Countercurrent extractors

The vegetable-tanning materials are commercially extracted using hot water. The extraction is normally done in countercurrent extractors that permit the final removal of the extracts with fresh water. The dilute extracts are then evaporated to the desired concentration in multiple effect evaporators. Some extracts may be further dried by spray drying or any other means that proves effective without overheating the extract. Extract preparation depends on the type of extract, the si2e of the operation, and the desired concentration of the final product. 

The value of K is one of the main parameters used to establish the minimum ratio of extraction solvent to feed solvent that can be employed in an extraction process. For exanmle, if the partition ratio K is 4, then a countercurrent extractor woula require 0.25 kg or more of extraction-solvent flow to remove all the solute from 1 kg of feed-solvent flow. 

Prepared cottonseed meats containing 35 per cent of extractable oil are fed to a continuous countercurrent extractor of the intermittent drainage type using hexane as the solvent. The extractor consists of ten sections and the section efficiency is 50 per cent. The entrainment, assumed constant, is 1 kg solution/kg solids. What will be the oil concentration in the outflowing solvent if the extractable oil content in the meats is to be reduced by 0.5 per cent by mass ... 

The capacity of a column operating as a countercurrent extractor, as shown in Figure 13.20, may be derived as follows. 

Fig. 9.13 Flooding point diagram of countercurrent extractors. The drawn lines for six different Ar values are valid for circulating drops. The dotted lines for three different values are valid for oscillating drops. (From Ref. 2.)... 

Fig. 9.15 Principle of residence time measurement of the continuous heavy phase in a countercurrent extractor. After feeding a short test signal, for example, a colored tracer, the color intensity c or Cq, respectively, is measured downstream at two cross sections ot and ft) as a function of time. The test signal curve widens downstream, owing to the various mixing phenomena.

Only a careful analysis of the differences that appear in construction, in the processes of flow and mass transfer and measuring and observing the behavior and the performance of large-scale extractors, can answer these questions. This provides formula of experience for the factors of scale-up that have been listed [4] for the countercurrent extractors described here. 

In this process developed by Lurgi [17], the phenolic effluent is contacted with the solvent in a multistage mixer-settler countercurrent extractor (Fig. 10.8). The extract, containing phenol, is separated into phenol and solvent by distillation and solvent is recycled to the extractor. The aqueous raffinate phase is stripped from solvent with gas, and the solvent is recovered from the stripping gas by washing with crude phenol and passed to the extract distillation column. 

As explained in the Introduction, conventional countercurrent extractors are frequently ineffectual because of the small difference in specific gravity which exists in most two-liquid systems. The centrifugal extractors overcome this problem by increasing the effective specific gravity difference with the help of centrifugal force. There have been many designs proposed, but only those which have had important commercial acceptance will be mentioned here. 

Such an assembly of mixing and separating equipment is represented in Figure 14.3(a), and more schematically in Figure 14.3(b). In the laboratory, the performance of a continuous countercurrent extractor can be simulated with a series of batch operations in separatory funnels, as in Figure 14.3(c). As the number of operations increases horizontally, the terminal concentrations E1 and R3 approach asymptotically those obtained in continuous equipment. Various kinds of more sophisticated continuous equipment also are widely used in laboratories some are described by Lo et at. (1983, pp. 497-506). Laboratory work is of particular importance for complex mixtures whose equilibrium relations are not known and for which stage requirements cannot be calculated. 

Axial Dispersion. Elementary texts assume that all the fluid in each phase has the same resident time in a countercurrent extractor. In practice. 

The variety of extractors used in liquid-solid extraction is diverse, ranging from batchwise dump or heap leaching for the extraction of low grade ores to continuous countercurrent extractors to extract materials such as oilseeds and sugar beets where problems of solids transport have dominated equipment and development. 

