Particle stratification

Jigging is a process of particle stratification in which the particle rearrangement results from an alternate expansion and contraction of a bed of particles by pulsating fluid flow. The vertical direction of fluid flow is reversed periodically. Jigging results in layers of particles arranged by increasing density from the top to the bottom of the bed. 

Patwardhan, V.S., and Tien, C., Effect of particle stratification on the performance of fluidized adsorption beds. AlChE J., 32(2), 321-324 (1986). 

It is often said that particle size analysis can be only as good as the sampling technique used for collecting the sample. As all laboratory techniques (and also some on-line techniques) use a small sample taken from a production stream in which particle stratification or segregation often takes place, the correct sampling technique is critical for the accuracy of the eventual particle size analysis data. 

A similar idea forms the basis for the operation of the Mintex/RSM slurry analyser based on the work of Holland-Batt the slurry is pumped through a single-turn helical tube of rectangular cross-section and yS-ray attenuation measurements before the helix and at different positions just after the helix (where particle stratification occurs due to centrifugal forces) yield a point on size distribution data. The instrument requires calibration, which can be made at any selected reference size in the range of 10 to 105 pm. 

Another approach to the problem of binary particle stratification is based on the more traditional concept of axial dispersion according to Pick s law of diffusion in competition with segregation, as formulated by Kennedy and Bretton (1966). Assuming semibatch liquid fluidization that results in partial segregation of the type represented by Fig. 8c or 8d, then at any bed level within the partially mixed region, the volumetric particle mixing flux for species i must equal the particle segregation flux for that species, i.e.. 

Because in several applications of three-phase fluidized beds a size distribution is commonly encountered. Fan et al. [75] analyzed the conditions of particle mixing and particle stratification using a binary mixture of solids. Using a 7.62 cm. diameter plexiglass column and two types of binary solid mixtures - 3mm and 4 mm glass particles, and 3 mm and 6 mm glass particles three possible mixing states were observed. These three states... 

Stratification of the particles making up the bed, caused by the fluidization (fines on top), is not desirable. The soflds holding capacity of the bed is best utilized if the filtration flow encounters progressively finer sand particles. This is achieved in upflow filters where the fluidization due to backwash produces the correct stratification in the bed. Unfortunately, the filtration flow and the backwash take place in the same direction the disadvantage is that the washwater goes to the clean side of the filter. 

A variation to the top-feed dmm filter is the dual dmm filter which uses two dmms of the same size in contact with each other and rotating in opposite directions. The feed enters into the V-shaped space formed on top of the two dmms and the cake that starts forming initially contains coarser particles due to the settling which takes place in the feed zone. This is beneficial to the clarity of the filtrate because the coarser particles act as a precoat. Erom the point of view, however, of the final moisture content of the cake the stratification of the soHds in the cake may lead to somewhat wetter cakes. Utilization of the area of the dmms is poor since there are dead spaces under the two dmms. The primary appHcation of the dual dmm filters is in dewatering coarse mineral or coal suspensions at feed concentrations greater than 200 kg/m. ... 

On a chute, higher drag results in lower particle velocity which can be accentuated by stratification on the chute surface because of the sifting mechanism. Concentrations of smaller particles close to the chute surface and larger particles at the top of the bed of material, combined with the typically higher frictional drag of finer particles, often result in a concentration of fine particles close to the end of the chute, and coarse particles farther away. This can be particulady detrimental if portions of the pile go to different processing points, as is often the case with multiple outiet bins or bins with vertical partitions. 

Fig. 14-5 Typical distribution of P and temperature in a temperate lake in summer. Thermal stratification restricts exchange between surface and deep wafers. Phosphorus is depleted in the surface waters by the sinking of biologically produced particles.

Use of bioflocs rather than supported film particles will maximize the effectiveness factor for a given particle, but uneven growth of floes can cause severe stratification in the bed. If stratification can be overcome by methods such as the use of a tapered bed to control porosity the removal, breaking up, and recycle of biomass at the bottom of the bed or, ideally, the use of microbial strains or species that will stop growing at a desirable floe size, such as a Zymomonas mobilis strain that stops growing at one millimeter in diameter (Scott, 1983), the use of bioflocs rather than support particles can improve reactor productivity. 

More recently, Wells and Goldberg (1991) report that very small marine colloids (d < 120 nm) are, by at least three order of magnitudes, more abundant. A vertical stratification of these particles was found (very high concentrations in the thermodine and season-dependent in the bottom-waters). This stratification indicates that these very small colloidal particles are reactive. The apparent close association of metals with these colloids suggests that they may play an important part in the transport and fate of trace elements in seawater (Wells and Goldberg,... 

Epstein, N. and Pruden, B.B. Chem, Eng. Sci. 54 (1999) 401. Liquid fluidisation of binary particle mixtures - III. Stratification by size and related topics. 

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