Adsorption continuous countercurrent systems

Fig. 17. The two basic modes of operation for an adsorption process (a) cycHc batch system (b) continuous countercurrent system with adsorbent...

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... 

FIGURE 8 Schematic diagram showing the two basic modes of operating an adsorption separation process (a) cyclic batch two-bed system (b) continuous countercurrent system with adsorbent recirculation. Concentration profiles through the adsorbent bed are indicated. Component A is more strongly adsorbed than B. (Reprinted with permission from Ruthven, D. M. (1984). Principles of Adsorption and Adsorption Processes, copyright John Wiley Sons, New York.)... 

Although the continuous-countercurrent type of operation has found limited application in the removal of gaseous pollutants from process streams (Tor example, the removal of carbon dioxide and sulfur compounds such as hydrogen sulfide and carbonyl sulfide), by far the most common type of operation presently in use is the fixed-bed adsorber. The relatively high cost of continuously transporting solid particles as required in steady-state operations makes fixed-bed adsorption an attractive, economical alternative. If intermittent or batch operation is practical, a simple one-bed system, cycling alternately between the adsorption and regeneration phases, 1 suffice. 

Adsorption Separation and Purification Processes. Adsorption processes can be classified according to the flow system (cyclic batch or continuous countercurrent) and the method by which the adsorbent is regenerated. The Iwo basic flow schemes arc illustrated in Figure 3 The cyclic batch scheme is simpler but less efficient. It is generally used where selectivity is relatively high. Countercurrent or simulated countercurrent schemes arc more expensive in initial cost and arc generally used only for difficult separations in which selectivity is limited or mass-transfer resistance is high. 

Large-scale adsorptive separation processes may be conveniently divided into two broad classes cyclic batch systems, in which the adsorbent bed is alternately saturated and regenerated in a cyclic manner, and continuous flow systems, generally involving continuous countercurrent contact between feed and adsorbent. The distinction between these two basic modes of operation is shown schematically in Figure 11.1. The present chapter is restricted to processes which operate in the cyclic mode while continuous contacting systems are discussed in Chapter 12. 

There is complete formal analogy between continuous countercurrent adsorption and other continuous countercurrent operations such as distillation or gas absorption. Such processes are normally operated at steady state and, for an isothermal system containing only a single adsorbable species, the analysis is straightforward. 

Steady-state operation of a continuous countercurrent adsorption system may be conveniently discussed in terms of the simple McCabe-Thiele analysis. The simplest type of continuous countercurrent process is illustrated in Figure 12.2, and it is evident that any of the separation processes discussed in Chapter 11 could, in principle, be carried out in such a system, rather than in a cyclic batch process. 

FIGURE 12.10. Comparison of continuous countercurrent, periodic countercurrent, and cyclic batch adsorption systems showing effectiveness of adsorbent utilization as a function of bed length. (From ref. 15, reprinted with permission.)... 

Large-scale adsorption processes can be divided into two broad classes. The first and most important is the cyclic batch system, in which the adsorption fixed bed is alternately saturated and then regenerated in a cyclic manner. The second is a continuous flow system which involves a continuous flow of adsorbent countercurrent to a flow of feed. 

Hassan, M.M. Rahman, A.K.M.S., and Loughlin, K.F., Numerical simulation of unsteady continuous countercurrent adsorption system with nonlinear adsorption isotherm, Sep. Technol., 4(1), 15-26 (1994). 

Figure 7.13 Removal of a single component contaminant from a feed continuous countercurrent adsorption system and McCabe-Thiele operating diagram (source Ruthven and Ching 1989).

As an alternative to the continuous countercurrent model an equilibrium stage model of the adsorptive fractionation system exists (Chinget a/. 1985). In effect the concentration ratio cl/co for each column is represented by the well-known equation (Kremser 1930, Souders and Brown 1932) ... 

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. 

Alternatively, one can resort to the use of moving- or fluidized-bed systems in which the adsorbent, and sometimes also the catalyst, is continuously withdrawn from the reactor to undergo an external regeneration. Ideally, one tries to achieve countercurrent adsorbent flow pattern to optimize utilization of the adsorptive capacity. The major problems of such arrangements are those of solids handling (e.g., gas-tight... 

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