For recovery, the broth is clarified by means of rotary vacuum filters. The penicillin, being acidic, is extracted from the aqueous phase into a solvent, such as methyl isobutyl ketone or amyl acetate, at a pH of 2.5 by means of a continuous countercurrent extractor, such as a Podbielniak. The penicillin is then reextracted with an aqueous alkaline solution or a buffer at a pH of 6.5-7.0. About 90 percent recovery yield is typical at this step. The... 

Unique Reaction Sequence Advantageous methods have been developed to improve esterification processes. In an early development, Glossop (81) reported the removal of fatty acids from high acid oils by washing with methanol or another solvent that was not miscible with oil in a countercurrent extractor. After washing, the oil phase had low free fatty acids and was suitable for base trans-esterification. The methanol phase was treated with acid to esterify the fatty acids. 

Flooding Phenomena The hydraulic capacity of a countercurrent extractor is constrained by breakthrough of one liquid phase into the discharge stream of the other, a condition calXeA flooding. The point at which an extractor floods is a function of the design of the internals (as this affects the pressure drop and holdup characteristics of the extractor), the solvent-to-feed ratio and physical properties (as... 

Flooding Velocity Maximum flow through a countercurrent extractor is limited by the flooding velocity. See Hydrodynamics of Column Extractors lor a general discussion of flooding mechanisms. Because of the many possible causes of flooding, published data and... 

A countercurrent extractor is used to lower the concentration of component f in a feed stream and produce a raffinate containing 12% mole component E. The compositions ofthe feed and of the solvent used for the extraction are given below ... 

Ethyl alcohol in water solution is to be treated with supercritical carbon dioxide solvent to lower by extraction the alcohol concentration in the water. The process is carried out in a six-stage countercurrent extractor. The stream data are as follows ... 

A multi-component countercurrent extractor model may be delined for the shortcut column section method in a manner very similar to the vapor-liquid model. The extractor model is developed for liquid-liquid equilibrium stages leading to component flow relationships comparable to the Kremser equation ... 

A schematic diagram of the Solexol processing of fish oils is shown in figure 7.4. The fish oils are fed to a train of countercurrent extractors where column-to-column variations in temperature as well as temperature variations within each specific column can affect the solvent power of propane in a manner similar to that seen with propane deasphalting. 

A true continuous extraction requires that fresh solvent continuously flow through the sample and that fresh sample be continuously flowing in the opposite direction. If the system is designed properly, the compound of interest will be completely extracted from the sample just as the sample reaches the end of the apparatus, and the extracting solvent will be saturated with the extracted compound when it reaches the end of the apparatus. Several such continuous countercurrent extractors are shown in Figure 10-2. 

Figure 10-3. Continuous countercurrent extractors - mechanically agitated. (Courtesy - R.E. Kirk and D.F. Othmer, Encyclopedia of Chemical Technology, John Wiley Sons, New York, NY, 1980)...

FIGURE 10.3 11 Countercurrent extractor for gelatin extraction. (Reproduced by permission. Chamcd engineering Progress, American Institute of Chemical Engineers.)... 

Y for extract entering fixed-bed or continuous countercurrent extractor (kg/m3)... 

F, p. plachco and l. H. Krasuk. Solid-Liquid Countercurrent Extractors, Ind. Eng. Chem. 

A waste solution containing nitrate salts and traces of uranium is to be purified by contacting it with initially pure ether in a countercurrent extractor. The flow rates of aqueous feed and ether are adjusted such that their ratio FfE is equal to the distribution coefficient D for uranium. 

Example 20.2. Oil is to be extracted from meal by means of benzene using a continuous countercurrent extractor. The unit is to treat 1000 kg of meal (based on completely exhausted solid) per hour. The untreated meal contains 400 kg of oil and 25 kg of benzene. The fresh solvent mixture contains 10 kg of oil and 655 kg of benzene. The exhausted solids are to contain 60 kg of unextracted oil. Experiments carried out under conditions identical with those of the projected battery show that the solution retained depends on the concentration of the solution, as shown in Table... 

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