as a function of column sequence number. This is the position relative to the beginning of Zone I, shown here for a 2-5-4-1 configuration. Brackets with labels indicate where feed and elution solvent are added to the loop and where raffinate and extract are removed. Solute concentrations in the extract and raffinate streams change dramatically over the course of a single step, as illustrated by the bracketed ranges.
FLOW RATE REQUIREMENTS
For economical operation, liquid flow rates must be carefully adjusted to obtain desired separation while minimizing solvent consumption. Flow rates are regulated to prevent slow and fast eluting solutes from lapping each other and to generate an internal profile in which, ideally, essentially all of the fast eluter exits in the raffinate and all of the slow eluter exits in the extract. For simplicity, consider a system with only two solutes, a fast eluter A and a slow eluter B. Starting with Zone II, liquid flow rate is chosen so the majority of A moves forward (clockwise) into Zone III but flow rate is limited so the majority of B doesn't enter Zone III. In Zone III, liquid flow rate is selected so B barely moves backward (counterclockwise relative to the inlet and outlet ports) and thus A (being faster) will continue to move forward. Flow rate in Zone III always will exceed that in Zone II because in Zone III flow rate equals the sum of flow rates of Zone II and the entering feed.
Zone IV uses the slowest liquid flow rate — chosen to be just slow enough to prevent A from moving from Zone IV into Zone I. It should be no slower than necessary, as this yields a more economical operation because more solvent will be recycled into Zone I, reducing need to add fresh solvent. Zone I uses the highest flow rate, to stop B from falling behind. Flow rate is selected to be just fast enough to force B to move forward. Making this flow too fast requires excess elution solvent.
Pulse tests can serve to estimate SMB flow rates. Optimal pulse test uses a single column of the same length and filled with the same media as envisioned for the commercial scale. For economical commercial-scale operation, particle diameters typically are on the order of 200 to 350 microns to avoid excessive pressure drop [5,7]. A column of 0.5-in. (1.3-cm.) diameter or larger is needed to avoid significant wall effects. To provide adequate separation performance with the larger particle diameters, column length usually is at least 3 ft. (approximately 1 m).
Figure 3 presents pulse test data generated in a study of protein separations. The fast eluting solute is BSA, the slow eluting solute is EHM and the eluent is a dilute buffered solution of NaCl in water. The graph shows solute concentration in the effluent [relative to that in the feed] versus the number of empty bed volumes (BV) of feed liquid that have passed through the column. The peaks in Figure 3 don't show baseline resolution, which is unneeded and, in fact, undesirable. Instead, the goal is to separate the leading edge of the first peak from the trailing edge of the second. Unlike analytical chromatography, peaks should overlap significantly while maintaining good purities within the leading-edge and trailing-edge regions. This facilitates a good binary separation at maximum productivity potential. If overlap is small, increase the concentration of solute in the feed pulse and repeat the test. Ideally, for most economical SMB operation, all peaks should elute within about 1 to 3 BV, as too much retention by the media is undesirable. Some applications require up to 7 or 8 BV for everything to elute; this may be acceptable — but only if the product is particularly valuable.