Sedimenting centrifuges centrifugal sedimentation

The water and sediment contents of crude oils is measured according to the standard methods NF M 07-020, ASTM D 96 and D 1796, which determine the volume of water and sediments separated from the crude by centrifuging in the presence of a solvent (toluene) and of a demulsifylng agent Table 8.13 gives the bottom sediment and water content of a few crude oils. 

Water and sediments NF M 07-020 ISO 3734 ASTM D 1796 Mix with toluene and centrifugation... 

The final and less commonly dealt-with member of the family of electrokinetic phenomena is the sedimentation potential. If charged particles are caused to move relative to the medium as a result, say, of a gravitational or centrifugal field, there again will be an induced potential E. The formula relating to f and other parameters is [72, 77]... 

Anotlier standard metliod is to use a (high-speed) centrifuge to sediment tire colloids, replace tire supernatant and redisperse tire particles. Provided tire particles are well stabilized in tire solvent, tliis allows for a rigorous purification. Larger objects, such as particle aggregates, can be fractionated off because tliey settle first. A tliird metliod is (ultra)filtration, whereby larger impurities can be retained, particularly using membrane filters witli accurately defined pore sizes. 

V is the velocity of sedimentation and a is the acceleration of free fall or centrifugation. 

If a sedimentation experiment is carried out long enough, a state of equilibrium is eventually reached between sedimentation and diffusion. Under these conditions material will pass through a cross section perpendicular to the radius in both directions at equal rates downward owing to the centrifugal field, and upward owing to the concentration gradient. It is easy to write expressions for the two fluxes which describe this situation ... 

Prethickening of filter feeds can be done with a variety of equipment such as gravity thickeners, hydrocyclones, or sedimenting centrifuges. Even cake filters can be designed to limit or completely eliminate cake formation and therefore act as thickening filters and be used in this thickening duty. 

Packed red cells are prepared from whole blood. These are collected ia blood coUectioa units having integrally attached transfer packs. The red cells are sedimented by centrifugation, and the plasma and huffy coat are expressed from the bag. Further processiag of the packed red cells may be needed for a number of clinical indications. To reduce the white blood cell (WBC) contamination in a red cell product, two separation techniques are used. 

The processing operations for fluid or manufactured milk products include cooling, centrifugal sediment removal and cream (a mixture of fat and milk semm) separation, standardization, homogenization, pasteurization or sterilization, and packaging, handling, and storing. 

Centrifugation. Centrifugal devices include clarifiers for removal of sediment and extraneous particulates, and separators for removal of fat (cream) from milk (see SEPARATION, CENTRIFUGAL). 

Sedimentation (qv) techniques, whether based on gravitational forces or centrifugation, derive the particle size from the measured travel rates of particles in a Hquid. Before the particle analysis is carried out, the sample is usually dispersed in a medium to break down granules, agglomerates, and aggregates. The dispersion process might involve a simple stirring of the powder into a Hquid, but the use of an ultrasonic dispersion is preferred. 

A larger sedimentation force can be developed by centrifuging the suspension. At an angular velocity of CO radians pet second the centrifugal... 

Sediment Volume. If the dispersion is unstable, the sediment bed will be quite deep and sedimenting particles will stick together where they first strike the sediment bed, thus forming an open stmcture with considerable occluded Hquid. If the dispersion is stable to reagglomeration, the particles will move freely past one another to avoid contact as long as possible. The result is a thin sediment bed with maximum soHds packing and minimum occluded hquid (12). Since dispersed particles setde more slowly than docs, centrifugation maybe needed to force sedimentation of small particles within a reasonable analysis time. 

Biomolecule Separations. Advances in chemical separation techniques such as capillary zone electrophoresis (cze) and sedimentation field flow fractionation (sfff) allow for the isolation of nanogram quantities of amino acids and proteins, as weU as the characterization of large biomolecules (63—68) (see Biopolymers, analytical techniques). The two aforementioned techniques, as weU as chromatography and centrifugation, ate all based upon the differential migration of materials. Trends in the area of separations are toward the manipulation of smaller sample volumes, more rapid purification and analysis of materials, higher resolution of complex mixtures, milder conditions, and higher recovery (69). 

