Moving-bed adsorption system

FIGURE 8.2 Schematic showing fixed-bed and moving-bed adsorption system. (After Knox et al., 1986.)... 

FIGURE 14.18 Simulated moving-bed adsorption system, after UOP Sorbex concept. 

Most adsorption systems use stationary-bed adsorbers. However, efforts have been made over the years to develop moving-bed adsorption processes in which the adsorbent is moved from an adsorption chamber to another chamber for regeneration, with countercurrent contacting of gases with the adsorbents in each chamber. Union Oil s Hypersorption Process (90) is an example. However, this process proved uneconomical, primarily because of excessive losses resulting from adsorbent attrition. 

The simulated moving bed adsorption technique is based on the movement of the stationary phase. The front and back ends of a series of columns are connected to form a circle, and during rotation of the columns a countercurrent movement of the phase relative to the liquid stream in the system is developed [158]. Injections of the fresh chiral analyte and the solvent are made at various coimecting points, and the separated enantiomers are withdrawn simultaneously at certain time intervals. This is a continuous process that provides certain advantages for enantiodiscrimination. The chiral selectors used in this technique are the same as those utilized in liquid chromatography and capillary electrophoresis. 

System type (2) Two different pre-existing immiscible phases introduced into the device/column move countercurrent to each other and leave the device/ column. Examples include gas-Uquid absorption/ stripping solvent extraction/back extraction moving-bed adsorption supercritical extraction. 

Another approach is the simulated moving-bed system, which has large-volume appHcations in normal-paraffin separation andpara- s.yXen.e separation. Since its introduction in 1970, the simulated moving-bed system has largely displaced crystallisation ia xylene separations. The unique feature of the system is that, although the bed is fixed, the feed point shifts to simulate a moving bed (see Adsorption,liquid separation). 

Continuous Countercurrent Systems Most adsorption systems use fixed-bed adsorbers. However, if the fluid to be separated and that used for desorption can be countercurrently contacted by a moving bed of the adsorbent, there are significant efficiencies to be realized. Because the adsorbent leaves the adsorption section essentially in equilibrium with the feed composition, the inefficiency of the... 

FIG. 16-46 General scheme of a true moving bed (TMB) adsorption system for binary separations. A is less strongly retained than B. 

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Activated carbon adsorption may be accomplished by batch, column, or fluid-ized-bed operations. The usual contacting systems are fixed-bed or countercurrent moving beds, as shown in Figure 8.2. The fixed beds may employ downflow or upflow of water. The countercurrent moving beds employ upflow of the water and downflow of the carbon, since the carbon can be moved by the force of gravity. Both fixed beds and moving beds may use gravity or pressure flow. 

Another approach to continuous reaction chromatography is the countercurrent moving-bed chromatographic reactor (CMCR). In this type of reactor the stationary (solid) phase travels in the opposite direction to the liquid phase. In practice this is performed by introducing the stationary phase from the top of the reactor. The stationary phase flows downwards under the influence of gravity while the liquid phase is pumped upwards from the bottom. A schematic presentation of such a system is shown in Fig. 7. Depending on the adsorption characteristics of the different components, they can travel in the direction of the liquid or the solid phase resulting in their separation. 

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