Equipment extraction

Extraction equipment can be divided into two broad groups  

Stage-wise extractors, in which the liquids are alternately contacted (mixed) and then separated, in a series of stages. The mixer-settler contactor, is an example of this type. Several mixer-settlers are often used in series to increase the effectiveness of the extraction. 

Differential extractors, in which the phases are continuously in contact in the extractor and are only separated at the exits for example, in packed column extractors. 

Extraction columns can be further sub-divided according to the method used to promote 

Various types of proprietary centrifugal extractors are also used. 

Extraction columns can be further sub-divided according to the method used to promote contact between the phases packed, plate, mechanically agitated, or pulsed columns. Various types of proprietary centrifugal extractors are also used. 

The following factors need to be taken into consideration when selecting an extractor for a particular application  

The chemistry of extraction frequently involves nonlinear equihbria between the raffinate and extract. This reflects the extraction chemistry, which is often more com-phcated than that for absorption or dilute distillation. The nonlinear equilibria frequently result from specific chemical reactions. For example, benzoic acid can be extracted from water into benzene. The benzoic acid in water can ionize to form a mixture of benzoic acid, benzoate anions, and protons. Benzoic acid in benzene can dimerize. The ionization and dimerization may result in an equihbrium that is nonlinear and strongly dependent on concentration, pH, and temperature. 

In a similar way, much metal purification involves ion exchange. For example, in the case of copper extraction mentioned above, the equihbrium is  

In spite of these complexities, most analyses of extraction assume linear equihbria. The detailed chemistry appears as a concentration-dependent partition coefficient. While such concentration-dependent partition coefficients are beyond the scope of this book, they are well understood and discussed in detail in more speciahzed references. 

Liquid liquid extraction and leaching use different equipment, even though the analysis of this equipment can be similar. Liquid-liquid extraction can be accomplished either in differential contactors or in staged extractors (Godfrey and Slater, 1995). The differential contactors are analyzed in ways that parallel the analysis of gas absorption the staged extractors depend heavily on ideas developed for distillation. In both cases, an enormous variety of equipment is used, with specific apparatus often being optimized for particular separations. 

Both spray columns and packed columns are seriously compromised by flooding. In flooding, the feed and solvent streams do not flow evenly and countercurrently past each other, but both simply gush out one end of the column. Flooding is a more serious risk in extraction than in absorption because of the smaller density difference between the two fluids. This density difference is typically less than 0.1 g/cm, about 10 times less than that common in gas absorption. As a result, countercurrent flows that are routine in gas absorption will be difficult to realize in liquid-liquid extraction. 

As in gas absorption and distillation, liquid extraction requires that two phases be brought into intimate contact with each other to ensure transfer of the solute from the diluent to the solvent, and then separated. In absorption and distillation there are two phases (liquid and vapor) with signiticantly different densities. This is not the case with extraction where both solvent and diluent solutions are liquids, often with similar densities. Because of this and the immiscibility requirement, the two liquids are often difficult to mix and even more difficult to separate. In addition, liquids have sufficiently high viscosities that they require pumps to maintain flow. Most extraction processes include mechanical energy for pumping, mixing, and separating the liquids. 

Regardless of which phase is dispersed there is a continuous transfer of solute from the diluent to the solvent. Equilibrium is never completely reached at each point in the column. Rather, the difference between equilibrium and operating conditions governs the driving force for mass transfer. 

In spray towers the highest rates of mass transfer tend to occur close to the distributor plates. At small distances above and below the plate the dispersed phase tends to recoalesce and mass transfer declines significantly, such that it is more effective to add distributors to redisperse the droplets than to increase the height of the tower. Another solution is to add packing similar to that used in absorption towers. The packing causes the drops to coalesce and reform and, thus, reduces the height of each theoretical transfer unit. 

It is important to note that neither group is dimensionless, so proper units must be used. 

The y-axis can be estimated from physical properties and the corresponding x-axis found. Then, the required dispersed or continuous solvent phase velocity can be found as a function of a specified diluent flowrate. If the diluent is the dispersed phase, the continuous solvent flowrate is found and likewise if the diluent is the continuous phase, the dispersed solvent flowrate is determined. Obviously, there will be some variation with column diameter and the best design is most easily determined with a spreadsheet 

Extractor equipment considerations are discussed here in the context of their effect on the process performance and not for the purpose of describing detailed design. The main parameter of interest at this point is the number of equilibrium stages that represent the process. Liquid-liquid extraction requires thorough mixing of two liquid phases to achieve thermodynamic equilibrium, followed by complete separation of the phases. The particular equipment selected for a given process is determined, in part, by the mixing and separation characteristics of the phases. 

In one arrangement, each stage of the extraction process consists of a mixing 

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Drop Diameter. In extraction equipment, drops are initially formed at distributor no22les in some types of plate column the drops are repeatedly formed at the perforations on each plate. Under such conditions, the diameter is determined primarily by the balance between interfacial forces and buoyancy forces at the orifice or perforation. For an ideal drop detaching as a hemisphere from a circular orifice of diameter and then becoming spherical ... 

The earliest large-scale continuous industrial extraction equipment consisted of mixer—settlers and open-spray columns. The vertical stacking of a series of mixer—settlers was a feature of a patented column in 1935 (96) in which countercurrent flow occurred because of density difference between the phases, avoiding the necessity for interstage pumping. This was a precursor of the agitated column contactors which have been developed and commercialized since the late 1940s. There are several texts (1,2,6,97—98) and reviews (99—100) available that describe the various types of extractors. 

Slater, Rate Coefficients in Liquid-Liquid Extraction Systems in Godfrey and Slater, Liquid-Liquid Extraction Equipment, Wiley, 1994, pp. 45-94. 

The other common objective for calculating the number of countercurrent theoretical stages (or mass-transfer units) is to evaluate the performance of hquid-liquid extraction test equipment in a pilot plant or to evaluate production equipment in an industrial plant. Most liq-uid-hquid extraction equipment in common use can oe designed to achieve the equivalent of 1 to 8 theoretical countercurrent stages, with some designed to achieve 10 to 12 stages. 

The mass transfer in extraction equipment using mixers requires careful study before scale-up. 

The fields of application of the various types of extraction equipment are also well summarised in Volume 2, Chapter 13. The basic principles of liquid-liquid extraction are covered in several specialist texts Treybal (1980), Robbins (1997), and Humphrey and Keller (1997). 

As with sampling, laboratory extraction equipment must be clean. In addition, extraction equipment must be compatible with the analyte of interest. It also must not add any of the analyte or an interfering analyte to the sample during extraction. This is particularly important because of the low levels of analyte being determined and because most laboratories will not have new, disposable equipment. 

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