Settling regimes

Design and operation of recirculation systems can be compHcated. Problems are avoided by using a sludge-blanket clarifier, in which feed enters below a blanket of accumulated and flocculated soflds which become fluidized in the zone-settling regime by the upflowing feed. Feed soflds are trapped in the blanket. The soflds content of the blanket continuously increases and part must be bled off in order to maintain the mass balance. 

There is a gradual transition from particulate setthug into the zone-settling regime, where the particles are constrained to settle as a mass. The principal characteristic of this zone is that the setthug rate of the mass, as observed in batch tests, will be a function of its solids concentration (for any particular condition of flocculation, particle density, etc.). 

Example Determine the dimensions of a simple settling chamber required to remove 50 ft size particles under the following conditions Gas capacity, q = 2400 mVhr Particle density, Pp = 2400 kg/m Gas temperature, t = 20 °C Gas density, p = 1.2 kg/m Gas viscosity, ft = 1.8x 10 N-s/m. The solution is as follows. The settling regime for the particles must be determined first. Hence, the critical particle diameter is computed first ... 

Starting point for evaluating the settling characteristics of suspended solids for dilute systems. Note that from the definition of the Reynolds number, we can readily determine the settling velocity of the particles from the application of the above expressions (u, = /xRe/dpp). The following is an interpolation formula that can be applied over all three settling regimes ... 

For more concentrated suspensions (q> >0.2), the sedimentation velocity becomes a complex function of At > 0.4, a hindered settling regime is usually entered whereby all of the particles sediment at the same rate (independent of size). A schematic representation for the variation of v with is shown in Figure 9.12, which also shows the variation of relative viscosity with rp. It can be seen from these data that v decreases exponentially with increase in approaches zero when cp approaches a critical value (the maximum packing fraction). The relative viscosity shows a gradual increase with increase in cp such that, when cp = the relative viscosity approaches infinity. 

MANDERSLOOT, W.G.B., SCOTT, K.J. 8c GEYER, C.P. 1986. Sedimentation in the hindered settling regime. In Advances in Solid-Liquid Separation (ed. H.S. Muralidhara), pp. 63—77. Columbus Battelle. 

FIGURE 7.50. Settling regimes as fimction of particle characteristics. 

The kinetic energy of the particles is defined by Equation 3.30, where the particle velocity Up is a function of the terminal settling velocity u depending itself on the settling regime, as explained in Section 3.5.5.4. Iterations are, therefore, needed. Assuming applicability of Equation 3.32 to calculate Up it is found that the value of the particle Reynolds number Rep surpasses the limit of 0.4, so a second approach using Equation 3.33 is done, that is. 

The design and scale-up of conventional gravity thickeners is usually based upon batch settling tests on suspensions in the zone settling regime as... 

Thickener Basin Area (Unit Area). The thickener basin area is determined from the solid flux rates at the critical concentration in the hindered settling regime. By definition, the critical point can be experimentally determined from the solids concentration just prior to the beginning of the compression zone. For a nonflocculated system, the basin area, expressed as the unit area (m /ton/d), Aq, can be calculated using the equation (Osborne, 1990)... 

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