Use of Centrifugation

The objectives of centrifugation are similar to (hose of o(her dewatering devices. It is necessary to obtain a dry cake, a clear centrate, and a reasonable throughput or, in the language of filtration, a centrifuge yield. 

The variables involved in centrifugation are listed in Table 3. These may be classified as machine variables or operational variables, as before. 

Increasing the bowl length will also increase (he residence time which will generally result in a high solids recovery, but (he changes in cake dryness are not always predictable. It is also possible under some circumstances, to also increase the cake solids by increasing the bowl length. 

The pool depfti is a variable that can be varied while the machine is in operation. Some machines require the removal of plugs at the end of the bowl to increase or decrease the pool depth. An increase in pool depth will result in higher retention time, hence better solids recovery and a wetter cake. 

Increasing (he conveyor speed will force die solids out of the machine more quickly, thus leaving behind some of die wetter solids and increasing the solids concentration in the cake while decreasing solids recovery. Similarly, (he conveyor pitch will influence solids recovery and cake dryness. If die scroll pitch is increased, the solids will be moved out faster but only (he larger, heavier solids will he pushed out leaving the wetter solids in (he centrate. Increasing (he number of leads in (he conveyor will likewise increase cake dryness at (he expense of solids recovery. 

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Centrifugal separators make use of the common principle that an object whirled about an axis at a constant radial distance from the point is acted on by a force. Use of centrifugal forces increases the force acting on the particles. Particles that do not settle readily in gravity settlers often can be separated from fluids by centrifugal force. 

In general, when sepiarating two liquids, they must be immiscible and have different spiecific gravities before a separation technique-such as oil/water separation-would be effective. In the case of finely dispersed liquids or finely dispersed solids, if the dispersed material is below one micron in particle size, centrifuging should be considered. The use of centrifugal force on the differing densities of the material can facilitate the separation technique. 

A steam-water separation device that functions by the use of centrifugal force and changes in direction. 

Settling to remove solids from aqueous effluent can be enhanced by the use of centrifugal forces in hydrocyclones. Their design was discussed in Chapter 8. For solids removal, the solids are driven toward the wall of the hydrocyclone and removed from the base. Centrifugation can also be used for solids removal, but restricted to smaller volumes. Solids removal by centrifugation can typically be in the range 50 to 80%, increasing to typically 80 to 95% with chemical addition. 

Significant effort has been devoted to the development of separation methods for the post-BDS stage. Simple approach employing two phase separation methods like filtration or use of centrifugal forces have been studied as well as newer approaches including modification of biocatalyst to facilitate separation have been studied. Information has been obtained from patents as well as publications in open literature. 

It is the aim of this section to offer but a brief introduction to this relatively new field of endeavour that will surely have a not inconsiderable role in shaping the whole future of both physical and chemical processing. It may be noted at the outset, however, that it is not the use of centrifugal forces which is a relatively new development, but rather their application in spinning discs and indeed, centrifugal devices are already widely used in processing, as indicated by the following examples ... 

The use of centrifuges for isotope enrichment was suggested by Lindemann and Aston as early as 1919, but success required the development of vacuum centrifuges in the late 1930s. The rotors in such devices are suspended in vacuum chambers and gas is introduced and withdrawn through metal capillaries. Introductory discussions have been given by London (1961) and Benedict et al. (1981). 

Often the products of nuclear reactions have very short half-lives. This is especially true for the heaviest elements obtained by bombardment of heavy targets with heavy ions. To identify and characterize such short-lived nuclides, fast separations are required solvent extraction techniques are well suited to provide the required fast separations. For example, the SISAK method [68] has been successfully used in conjunction with in-line gas jet separators at heavy ion accelerators to identify short half-life actinide isotopes produced by collision of heavy atoms. The Sisak method involves use of centrifugal contactors, with phase residence times as low as tenths of a second, in conjunction with in-line radiometric detection equipment. 

Dekker et al. [170] have also shown that the steady state experimental data of the extraction and the observed dynamic behavior of the extraction are in good agreement with the model predictions. This model offers the opportunity to predict the effect of changes, both in the process conditions (effect of residence time and mass transfer coefficient) and in the composition of the aqueous and reverse micellar phase (effect of inactivation rate constant and distribution coefficient) on the extraction efficiency. A shorter residence time in the extractors, in combination with an increase in mass transfer rate, will give improvement in the yield of active enzyme in the second aqueous phase and will further reduce the surfactant loss. They have suggested that the use of centrifugal separators or extractors might be valuable in this respect. 

Toss bombing. A bombing action that involves the use of centrifugal force to carry the bomb dropped away from die plane (Ref 2, p297-L)... 

The use of centrifugal force by itself for particle removal in the oilfield is usually limited to the initial separation of large volumes and large drops of liquid. This principle, how -... 

D. Voet andj. Voet, Biochemistry (1995), John Wiley Sons (New York), pp. 97-101 Use of centrifugation in the purification of proteins. 

Arafat, H. A., M. C. Hash, A. S. Hebden, and R. A. Leonard. 2001. Characterization and Recovery of Solvent Entrained During the Use of Centrifugal Contactors. Argonne National Laboratory Report ANL-02/08, Argonne, IL. 

Cell-enrichment processes not using fluorescent or magnetic technology are based on cell physicochemical properties, such as density, size, electrophoretic mobility, or surface composition (phenotype) (1). One of the earliest and still most commonly used methods is based on the use of centrifugal force to exploit density differences and deplete erythrocytes from whole blood. Further... 

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