Multistage countercurrent contacting

Equipment in this category is usually arranged for multistage countercurrent contact of the insoluble hquids, without repeated complete separation of the hquids from each other between stages or their equivalent. Instead, the liquids remain in continuous contact throughout their passage through the equipment. 

In most industrial applications, multistage countercurrent contacting is required. The hydrodynamic driving force necessary to induce countercurrent flow and subsequent phase separation may be derived from the differential effects of either gravity or centrifugal force on the two phases of different densities. Essentially there are two types of design by which effective multistage operation may be obtained ... 

Multistage countercurrent contacting is the most effective mode for carrying out separation processes. It reduces the amount of solvent and makes possible the continuous production of extract. Real countercurrent contact is not easily established for solids, because special effort is necessary for moving the solid, with increased difficulties at elevated pressure. Therefore, it is easier not to move the solid material and to achieve countercurrent contact by other measures. For SFE from solids, a well known and often used configuration is to have several fixed beds in countercurrent contact with the solvent. 

For an earlier introduction to column crystallization vis-a-vis the variety of equipment and patents, see Albertins et al. (1967). Ulrich (1993) provides a more modern version of melt crystallization devices, plants and processes. For mathematical modeling and optimization of multistage crystallization processes, see Gilbert (1991). A more Involved model of continuous countercurrent contacting with axial dispersion in melt crystallization has been illustrated in Hemy and Moyers (1984). 

The need for a continuous countercurrent process arises because the selectivity of available adsorbents in a number of commercially important separations is not high. In the -xylene system, for instance, if the Hquid around the adsorbent particles contains 1% -xylene, the Hquid in the pores contains about 2% xylene at equiHbrium. Therefore, one stage of contacting cannot provide a good separation, and multistage contacting must be provided in the same way that multiple trays are required in fractionating materials with relatively low volatiHties. 

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. 

In leaching processes, finely divided solids are contacted with solvents to remove soluble constituents. Usually some kind of multistage and countercurrent operation is desirable. The most bothersome aspect is handling of the wet solids. 

Many chemical and biological systems include multistage processes rather than only continuous contact ones. The most common multistage systems are absorption and distillation columns. Most of these systems involve more than one phase and they therefore fall under the category of heterogeneous multistage systems. Multistage systems can be cocurrent or countercurrent. 

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