Countercurrent system

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

Extraction. Traditionally tea leaf is extracted with hot water either in columns or ketdes (88,89), although continuous Hquid soHd-type extractors have also been employed. To maintain a relatively low water-to-leaf ratio and achieve full extraction (35—45%), a countercurrent system is commonly used. The volatile aroma components are vacuum-stripped from the extract (90) or steam-distilled from the leaf before extraction (91). The diluted aroma (volatile constituents) is typically concentrated by distillation and retained for davoring products. Technology has been developed to employ enzymatic treatments prior to extraction to increase the yield of soHds (92) and induce cold water solubiUty (93,94). 

Displacement-purge forms the basis for most simulated continuous countercurrent systems (see hereafter) such as the UOP Sorbex processes. UOP has licensed close to one hundred Sorbex units for its family of processes Parex to separate p-xylene from C3 aromatics, Molex tor /i-paraffin from branched and cyclic hydrocarbons, Olex for olefins from paraffin, Sarex for fruc tose from dextrose plus polysaccharides, Cymex forp- or m-cymene from cymene isomers, and Cresex for p- or m-cresol from cresol isomers. Toray Industries Aromax process is another for the production of p-xylene [Otani, Chem. Eng., 80(9), 106-107, (1973)]. Illinois Water Treatment [Making Wave.s in Liquid Processing, Illinois Water Treatment Company, IWT Adsep System, Rockford, IL, 6(1), (1984)] and Mitsubishi [Ishikawa, Tanabe, and Usui, U.S. Patent 4,182,633 (1980)] have also commercialized displacement-purge processes for the separation of fructose from dextrose. 

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... 

Disadvantages of these continuous countercurrent systems are associated primarily with the complexity of the equipment required and with the attrition resulting from the transpoiT of the ion exchanger. An effective alternative for intermediate scale processes is the use of merry-go-round systems and SMB units employing only packed-beds with no movement of the ion-exchanger. 

Fast Rinse Cycle - The fast rinse step is essentially the service cycle except that the effluent is diverted to waste until quality is proven. This final rinse is always in the same direction as the service flow. Therefore, in countercurrent systems the displacement flow and rinse flows will be in opposite directions. 

The modeling of SMB can be addressed by simulating the system directly, taking into account its intermittent behavior, or by representing its operation in terms of a true countercurrent system. The first model represents the real SMB and considers the periodic switch of the injection and collection points. The second is developed assuming the equivalence with the TMB, where solid and fluid flow in opposite directions. 

In practice, a small number of beds in series in each zone provide a close approach to the performance of ideal, true countercurrent system. Industrial SMB systems normally use 2 to 3 beds per zone. 

In a simple countercurrent system, the solid is contained in a number of tanks and the solvent flows through each in turn. The first vessel contains solid which is almost completely extracted and the last contains fresh solid. After some time, the first tank is disconnected and a fresh charge is introduced at the far end of the battery. The solvent may flow by gravity or be fed by positive pressure, and is generally heated before it enters each tank. The system is unsatisfactory in that it involves frequent interruption while the tanks are recharged, and countercurrent flow is not obtained within the units themselves. 

In the systems considered so far, the quantity of solvent, or of solution, removed in association with the insoluble solids has been assumed to be constant and independent of the concentration of solution in the thickener. A similar countercurrent system is now considered in which the amount of solvent or solution in the underflow is a function of the concentration of the solution. This treatment which is equally applicable to the washing thickeners alone, or to the whole system involving agitator and thickeners, is attributable to Ruth 7 ... 

Considering a countercurrent system consisting of n thickeners, as shown in Figure 10.19, the net flow to the right must necessarily be constant throughout the system, if no material enters or leaves at intermediate points. 

Penicillin acylase (immobilized) Penicillin G hydrolysis in countercurrent system, in water (low pH, 4-5.6)/butyl acetate [73]... 

The third-power TBP dependence has been verified by Peppard et al. [55, 56] for lower Z rare earths and toluene diluent, and for Y and Ce by Scargill and his coworkers [57—59]. However, there is a lack of direct evidence for the third-power nitrate dependence. In a nitrate medium with TBP, using a countercurrent system Weaver, Kappel-mann and Topp [60] were able to prepare 95 per cent pure Gd in kilogram quantities. Additional equilibrium data on rare earth nitrate-TBP system has recently appeared [61]. 

Dynamic programming in countercurrent systems (with D. Yesberg). J. Math. Anal. Appl. 7,421-424 (1963). 

Martin and Porter (19) described a partition chromatographic procedure and first demonstrated the presence of at least one minor active component in the crystalline enzyme preparation. King and Craig (20) found a solvent system permitting effective countercurrent distribution of ribonuclease, ethanol water ammonium sulfate in the ratios 25.9 -57.6 16.5. The principal component of the Kunitz preparation behaved as an almost ideal solute with a partition ratio of 0.8. Albertsson has provided a liquid polymer countercurrent system based on dextrari and methyl cellulose (21). 

Let us examine the equilibrium curve in somewhat more detail. The countercurrent system defined in Figure 5.6 is restated in Figure 5.12 in a slightly more simplified form to illustrate some important features on the enthalpy-temperature plot. In this figure, T denotes air temperature and t water temperature. The following curves are of importance ... 

Therefore, further reduction of adsorbent usage can be realized with the countercurrent system. 

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.)... 

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