SMB – Simulated Moving Bed
The Simulated Moving Bed, abbreviated SMB, is a continuous process of column chromatography. It offers some advantages compared to the purification in a batch process e.g. lower solvent consumption and better utilization of the stationary phase.
"Time is money" - says an old proverb which also has its importance in chromatography. An automatic substances purification process saving solvents as well as manpower and delivering the best results is always welcome, both in the laboratory and in production.
The goal of column chromatography purification is the generation of clearly separated components of a compound mixture by the HPLC column. Unfortunately, this is not successful with every type of substance within one run, so that several subsequent purifications have to be carried out, which inevitably lead to considerable costs concerning material and working hours.
The SMB chromatography avoids these disadvantages of simple batch process purifications by multi-zone separation processes, and by achieving higher purities with less solvent consumption. However, the simulated moving bed chromatography is limited to two factions and restricted to binary substance separations.
The principle of SMB chromatography is based on the movement of the solid phase in the opposite direction to the liquid phase. However, the opposite movement of the liquid to the solid phase is technically as good as not feasible, so that this process is simulated in the SMB procedure.
For this purpose, instead of one HPLC column numerous small HPLC columns are interconnected in series so that the liquid phase can flow through these in a closed circuit. The movement of the solid phase is achieved either by switching the columns against the flow direction of the liquid phase or by switching the inputs and outputs in the flow direction of the liquid phase. Actually the solid phase is not in motion but by column switching the movement can be simulated.
By the use of various inputs and outputs the solid phase is divided into four zones. Between zones 2 and 3, the substance to be separated (feed) is injected and here the separation of the components takes place. For this reason the feed is transported to zone 3 with the liquid phase. The idea is realizing a complete adsorption of the stronger adsorbing component A, so that it can be transported to zone 2 with the solid phase. The weaker adsorbing purified component B can be removed from the process at the output between zones 1 and 2. In zone 2 the complete desorption of the weaker adsorbing component B takes place, so that it can be transported back to zone 3 with the liquid phase, while the stronger adsorbing component A can be removed purified at the output between zones 3 and 4.
In zone 1 fresh eluent addition generates an elution of the stronger adsorbing component, which is subsequently transported to zone 2 with the liquid phase. The solid phase is regenerated and can be reused in zone 4.
In zone 4, the adsorption of the weaker adsorbing substance takes place. This substance is then transported in adsorbed form with the stationary phase to zone 3. Also the liquid phase in zone 4 is regenerated now and can be reused in zone 1. The extracted raffinate is returned to the system as solvent between zone 1 and 4.
In order to achieve a complete separation of a substance mixture, the flow rate in the separation zones must be adjusted in such way that the more retarded component A is delivered to the output A while the less retarded component B is delivered to the output B.
After a certain start-up time adjusted individual flow rates in the different zones and optimized velocity of the solid phase lead to optimal separation efficiency.