Decantation, continuous design

The vessels are often referred to as decanters and may operate as a batch or a continuous operation. The emphasis here will be on continuous designs. The basic design approach is as foilows ... 

The cake discharge is an important part of the decanter process design, especially because the ability to separate solids continuously is the prime reason for the existence of the decanter. An inadequate design will reduce process performance. 

Cross-sectional aiea allocated to light phase, sq ft Area of particle projected on plane normal to direction of flow or motion, sq ft Cross-sectional area at top of V essel occupied by continuous hydrocarbon phase, sq ft Actual flow at conditions, cu ft/sec Constant given in table Volume fiaction solids Overall drag coefficient, dimensionless Diameter of vessel, ft See Dp, min Cyclone diameter, ft Cyclone gas exit duct diameter, ft Hy draulic diameter, ft = 4 (flow area for phase in qiiestion/wetted perimeter) also, D in decanter design represents diameter for heavy phase, ft... 

Decanters are normally designed for continuous operation, but the same design principles will apply to batch operated units. A great variety of vessel shapes is used for decanters, but for most applications a cylindrical vessel will be suitable, and will be the cheapest shape. Typical designs are shown in Figures 10.38 and 10.39. The position of the interface can be controlled, with or without the use of instruments, by use of a syphon take-off for the heavy liquid, Figure 10.38. 

Separation Techniques for Chemical Engineers, 3d ed., Schweitzer, ed. (McGraw-Hill, 1997)] and Jacobs and Penney [Chap. 3 in Handbook of Separation Process Technology, Rousseau, ed. (Wiley, 1987)] for design of continuous decanters. It is based on the dispersed-phase data of Selker and Sleicher [Can. J. Chem. Eng., 43, pp. 298-301 (1965)]. 

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... 

Without using any capability for mixing, the process designer will strive to make control of the temperature in the decanter as continuously close to a laboratory-determined goal as possible. Temperature variation from goal means that the integrity of phase separation will vary from goal as well. ... 

Design Scope. Stirred vessels, rotor-stator mixers, static mixers, decanters, settlers, centrifuges, homogenizers, extraction colunms, and electrostatic coalescers are examples of industrial process equipment used to contact liquid-liquid systems. Although this chapter emphasizes stirred vessels, the fundamentals of phase behavior are applicable to a broad range of other equipment types. Immiscible liquid-liquid systems are processed in batch, continuous, and semicontinuous modes. 

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