Extractor cascade

Dual solvent fractional extraction (Fig. 7b) makes use of the selectivity of two solvents (A and B) with respect to consolute components C and D, as defined in equation 7. The two solvents enter the extractor at opposite ends of the cascade and the two consolute components enter at some point within the cascade. Solvent recovery is usually an important feature of dual solvent fractional extraction and provision may also be made for reflux of part of the product streams containing C or D. Simplified graphical and analytical procedures for calculation of stages for dual solvent extraction are available (5) for the cases where is constant and the two solvents A and B are not significantly miscible. In general, the accurate calculation of stages is time-consuming (28) but a computer technique has been developed (56). 

The modelling approach to multistage countercurrent equilibrium extraction cascades, based on a mass transfer rate term as shown in Sec. 1.4, can therefore usefully be applied to such types of extractor column. The magnitude of the... 

This model is described in Sec. 3.3.1.5 and consists of a countercurrent stagewise extractor cascade with backmixing in both phases. 

The modelling approach to multistage countercurrent equilibrium extraction cascades, based on a mass transfer rate term as shown in Section 1.4, can therefore usefully be applied to such types of extractor column. The magnitude of the mass transfer capacity coefficient term, now used in the model equations, must however be a realistic value corresponding to the hydrodynamic conditions, actually existing within the column and, of course, will be substantially less than that leading to an equilibrium condition. 

These plants, with the greatest number of industrial application so far, are used mainly for the extraction of plant materials such as spices and herbs. The extraction volumes are in the range of 200 to 800 1 in which, in most cases, cascade-mode operation with 2 to 4 extractors is applied. The design pressure-range is up to 550 (800) bar. Fig. 8.1-2 illustrates a multipurpose extraction unit for spices and herbs. 

A comparison for the different cases of the production costs and dependence on the annual capacity is given in Fig. 8.1-4. The results are based on the cascade-operation mode, with three extractors, extraction at 280 bar and 65°C, cycle-times of 7.5 hours, and a separation pressure of 60 bar for the non-isobaric process. 

In our case, the 2 x 300 1 cascade could be enlarged with one additional extractor, thereby increasing the capacity by 50%, or the single mode (lx 600 1) capacity could be doubled or tripled by adding one or two extractor units. In Fig. 8.1-6 the effect on production costs of adding two extractors is projected, while Table 8.1-2 informs about the other possibilities. 

As a result of this investigation it can be mentioned that changing the single-mode to a cascade of two extractors reduces the production costs remarkably, down to about 54%. Further enlargement to three extractors reduces them to about 38%, always compared with the single-mode costs. Enlarging a cascade from two- to three extractors reduces production costs by about 30%. 

Extraction involves the transfer of components between two liquid phases, much as absorption or stripping involves the transfer of components from liquid to vapor phase or vice versa. As in vapor-liquid multistage separation processes, the device employed to carry out liquid-liquid extraction is usually a counterflow column that performs the function of a number of equilibrium stages interconnected in counterflow configuration. In each stage, two inlet liquid streams mix, reach equilibrium, and separate into two outlet liquid streams. As in vapor-liquid columns, the lack of complete equilibrium in liquid-liquid extractors is accounted for by some form of tray efficiency. Liquid-liquid extraction may also be carried out in a cascade of mixing vessels connected in series in counterflow. 

There should be a proportional relationship between the primary and secondary loop. For example, when the primary loop is linear (such as with temperature or pressure), the secondary loop should also be linear. This is true with the TRC/ PRC and the TRC/TRC shown in Figures 10-3 and 10-4. However, when flow is selected as the secondary loop, it has a square-root scale which must be linearized with a square-root extractor. Therefore, the TRC/FRC cascade loop shown in Figure 10-2 requires special square root extractor instrumentation. 

Output variables for this specification include missing mole fractions for Vm and Lin, stage temperatures, and the variables associated with the Vout stream. Loot stream, and interstage streams. The results obtained in this example are included in Table 6.1. The N-stage cascade unit can represent simple absorbers, strippers, or liquid-liquid extractors. 

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