Settling velocity solids concentration

Of these factors, the size, density, and shape of a particle are the most important determinants of settling velocity. Solids concentration and turbulence indirectly affect settling velocity by influencing formation of floes, while sediment bed roughness is a factor in deposition. Floe formation is also strongly influenced by particle and surface chemistry. Chemical properties which factor into the process of floe formation include... 

Due to density differences the particles have the tendency to settle. Thus, solid concentration profiles result which can be described on the basis of the sedimentation-dispersion model (78,79,80). This model involves two parameters, namely, the solids dispersion coefficient, E3, and the mean settling velocity, U5, of the particles in the swarm. Among others Kato et al. (81) determined 3 and U3 in bubble columns for glass beads 75 and 163 yum in diameter. The authors propose correlations for both parameters, E3 and U3. The equation for E3 almost completely agrees with the correlation of Kato and Nishiwaki (51) for the liquid phase dispersion coefficient. 

It is desirable to measure the fluidization behavior of the particular solids in the column, ideally in actual mother liquors. A typical set of curves is shown in Fig. 1 l-22a. Each particle size is a relatively monosized sieve fraction. The x-intercept of each curve is the terminal settling velocity (slurry concentration 0 gm/liter), but it is more accurate to measure the tenninal settling velocity separately with a stopwatch and a graduated cylinder. Such a data set is shown in Fig. ll-22b. 

A simple one-dimensional diffusion sedimentation model works quite well for solids distribution. This model shows that large settling velocity solids at high concentration are much more difficult to homogenize than those with a low settling velocity at low concentration. Experimental data fit this model quite well and probably help explain the old rule of thumb, solids with a settling velocity less than 5 mm/s represent an easy suspension problem and if greater than 30 mm/s, a difficult one . ... 

An increase in the suspension s particle concentration also enhances accumulation in "dead zones with subsequent bridging. Hence, both high particle settling velocity increases and higher solids concentrations create favorable conditions for cake filtration. In contrast, low settling velocity and concentration results in favorable conditions for gradual pore blocking. 

The design and development of a novel vibrating reed technique for on-line measurement of the sedimentation kinetics of two-phase dispersions is described. The technique has been tested in conjunction with a variety of solid/liquid and liquid/liquid dispersions with dense phase concentrations in the range 0-50 % v/v. Typical output include settling velocities, solids flux profiles as well as solids throughputs. Additionally, the performance of a number of sedimentation kinetics models proposed for dilute systems (0 - 2.81 % v/v) are evaluated by comparison with data obtained using the device. 

FIG. 18-85 Depth correction factorto he applied to unit areas determined with Wilhelm-Naide and direct methods. Velocity ratio calculated using tangents to settling cun e at a particular settled solids concentration and at start of test. 

The thickener must be designed so that the settling velocity of particles is significantly greater than the upward fluid velocity, to minimize any increase in the solids concentration in the clarification zone. 

In this diagram the key features are A - Diffusion baffle this serves four roles. First to dissipate the velocity head, thereby improving the overall hydraulic characteristics of the separator. Next, to direct incoming flow downward and outward maximizing the use of the separator volume. Third, to reduce flow turbulence and to distribute the flow evenly over the separator s cross-sectional area. Finally, to isolate inlet turbulence from the rest of the separator. B- Internal chambers In the sediment chamber, heavy solids settle out, and concentrated slugs of oil rise to the surface. As the oily water passes through the parallel corrugated... 

Determine the capacity, cross-sectional area and diameter of a continuous sedimentation tank for liquid suspension clarification in the amount of = 20,000 kg/hr. The concentration of solids is x, = 50%, the settling velocity is Uo = 0.5 m/hr, and the density of liquid phase is 1,050 kg/mT... 

In the top (clarifying) zone the relatively clear liquid moves upward and overflows the top. In the middle zone the solid particles settle as the displaced liquid moves upward, and both the local solids concentration and the settling velocity vary from point to point. In the bottom (compressed) zone, the solids and liquid both move downward at a rate that is determined mainly by the underflow draw-off rate. For a given feed rate and solids... 

The solids flux depends on the local concentration of solids, the settling velocity of the solids at this concentration relative to the liquid, and the net velocity of the liquid. Thus the local solids flux will vary within the thickener because the concentration of solids increases with depth and the amount of liquid that is displaced (upward) by the solids decreases as the solids concentration increases, thus affecting the upward drag on the particles. As these two effects act in opposite directions, there will be some point in the thickener at which the actual solids flux is a minimum. This point determines the conditions for stable steady-state operation, as explained below. 

When bubbles burst at the surface of the fluidized bed, solid material carried along in their wake is ejected into the freeboard space above the bed. The solids are classified in the freeboard particles whose settling velocity ut is greater than the gas velocity fall back into the bed, whereas particles with u < u are elutriated by the gas stream. As a result, both the volume concentration of solids cy and the mass flow rate of entrained solids in the freeboard show a characteristic exponential decay... 

Provided the particle settling velocities vt are known, this equation allows the calculation of )e,s Usually, experiments at non-zero liquid rates are used to evaluate t , and )e,s separately. A similar concentration profile might occur in practice if slurry column reactors are operated close to the conditions given by the minimum suspension criterium. In this case, reactor calculations should take the solids concentration profiles into account. A recommended correlation for the solids dispersion coefficient for small particles is given by Kato et al. [15] ... 

In the design of upflow, three phase bubble column reactors, it is important that the catalyst remains well distributed throughout the bed, or reactor space time yields will suffer. The solid concentration profiles of 2.5, 50 and 100 ym silica and iron oxide particles in water and organic solutions were measured in a 12.7 cm ID bubble column to determine what conditions gave satisfactory solids suspension. These results were compared against the theoretical mean solid settling velocity and the sedimentation diffusion models. Discrepancies between the data and models are discussed. The implications for the design of the reactors for the slurry phase Fischer-Tropsch synthesis are reviewed. 

Cova (3 ) measured the solid concentration profiles of a Raney nickel catalyst with an average diameter of 15.7 ym in a h.6 cm id reactor, using water and acetone as the liquids. He developed a sedimentation diffusion model, assuming solid and liquid dispersion coefficients were equal, and slurry settling velocities were independent of solid concentration. The model was then applied to data for Raney nickel in 6.35 and kk.J cm id bubble columns, in both cocurrent and countercurrent flow. 

Kato, et al (5 ) measured solid concentration profiles, solid dispersion coefficients and terminal settling velocities for glass spheres in water, using 6.6, 12.2 and 21.k cm bubble columns. They developed a dimensionless, empirical correlation for the solid dispersion coefficients which agreed with their observed values to within 20%. 

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