Section 5.8 Settlers

Preliminary treatment unit, bar screen, grit chamber, overflow and bypass chamber and Parshall flume, PM cost 270 000 at design flow rate = 58 L/s with n = 0.64 for the range 1-10000 L/s. 

Primary clarifier, cylindrical c/s for above ground installation excluding painting and mechanism. FOB 130000 at a surface area = 100 m with n = 0.46 for the range 50-800. Factors including painting, X 1.2 installed below grade X 1.3-1.5. L-i-M = 3.2-4.3. 

Primary clarifier rectangular basin, concrete excluding the earthwork and mechanism. Installed cost 160000 at surface area = 400 m with n = 0.56 for the range 60-400 and n = 1.45 for the range 400-800. 

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Liquid-Liquid Extraction The actual configuration of mixers in multistage mixer-settlers and/or multistage columns is summarized in Section 15. A general handbook on this subject is Handbook of Solvent Extraction by Lowe, Beard, and Hanson. This handbook gives a comprehensive review of this entire operation as well. 

The use of these settlers is not usually practical for most situations. The diameters or cross-section areas become too large for the handling of anything but the very smallest of flowing vapor streams. In general, gravity settlers of open box or tank design are not economical for particles smaller than 325 mesh or 43p [23]. 

A gravity settler is to be used to separate particles less than 75 qm from a flowrate of gas of 1.6 m3s 1. The density of the particles is 2100 kg-rn 3. The density of the gas is 1.18 kg-m-3 and its viscosity is 1.85x 10 5 kg-rn 1 -s 1 Estimate the dimensions of the settling chamber assuming a rectangular cross section with length to be twice that of the breadth. 

Ignoring the quite distinct functions and hydrodynamic conditions which exist in the actual mixer and settler items of the combined mixer-settler unit, it is possible, in principle, to treat the combined unit simply as a well-mixed equilibrium stage. This is done in exactly the way as considered previously in Sections 3.2.1 to 3.2.6. A schematic representation of an actual mixer-settler device is shown in Fig. 3.38 and an even more simplified representation of the equivalent simple well-mixed stage is given in Fig. 3.39. 

In addition to the rotating columns previously described, there are a number of other designs for centrifugal extractors, many originally developed for the separation of radioactive wastes in nuclear processes (see Chapter 12). They are both of the mixer-settler type, as discussed in section 9.3.3, and of the rotating column types. 

The factor C and the exponent y must be determined by experiments with pilot settlers. These terms combine all the unknown parameters. The cross-sectional area, S, of the settler should be chosen in such a way that the superficial velocity, of the dispersed phase does not exceed 5-9mms . Often aids for coalescing (e.g., packages of inclined plates or filter cartridges made of metal or plastic) (Fig. 9.23) are used. There are several ways of predetermining coalescence [31-33]. Furthermore, in special cases electrical fields and centrifuges are used to support coalescence [34]. 

The mixture leaving the reaction zone is in the form of a hydrocarbon-acid emulsion and passes to an acid settler for separation of acid and hydrocarbon phases. This acid settler is usually a separate vessel from the reactor itself, although it is an integral part of one type of system. The hydrocarbon-free acid from the acid settler recirculates to the reactor. The hydrocarbon layer, which consists of alkylate, excess isobutane, and the inert diluents introduced with the feed, receives a caustic treatment and goes to the fractionating section of the plant. Caustic treatment is necessary at this stage of the process to neutralize acidic components, such as sulfur dioxide, which are formed in small quantities by catalyst degeneration. 

The equations needed to design a vertical separator were presented in the sections on vapor- and liquid-phase separation in gravity settlers. 

The analysis in this section and the next can be also applied for a cell settler as a cell separator. The outlet stream of the cell settler will be equal to F = B + L and its concentration will be (B/F)CXy = /5CSf. 

The centrifugal contactor was first used to reprocess spent nuclear fuel at the SRS in 1966 (Webster et al., 1969). For almost 40 years, this 18-stage 25-cm SRL contactor was used for the extraction and scrub sections (the A-bank) of the PUREX (plutonium-uranium extraction) process at the SRS. Contactor operation stopped when the facility in which they were housed was shut down in 2003. This 18-stage contactor replaced a 24-stage mixer-settler. Mixer-settlers continued to be used for the rest of the processing, as most of the radiation was removed in the A-bank. The ability to... 

Monitoring the separation efficiency is an important way to evaluate the performance of a given separator. The efficiency concepts described in this section apply not only to animal cell separation, but also to the separation of any particles and bioparticles, such as microorganisms, cellular debris, nuclei, etc. These concepts apply to any separation device whose performance remains constant if the operational conditions do not change. This happens, for instance, in gravity settlers, centrifuges, and hydrocyclones. 

Due to the good performance of the single stage unit (see section 3) an extraction plant has been designed and built for pressures up to 30 MPa and temperatures up to 120 °C. The apparatus consists of five mixer-settler modules, as described above, which are composed in counter current flow (fig. 2). 

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