Staged extraction columns

A wide variety of extraction column forms are used in solvent extraction applications. Many of these, such as rotary-disc contactors (RDC), Oldshue-Rushton columns, and sieve-plate column extractors, have rather distinct compartments and a geometry which lends itself to an analysis of column performance in terms of a stagewise model. As the compositions of the phases do not come to equilibrium at any stage, however, the behaviour of the column is therefore basically differential in nature. 

At the prevailing high levels of dispersion normally encountered in such types of extraction columns, the behaviour of these essentially differential type contactors, however, can be represented by the use of a non-equilibrium stage-wise model. 

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. 

In Fig. 3.47 the column contactor is represented by a series of N non-equilibrium stages, each of which is of height H and volume V. The effective column height, Z, is thus given by Z = N H. 

The stagewise model with backmixing is an essential component of any model representation of a stagewise extraction column. As shown in Section 3.3.1.5 the non-ideal flow behaviour is represented by the presence of the N stages 

The modelling approach to multistage countercurrent equilibrium extraction cascades, based on a mass transfer rate term as shown in Sec. 1.5, can therefore usefully be applied to such types of extractor column. The magnitude of the 

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Figure 3.52. Model representation of a non-equilibrium staged extraction column.

A five-stage extraction column with control on the outlet raffinate phase is to be simulated. The solute balance equations for each phase are formulated according to Sec. 3.3.1.5. 

Figure 5.185. Five stage extraction column with control.

Fig. 3.46 Computer simulation output for a five-stage mixer-settler cascade with entrainment. 3.3.1.10 Staged Extraction Columns...

In this example of a five stage extraction column with backmixing, proportional plus integral control of the exit raffinate concentration is to be achieved by regulating solvent flowrate. 

A 15-stage extraction column similar to the one in Example 2.22 is set up to separate acetone and ethanol using two solvents, pure water and pure chloroform. The only added features to Fig. E2.22 in Example 2.22 are (1) that you must consider two solutes in each solvent instead of one solute in the two solvents and (2) the acetone-ethanol feed is introduced into stage 6. You know that the feed composition is 50 mol % ethanol and 50 mol % acetone and that the feed rate is 20 lb mol/hr. Assume that the water and chloroform are not soluble in each other. You want to have 1.50 mol % ethanol in the exit stream from the bottom of the column, and 2 X 10 mol % acetone in the exit stream from the top of the column. Determine the rates of flow of the two solvents. In each stage you are given a relationship between Xf and Xf (/ = 1, 2, 3) in the two phases in terms of mole fractions ... 

The distillate D has a methanol composition (28mol% methanol) that is near the azeotrope at 4 bar. It is fed at a rate of 1122kmoiyh to Stage 6 of a 12-stage extraction column. Water is fed on the top tray at a rate of 1050kmol/h and a temperature of 322 K, which is achieved by using a cooler (heat removal 1.24 MW). The column is a simple stripper with no reflux. The column operates at 2.5 atm so that cooling water can be used in the condenser (reflux drum temperature is 326 K). Reboiler heat input is 5.96 MW. The overhead vapor is condensed and is the C5 product stream. 

One of the components, A (not necessarily the most volatile species of the original mixture), is withdrawn as an essentially pure distillate stream. Because the solvent is nonvolatile, at most a few stages above the solvent-feed stage are sufficient to rectify the solvent from the distillate. The bottoms product, consisting of B and the solvent, is sent to the recoveiy column. The distillate from the recoveiy column is pure B, and the solvent-bottoms product is recycled back to the extractive column. 

Extractors with mechanical agitation, such as mixer-settlers, Kuhni columns, York-Schiebel columns, etc., should be avoided as much as possible. Up to seven theoretical stages packed extraction columns can be conveniently adopted. Sieve-plate extractors can be used up to 20 stages. When a very efficient extraction has to be carried out with expensive solutes, and for reasons of material stability and requirements of low expensive product inventory, we may have to use centrifugal extractors or hollow-fibre extractors. 

Figure 3.41. Concentrations in the solvent phases of stages 1, 2 and 3 in a countercurrent extraction column with slow chemical reaction.

In extraction column design, the model equations are normally expressed in terms of superficial phase velocities, L and G, based on unit cross-sectional area. The volume of any stage in the column is then A H, where A is the cross-sectional area of the column. Thus the volume occupied by the total dispersed phase is h A H, where h is the fractional holdup of dispersed phase, i.e., the droplet volume in the stage, divided by the total volume of the stage and the volume occupied by the continuous phase, in the stage, is (1-h) A H. 

Under changing flow conditions, it can be important to include some consideration of the hydrodynamic changes within the column (Fig. 3.53), as manifested by changes in the fractional dispersed phase holdup, h , and the phase flow rates, Ln and G . which, under dynamic conditions, can vary from stage to stage. Such variations can have a considerable effect on the overall dynamic characteristics of an extraction column, since variations in hn also... 

Figure 5.195. Flows of a single stage agitated extraction column.

Nonlinear programming Staged-Distillation column (12.1) < Liquid extraction column (12.2) Gas transmission network (13.4) Ammonia reactor (14.2) Alkylation reactor (14.3) CVD reactor (14.5) Refrigeration process (15.2) Extractive distillation (15.3) Operating margin (15.4) Reactor control (16.3)... 

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