Compared to traditional batch chromatography, Simulated Moving Bed (SMB) chromatography leads to greater yields of purified substances while consuming less eluent and packing material.
Unlike batch chromatography, in a SMB process not only the liquid phase is in motion but the solid phase also moves in counter-current mode to the liquid. At the same time, the individual areas are operated with different volume currents. The concept that is demonstrated in Figure 1 refers to the classic four zone version SMB configuration.
SMB process scheme.
Separation Zones
the solid phase is regenerated. The feed (component A + B) is fed in the middle of the illustrated “SMB column”. The liquid phase (desorbent or eluent) travels from the bottom to the top and transports component A (blue).
Component A is continuously led out of the system in the raffinate as a pure substance. The solid phase (solid) passes from the top to the bottom and transports component B (red). Component B is pure when it is led out of the system as an extract. The distribution of both components over the whole column system is effortlessly recognized by the concentration profile.
The movement of the solid phase is recognized by connecting a specific number of columns in series in a closed circuit and by switching the columns counter-clockwise in defined time intervals one position upstream. The build-up of the process is schematically displayed in Figure 2. Four pumps are combined into the system.
Two pumps are employed for the add-on of feed and fresh eluent and two pumps are employed for the maintenance of the internal flow. Generally 8, 12, or 16 columns are employed based on the need. The columns in general are equally distributed in the individual zones (for example, 2/2/2/2). However, based on the separation problem, they can be asymmetrically distributed (for example, 1/3/3/1).
In addition to the four zones, concepts with three, five or eight zones can also be recognized. The latter two can be drawn for a three component separation. Complex mixtures can also be separated into two valuable substance fractions (pseudo-components).
In standard chromatographic separations with just one column, a small part of the column takes part in the separation, while the predominant part does not make a contribution to the productivity. This SMB solution ensures the whole column bed is efficiently used. A stationary state is developed when all volume currents are optimized. This means that the middle concentrations of the purified extract and raffinate currents no longer change. The system can be continuously operated under these conditions.
Classical 4 zone SMB configuration.
Conclusion
SMB was originally used to separate xylene isomers in the oil refining industry as they could not be efficiently separated through distillation.
Today, this technology has been established and is employed in the sugar industry, for the production of pharmaceutical ingredients and in chiral separations
Reference
en.wikipedia.org/wiki/Simulated_moving_bed
This information has been sourced, reviewed and adapted from materials provided by KNAUER Wissenschaftliche Geräte GmbH.
For more information on this source, please visit KNAUER Wissenschaftliche Geräte GmbH.