Segregation mechanisms

1 Momentum. This mechanism occurs when a flowing bulk solid impacts a pile that is formed below the bulk sohd. Because of differences in the momentum and in some cases the cohesion between the coarse and fine fractions, the coarse particles tend to gather at the outer periphery of the pile while the fines are deposited directly below the point of impact. This mechanism is quite common and has been shown to occur when the ratio of particle diameters is as small as 1.3. This mechanism would probably occur when, for example, a batch mixer is discharged into a hopper below it. Fortunately, the use of mass flow hoppers can effectively combat this radial segregation. 

Here fine particles sift in between the coarse as the bulk solid avalanches down toward the center of the silo. This usually manifests itself by showing an increase in fines content in the material exiting the silo at the end of the discharge of the silo. Another common cause of sifting is vibration. In this case, the finer fraction can sift into the interstices of the coarse. This occurs frequently during product shipment. 

4 Fluidization. When conveying material into a silo, it is not unusual for the fines to remain suspended in the headspace above the material for a long period of time, eventually settling as a layer on the material conveyed. This mechanism is likely to occur when mixmres containing slow settling particles (usually less than 100 p,m) are conveyed. 

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Particle Segregation Mechanisms. Segregation is the process by which an assembly of soHd particles separates as it is being handled. This often results in cosdy quaUty control problems due to the waste of raw or finished materials, lost production, increased maintenance, and capital costs required to retrofit existing faciUties. 

Segregation is the unwanted separation of differing components of the blend. This separation action is often referred to as a segregation mechanism. A second action is required for segregation to manifest itself, specifically, the flow from the blender to the creation of the dose. As material flows, the segregated zones may be reclaimed in such a way as to be effectively reblended or these zones may be reclaimed one at a time, exacerbating segregation. 

Minimize drop height. Drop height serves to aerate the material, induce dust, and increase momentum of the material as it hits the pile, increasing the tendency for each of the three segregation mechanisms described earlier. 

Segregation is a real problem in industry. Some mixers may not be able to form a random mixture of some powder mixes because of powder segregation mechanisms occurring within the mixer. Some mixers may even demix a mixture due to the dominance of segregation mechanisms acting on a particular mixer, for example, a... 

Three primary segregation mechanisms are of interest in typical pharmaceutical blend handling operations. Other mechanisms exist (46), but are less frequently encountered. The segregation mechanisms of interest are ... 

Of all of these, segregation based on particle size is by far the most common (47). In fact, panicle size is the most important factor in all of the primary segregation mechanisms considered here. 

Ruidization is common in materials that contain a significant percentage of particles smaller than 100 pm (50). Fluidization segregation is likely to occur when fine materials are pneumatically conveyed when they are filled or discharged at high rates, or if gas counter-flow occurs. As with most segregation mechanisms, the more cohesive the material, the less likely it will segregate by this mechanism. 

From the previous discussion about segregation mechanisms, it can be concluded that certain material properties as well as process conditions must exist for segregation to occur. Elimination of one of these will prevent segregation. It stands to reason then that if segregation is a problem in a process, one should look for opportunities to either change the material or change the process. 

Dust generation and fluidization of the material should be minimized during material movement. Dust can be controlled by way of socks or sleeves, to contain the material as it drops from the blender to the bin, for example. Some devices are commercially available. An example of this is a solids decelerator shown in Figure 11. Drop heights should be minimal, as they aerate the material, induce dust, and increase momentum of the material as it hits the pile, all of which can increase the tendency for each of the three segregation mechanisms to occur. Valves should be operated correctly. Butterfly valves should be operated in the full open position, not throttled to restrict flow. 

Mass flow is usually beneficial when handling segregation-prone materials, especially materials that exhibit a side-to-side (or center-to-periphery) segregation pattern, with overall uniformity in the vertical direction. Sifting and dusting segregation mechanisms fit this description. 

Johanson K. Eckert C, Ghose D, et al. Quantitative measurement of particle segregation mechanisms. Powder Technol 2005 159(11) 1-12. 

De Silva S, Dyroy A, Enstad G. Segregation mechanisms and their quantiHcation using segregation testers. In Solids Mechanics and Its Applications, Vol. 81. lUTAM Symposium on Segregation in Granular Flows, 1999 11-29. 

The gas is applied as a mixture to the retentate (high pressure) side of the membrane, the components of the mixture diffuse with different rates through the membrane under the action of a total pressure gradient and are removed at the permeate side by a sweep gas or by vacuum suction. Because the only segregative mechanisms in mesopores are Knudsen diffusion and surface diffusion/capillary condensation (see Table 9.1), viscous flow and continuum (bulk gas) diffusion should be absent in the separation layer. Only the transition state between Knudsen diffusion and continuum diffusion is allowed to some extent, but is not preferred because the selectivity is decreased. Nevertheless, continuum diffusion and viscous flow usually occur in the macroscopic pores of the support of the separation layer in asymmetric systems (see Fig. 9.2) and this can affect the separation factor. Furthermore the experimental set-up as shown in Fig. 9.11 can be used vmder isobaric conditions (only partial pressure differences are present) for the measurement of diffusivities in gas mixtures in so-called Wicke-Callenbach types of measurement. 

The lateral segregation mechanism of FPR desensitization in neutrophils (Figure 2) may be an attractive model that integrates the receptor-mediated actin polymerization/depolymerization and feed-back regulation of receptor fimction. What do we know about FPR binding to cytoskeletal proteins and membrane disribution of FPR as a function of the activation state of neutrophils to support such an idea ... 

The significance of this segregating mechanism in controlling metal solubility and bioavailability in soils can be easily imagined, as it provides a means of literally burying toxic or essential metals in a form inaccessible to desorption. Furthermore, the solubility of a toxic or essential metal could be lowered well below that predicted from the solubility of the pure solid phase. That is, if the trace metal of interest is symbolized by B, the solubility of B in equilibrium with the solid solution would be lower than that in equilibrium with pure BY. This fact is demonstrated by combining equations 4.78 and 4.83 to give... 

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