Dispersion zone settling

Decanters are used to separate liquids where there is a sufficient difference in density between the liquids for the droplets to settle readily. Decanters are essentially tanks which give sufficient residence time for the droplets of the dispersed phase to rise (or settle) to the interface between the phases and coalesce. In an operating decanter there will be three distinct zones or bands clear heavy liquid separating dispersed liquid (the dispersion zone) and clear light liquid. 

FIG. 15-63 Batch settling profile showing four regions a top clarified phase, a sedimentation zone, a dense-packed dispersion zone, and a bottom clarified phase. [Reprinted from Jeelani, Fanoussopoulos, and Hartland, Ind. Eng. Chem. Res., 38(2f pp. 493-501 (1999), with permission. Copyright 1999 American Chemical Society. ] Consult the original article for a detailed description. 

As noted, the weir crest is calculated on an equivalent clear-liquid basis. A more realistic approach is to recognize that in general a froth or spray flows over the outlet weir (settling can occur upstream of the weir if a large calming zone with no dispersers is used). Bennett et al. [AIChE J., 29, 434 (1983)] allowed for froth overflow in a comprehensive study of pressure drop across sieve plates their correlation for residual pressure drop /jf in Eq. (14-87) is represented by Eqs. (14-115) through (14-120) ... 

Adequate dispersion must be obtained for the measurement to be successful because the technique is based upon the Brownian motion of the particles in a liquid medium. If the particles should flocculate in the liquid, gravitational settling will occur, thereby removing the particles from the measurement zone in the sample cell. For this measurement to be successful, the refractive index of the material, both the real and the imaginary component, must be known. In the particle size regime where PCS can be employed, the refractive index has a very significant effect on the measured average particle size. 

Steady-state control of a continuously fed extraction column requires maintenance of the location of the liquid-liquid interface at one end of the column. The main interface will appear at the top of the column when the light phase is dispersed and at the bottom of the column when the heavy phase is dispersed. If needed, extraction columns can be designed with an expanded-diameter settling zone to facilitate liquid-liquid phase separation by reducing liquid velocities. If sufficient clarification of the phases cannot be achieved, then it may be necessary to add an external device such as a gravity decanter or centrifuge. (See Liquid-Liquid Phase Separation Equipment. ) Sometimes a column is built with expanded ends at... 

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