Equation 8 provides the basis of comparison for the performance of various bottle centrifuges having the same material, and also, under certain circumstances, of other types of sedimentation centrifuges, if geometric dissimilarities are also considered. 

The capacity factor, Zg, defined by equation 7, is derived from a set of assumptions. An additional assumption is specific to the botde centrifuge. Namely, a particle is considered sedimented when it reaches the surface of the cake without contacting the tube wall. 

Equation 12 shows that 2 can be expressed as the product of a mean sedimentation area (2 7Z r l) and the G level (co r /s), and therefore reflects the increased sedimentation rate expected through a defined area having centrifugal acceleration instead of gravity. 

Few of the assumed conditions are fully satisfied in practice. The last three items relate to potential interference between separated phases. Such interference can occur and leads to poor sedimentation performance if an excessive volume of the sedimented phase is retained in the centrifuge. 

Excessive volume of soHds may be retained in the bowl of conveyor centrifuges if (/) the conveyor volumetric displacement is not sufficient to handle the sedimentation rate of soHds (2) the sedimented soHds cannot be successfully conveyed and discharged over the soHds port until a sufficient layer has been built up inside the bowl and (3) soHds do not easily sHde outwardly on the underside of the disk of a disk centrifuge. 

In the case of the nozzle disk centrifuge, the flow of the soHds phase through the discharge nozzles may be so restricted that an excessive layer can accumulate inside the bowl sheU. When this layer reaches the zone utilized by the fresh feed stream entering the disk stack, reentrainment of the sedimented soHds by the fresh feed may lead to poor sedimentation performance. 

The sedimentation phenomenon that the Z-concept attempts to describe quantitatively is only part of the total task that the centrifuge has to accomplish. Thus, attempts to predict separation performance solely on the basis of Z-concepts have sometimes given disappointing results. 

The relation of equation 21 for similar centrifuges requires identical sedimentation performance characteristics when operating on the same material. 

The Z-concept permits scale-up between sinulat centrifuges solely on the basis of sedimentation performance. Other criteria and limitations, however, should also be investigated. Scale-up analysis for a specified sohds concentration, for instance, requires knowledge of sohds residence time, permissible accumulation of sohds in the bowl, G level, sohds conveyabihty, flowabihty, compressibihty, limitations of torque, and sohds loading. Extrapolation of data from one size centrifuge to another calls for the apphcation of specific scale-up mechanisms for the particular type of centrifuge and performance requirement. 

Other Sedimentation Seale-Up Equations. Some centrifuge supphers use an area-equivalent, description instead of S others use KQ or values. All of these ate in urhts of area. For a disk centrifuge,... 

Factors Influencing Centrifugal Sedimentation. The sedimentation velocity of a particle is defined by equations I and 2. Each of the terms therein effects separation. 

Particle Size Distribution. Almost every feed slurry is a mixture of fine and coarse particles. Performance depends on the frequency of distribution of particle size ia the feed. Figure 5 shows that whereas all of the coarse particles having a diameter greater than some are separated, fewer of the very fine particles are, at any given feed rate. The size distribution frequency of particles ia feed and centrate for a fine and coarse feed are quite different. More coarse particles separate out than fine ones. Classification of soHds by size is often done by centrifugal sedimentation. 

Manufacturers of sedimentation and filtration centrifuges can be found in Reference 13. 

Sedimentation Equipment. Centrifugal sedimentation equipment is usually characterized by limiting flow rates and theoretical settling capabihties. Feed rates in industrial appHcations may be dictated by Hquid handling capacities, separating capacities, or physical characteristics of the soHds. Sedimentation equipment performance is illustrated in Figure 8 on the basis of nominal clarified effluent flow rates and the appHcable values. The... 

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