True Moving Bed is a continuous chromatographic process whose principle has been adopted in Simulated Moving Bed process in which liquid and solid phase flow in opposite direction. For production processes, the productivity of the classical (batch) chromatography is too small and the consumption of solvents is too high. Therefore, true moving bed chromatography was invented as an approximation to a continuous counter- current operation. The concrete example of a True Moving Bed system is given in Hypersorption process by Union Oil Company in California, USA (Berg, 1946, 1951;
Kehde et al., 1948). The technique was first applied for gas separation with activated carbon as solid phase flowing continuously downward through a rising gas stream containing methane, hydrogen, ethylene and other gases lighter than ethane.
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In the area of bioseparation, Barker et al. (1992) use a 12-column preparative semi- continuous counter current chromatographic bioreactor separator in biosynthesis of dextran from sucrose in the presence of enzyme dextransucrase and the continuous production of maltose from modified starch. They are able to overcome viscosity problems and displacement of Ca2+ from the resin but product contamination were encountered after 50 hours of operation due to formation of levan and glucose. For maltose production, high purity maltose can be produced while keeping enzyme usage as low as 50% of the theoretical requirement for a conventional batch bioreactor.
Figure 2.2 Schematic diagram of TMB chromatographic process
The main principle of this system is adjusting the relative velocity between the descending stationary phase and the ascending mobile phases to ensure that the more
D
A
B Zone
I
Zone II
Zone III
Zone IV
Solid flow Liquid
flow Extract, A+D
Desorbent, D
Feed, A+B
Raffinate, B+D
Liquid Composition
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Schematic representation of TMB process is depicted in Figure 2.2 for binary separation of species A and B, with A selectively adsorbed in the solid phase. Feed is continuously injected into the middle of the system with two product withdrawal ports: the extract port, rich in the strongly adsorbed species A and the raffinate port, rich in the weakly adsorbed species B. Continuous recovery of relatively pure products is achieved with appropriate regulation of internal liquid and solid flow rates thus eliminating the drawback of species dilution and low adsorbent utilization encountered in batch chromatography.
There exist four intrinsic zones in the bulk of the system and the governing role that allows chromatographic separation of any of two species is detailed as follows:
1. Zone I (Desorption of A), is between point of eluent injection and extract withdrawal. The solid entering zone I contains the more retained component A and as the fresh desorbent D stream flows in the opposite direction with the solid phase, component A are displaced by desorbent and a portion of liquid leaving this zone is withdrawn as extract and the remainder flows to zone II as reflux. In other words, zone I allows solid regeneration.
2. Zone II (Desorption of B), is between point of feed injection and extract withdrawal. At the fresh feed point, the upward flowing solid adsorbent contains the quantity of component A that was adsorbed in zone I. However, the pores will also contain a large amount of B, because the adsorbent has just been in contact with fresh feed. The liquid entering the top of zone II contains no B, only component A and D. In this way, component B is gradually displaced from the pores of A and as the adsorbent moves up through zone II. At the top of zone II the pores will contain only A and D.
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3. Zone III (Adsorption of A), is between the point of feed injection and raffinate withdrawal. As fresh feed flows down through zone III, countercurrent to the solid adsorbent flowing upward, component A is selectively adsorbed from the feed into the pores of the adsorbent. At the same time, the desorbent is desorbed from the pores of the adsorbent to the liquid stream in order to provide room for component A in the pores.
4. Zone IV (Desorption of D), is where the feed components in zone III are segregated from extract in zone I. At the top of zone I, the adsorbent pores are completely filled with D. The liquid entering the top of zone IV consists of B and D. It is possible to prevent the flow of component B into zone I and avoid contamination of the extract by properly regulating the flow rate of zone IV.
This TMB approach, despite persistent work on moving system of this type, suffers from mechanical problem such as inconsistent flow caused by solids circulation, low mass transfer coefficient at uneven column packing, solid attrition due to shear forces and low mobile phase velocities to prevent bed fluidization. Low temperature distillation replaces this technique due to economical consideration after all.
In further development, the solid bed is kept stationary in practical applications and the continuous movement of solid phase is simulated by a periodic shift of inlet and outlet ports in the direction of mobile phase flow yielding Simulated Moving Bed Chromatography as described in the following sub chapter.
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