Differential extractors

Differential extractors, in which the phases are continuously in contact in the extractor and are only separated at the exits for example, in packed column extractors. 

FIGURE 13 Differential extractors. Reprinted from Treybal5 McGraw-Hill Companies. 

In continuous differential extractors (columns) it has been convenient to think in terms of a height equivalent to a theoretical stage (HETS), and to correlate HETS as a function of system and equipment variables. Alternately, correlations may be obtained on the basis of the height of a transfer unit (HTU), which is more amenable to calculations which separately include the effects of backmixing.l H ... 

Spray Column. The simplest differential extractor is the spray column (Fig. 18a.), which depends upon the initial dispersion of the dispersed phase to create favorably sized droplets. There is no means provided to redisperse this phase if any coalescence occurs. Although the equipment is simple and inexpensive, it is difificultto obtain more than one stage extraction. The passage of the dispersed phase ind uces considerable backmixing of the continuous phase, particularly in larger diameter columns. 

If a deep bed u used in a reflux extractor the exlractur can be analyzed tike fixed-bed extractors operating with dtiwnftow and yj0 0. In such a case V = Ee. However, if the bed is Hooded and consequently part of the reflux bypasses the bed. V should be based on an Eh that barely results in bed saturation rather than the actual Efe. If the refluxed solvent b not passed throngh a bed but is edded 10 a well-mixed batch of extract and solid and the rate of extract discharge is E . the extractor can be treated like a diflcrtmriel extractor. Reflux extractors in which very short beds are used can also be treated like differential extractors as a mesonablc approximation. 

This slice-wise calculation enables us to directly transpose, to the side of the liquid, the calculation of the differential extractors to express the influence of backmixing on the plate. For the axial dispersion coefficient, we could try a modified expression of that of the A.I.Ch.E journal. 

The differential extractors are also subject to axial mixing (see Chap. 6) it severely reduces extraction rates because of the deterioration of the concentration differences between phases which is the driving force for mass transfer. This is illustrated in Fig. 10.50, where the real (axial-mixing) concentration profiles show a substantially smaller concentration difference than those for plug flow. If the flow ratio of the liquids is not unity, and it very rarely is, dispersing the liquid flowing at the lower rate will lead to small numbers of dispersed-phase drops, small interfacial areas, and small mass-transfer rates. On the other hand, if the majority liquid is dispersed, the axial-mixing problem is exacerbated. The... 

Under Unit Procedure, go to Extraction then Liquid Extraction, select Differential Extractor (Eigure 8.15). 

At the same time, we want to focus on the role of diffusion in extraction, for that is the subject of this book. As a result, we emphasize the case of a dilute solute being extracted between two immiscible liquids. This defers complicated issues of ternary phase equilibria to more specialized texts and lets us focus on the issues of mass transfer, which can be obscured in those texts. Specifically, we discuss extraction equipment in Section 14.2, we analyze differential extractors as a parallel to gas absorption in Section 14.3, and we describe staged extraction in Section 14.4. Leaching, which can be either staged or differential, is treated in Section 14.5. The result is a brief summary that emphasizes the role of mass transfer. 

Differential extractors are much less common than staged extractors. Discuss why. 

Because of this, you plan to use a differential extractor 1.85 m tall. When the hexane solution flows through at 0.16 m/s, you know from the model experiments that the overall mass transfer coefficient KfjU (based on the hexane feed) is 0.14 s. What percent recovery of the flavor do you expect ... 

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