Countercurrent extraction dilute systems

Often solvent extraction is carried out continuously in a countercurrent multistage device/cascade. The sieve-plate tower is an example of one multistage device, whereas Figure 8.1.37(a) illustrates a multistage arrangement of N mixer-settler devices (one such device is studied in Section 6.4.1.2). First, we analyze a dilute solution of solute i in a feed-extract phase system assumed to be essentially insoluble in each other (Cussler, 1997). Then we will analyze extraction systems with some mutual solubility of the feed and the extraction solvent. 

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). 

Numerous reports have also been published on the separation of the individual alkaloids by using ion exchangers (301, 302), various solvents of different pH (303), or multibuffered extraction systems (102, 103, 304-306). The isolation of minor bases from extracts of P. somniferum (254,282, 307-311) and of Fumaria officinalis (166) has been reported. The separation of the bases obtained from this plant material was also carried out by countercurrent distribution (312) and column chromatography (313, 314). The alkaloid coreximine was detected in P. somniferum by an isotope dilution method which was based on the biosynthesis of this alkaloid from reticuline (249). The isoquinoline type of quaternary alkaloids can be... 

We will now consider a dilute solution of solute i in a countercurrent continuous-contact solvent extraction system consisting of feed and extract phases that are essentially insoluble in each other (Figure 8.1.36). We can now ignore the variation in the flow rate of either phase along the length of the counterflowing extraction column. The situation is similar to that of a steady state gas absorption system for a dilute species in a gas stream. The general... 

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