Settling velocity experiments

In connection with terminal settling velocity experiments at the axis of a long circular cylinder, Eq. (135) may be written in the equivalent form... 

Figure 4. Particle settling velocity experiment curves for the effluents mentioned in Table IV.

EXAMPLE 2.1 Analyzing Cumulative Sedimentation Data for Most Probable Settling Velocity. The following data show—as a function of time—the weight (as percentage of total) of suspended clay particles W, which has accumulated on a plate submerged 20 cm beneath the surface in a sedimentation experiment (Oden 1915). 

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] ... 

The simplest case to consider is the settling of a single homogeneous sphere, under gravity, in a fluid of infinite extent. Many experiments have been carried out to determine the relationship between settling velocity and particle size under these conditions and a unique relationship between drag factor (C ) and Reynolds number Re) has been found that reduces to a simple equation, known as the Stokes equation, at low Reynolds number. 

The second step was to examine the effect of particle size on the calibration curve. This step was not possible by sedimentation, because coarser particles have higher settling velocities. Therefore, a liquid-solid fluidized bed was used. A fluidization column was constructed with a 5-cm acrylic pipe. Weighed quantities of solids were used, and solids concentration was varied by changing the liquid flow rate. Measurements for these experiments included voltage, bed height, and temperature. To allow a precise determination of concentration from bed height, narrow sizes of particles were used. 

It was further demonstrated that this theoretical parameter was the right one to use, as Menet et al. have carried out experiments on the evolution of the hydro-dynamic behavior of the butanol-1-water system with the temperature [3]. They showed that the observed change in behavior was explained by an increase in the y p settling velocity with the temperature, and, thus, a decrease of the settling time, explaining the hydrophobic behavior of the solvent system. 

If the skin friction exceeds the critical threshold for resuspension, sedimentary material is I ifted off from the bottom and is transported into the water body. Grainy particles may also be moved by the so-called bed-load transport that occurs already at a lower threshold. Deposition results from the settling of the sediment particles, if the shear stress falls below a certain limit. The critical thresholds, the settling velocities, and the erosion and deposition rates are material constants derived from experiments (Soulsby, 1997). 

RBCs have been used widely used for wastewater treatment in the past and are well understood furthermore, abundant information is available in the literature on their operation and design [54], The upflow sludge bed reactor has also been apphed successfully in the past. However, experience is only widely available on its apphcation to anaerobic wastewater treatment [55,56]. One of the key requirements for apphcation of the AUSB for MTBE bioremediation, however, depends on the abihty of the MTBE bacteria to agglomerate and form dense granules [57]. The granules also need to attain a settling velocity in the range of 40-100 m/h to function properly inside a reactor [58]. 

This concept was tested on the basis of experimental data for a onedimensional system. Figure 4 shows the time-dependent decrease of the particulate concentration in an annular flume observed values and those computed according to Equation 12 show good agreement. In the experiments neither the hydrodynamic system nor the characteristics of the suspended matter were varied. One can see that the transport capacity Ceq/Co is constant (always about 50% of the initial concentration C is sedimenting) and independent of the absolute value of Co. This supports the hypothesis that there exists a critical settling velocity t s.cr, which divides the entire amount of particulates into sedimentous and nonsedimentous parts. 

Values have been corrected for seawater blank values but not for the concentration of the various species that are contributed by the same settling velocity fraction of the natural particles in the seawater used for the experiment. In the case of chlorinated hydrocarbons these natural values should be negligible hence all of the reported concentrations can be taken as effluent-derived. The rapid settling fraction is isolated according to the techniques of the PLOP experiment, which is described in the text, and chemical analysis for the various trace constituents is performed as described in Ref. 10. 

These kinds of experiments are without exception carried out in a column of fluid, usually of the same composition as that from which the aggregates were sampled. The aggregates are introduced into the top of the column and one or more microscopes are used to measure the settling velocity. Farrow and Warren describe a floe density analyser [69] which may be used to determine the fractal dimension. Nobbs et al. [70] review many of the practical aspects involved in performing this type of experimental investigation. 

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