Sedimentation free settling

Sohds in either phase are sedimented to the underside of the disks and shde outward along the surfaces because of their density. The aggregated sohds must move by free settling from the outer edges of the disks to the bowl wall some may be reentrained into new feed material, and carried into the disk stack, which accounts in part for actual performance falling short of theoretical prediction. 

Satzbrauen, n, brewing with cold malt extract, satzfrei, a. deposit-free, sediment-free. Satz-kriicke,/. (Brewing) yeaat rouaer. -mehl, n. fecola, starch fiour. -schale, /. settling dish. 

Richardson and Zaki(11) found that m, corresponded closely to u0, the free settling velocity of a particle in an infinite medium, for work on sedimentation as discussed in Chapter 5, although u, was somewhat less than n0 in fluidisation. The following equation for fluidisation was presented ... 

The sediment mostly settles in the plain a thinner sediment cover is on the slopes of the hills and the steep parts of the hills and their tops are free of sediment. 

Usnally, there are fonr zones in a thickener the clarification zone, the feed zone, the transition zone, and the compression zone. Not all of these zones will be present for all types of slurries. Liquid flows out of the feed zone, up through the clarification zone, and toward the overflow. Below the feed is the transition zone, where the particles start a downward motion. The concentration just below the supernatant-suspension (or sediment) interface assumes a value dictated by the dominance of either free settling or the presence of a compacting sediment. The region containing the particles offers resistance to liqnid flow and accepts only a portion of the liquid. The remainder reverses direction and exits as overflow. Under steady-state conditions, both solids and liquid fluxes are constant and independent of depth in the snspension and/or sediment zone. No liquid is squeezed out and upward in the sediment unless channels exist. 

Eor even higher solid concentrations where the particles are in contact with each other, free settling phenomena is replaced by compression or consolidation. The Coe and Clevenger-type design methods are not sufficient. Sediment height and consolidation effect need to be included in the thickener design (Tiller and Tamg 1995). 

Electrokinetics has also been tested as a means to increase the free settling velocity and the solids concentration of river sediments (Buckland, Shang, and Mohamedelhassan, 2000). The best results were obtained through a combination of electrokinetics and conditioning with FeCb. It was concluded that electrokinetic sedimentation is more effective than chemical coagulation. 

When a particle is at a sufficient distance from the walls of the container and from other particles so that its fall is not affected by them, the process is called free settling. Interference is less than 1% if the ratio of the particle diameter to the container diameter is less than T.200 or if the particle concentration is less than 0.2 vol % in the solution. When the particles are crowded, they settle at a lower rate and the process is called hindered settling. The separation of a dilute slurry or suspension by gravity settling into a clear fluid and a slurry of higher solids content is called sedimentation. 

Sedimentation or free settling refers to the sinking of solid particles in a volume of liquid which is large with respect to the total volume of particles, hence particle crowding is a neg-Hgible phenomena. Usually, free setthng predominates when the mass fraction of solids is less than 15 wt.%. 

In a solution of molecules of uniform molecular weight, all particles settle with the same value of v. If diffusion is ignored, a sharp boundary forms between the top portion of the cell, which has been swept free of solute, and the bottom, which still contains solute. Figure 9.13a shows schematically how the concentration profile varies with time under these conditions. It is apparent that the Schlieren optical system described in the last section is ideally suited for measuring the displacement of this boundary with time. Since the velocity of the boundary and that of the particles are the same, the sedimentation coefficient is readily measured. 

The particle size deterrnined by sedimentation techniques is an equivalent spherical diameter, also known as the equivalent settling diameter, defined as the diameter of a sphere of the same density as the irregularly shaped particle that exhibits an identical free-fall velocity. Thus it is an appropriate diameter upon which to base particle behavior in other fluid-flow situations. Variations in the particle size distribution can occur for nonspherical particles (43,44). The upper size limit for sedimentation methods is estabHshed by the value of the particle Reynolds number, given by equation 11 ... 

Free-falling diameter Also known as sedimentation or Stokes diameter, the diameter of a sphere with the same terminal settling velocity and density as a nonspherical or irregular particle. 

Elutriation differs from sedimentation in that fluid moves vertically upwards and thereby carries with it all particles whose settling velocity by gravity is less than the fluid velocity. In practice, complications are introduced by such factors as the non-uniformity of the fluid velocity across a section of an elutriating tube, the influence of the walls of the tube, and the effect of eddies in the flow. In consequence, any assumption that the separated particle size corresponds to the mean velocity of fluid flow is only approximately true it also requires an infinite time to effect complete separation. This method is predicated on the assumption that Stokes law relating the free-falling velocity of a spherical particle to its density and diameter, and to the density and viscosity of the medium is valid... 

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