Cross section extractor

To use the flooding point diagram, first it is necessary to decide whether the drops produced in the extractor are circulating or oscillating. The mean diameter di,2 (see Eq. 9.1) is used for the characteristic drop size. If the flow rate ratio is known from the thermodynamic design, the superficial velocities of both phases can be determined at the flooding point. The minimum column cross-sectional area and diameter necessarily follows directly from the superficial velocity at the flooding point with Eq. 9.19. 

Fig. 9.15 Principle of residence time measurement of the continuous heavy phase in a countercurrent extractor. After feeding a short test signal, for example, a colored tracer, the color intensity c or Cq, respectively, is measured downstream at two cross sections ot and ft) as a function of time. The test signal curve widens downstream, owing to the various mixing phenomena.

Equation (9.27) defines the so-called axial dispersion coefficient Dax as a model parameter of mixing. Nd is the dispersion flow rate, c the concentration of the tracer mentioned earlier, and S the cross-sectional area of the column. The complete mole flow rate of the tracer consists of an axial convection flow and the axial dispersion flow. The balance of the tracer amount at a cross section of the extractor leads to second-order partial differential equations for both phase flows at steady state. For example, for continuous liquids ... 

The arrangement used is shown in Fig. 12(a). The horizontal plates are pierced with holes of small diameter (approximately J -in. diameter), whose total cross section may be 20 to 25% that of the column. No downspouts are provided, as for ordinary perforated-plate extractors, and both liquids must pass through the same holes. The plate spacing is usually small, 2 in. in large-diameter columns, for example. Columns of this sort may be pulsed at amplitudes of 1 in. and frequencies up to 150 cycles/min., depending upon the circumstances. 

Holdup and Flooding. The volume fraction of the dispersed phase, commonly known as the holdup h. can be adjusted in a batch extractor by means of the relative volumes of each liquid phase added. However, in a countercurrent column contactor, the holdup of the dispersed phase is considerably less than this, because the dispersed drops travel quite fast through the continuous phase and therefore have a relatively short residence time in the equipment. The holdup is related to the superficial velocities V of each phase, defined as the flow rate per unit cross section of the contactor, and to a slip velocity Us ... 

Equations (7.26) and (7.27) are based on 80% extractor column flood. (Note the 0.8 factor in both equations.) Please note that Vc and VD are the superficial full-column internal diameter velocities. Superficial simply means that you calculate these velocities as though the respective liquid phase were the only material in the full-column cross-sectional area... 

Anderson improved their extractor with the addition of a fines separator on top (195, 196). The fines separator was a bundle of tubes through which the miscella ascended before leaving the extractor. The combined-cross sectional area of all the tubes was greater than the cross-sectional area of the column. This allowed the rate... 

High Cross-flow of the Continuous Phase Miniplant tests of sieve tray extractors are often performed prior to the final design of a commercial-scale column. The design often is scaled up based on superficial velocities of the dispersed and continuous phases calculated from the volumetric flow rates and the column cross-sectional area. However, in scaling up one must be careful about the cross-flow velocity (V eow) of the continuous phase. A value may be estimated from... 

The first HTU term contains the physical and fluid-dynamic parameter and the second NTU term expresses the number of theoretical stages as function of the solute concentration difference. The extractor-specific HTU value is, on the one hand, described by the quotient of flow rate and cross-sectional area of the column, and, on the other hand, it is characterised by the interfacial area per unit volume and the mass transfer coefficient. The former is mainly influenced by drop size and phase hold-up, the latter by the relative movement of the dispersed phase. These characteristic HTU values can be experimentally measured for a certain extractor type and are used for comparison with other extractors or for the projection of larger units. 

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