Settling particles, rate

Berelson W. (2002) Particle settling rates increase with depth in the ocean. Deep-Sea Res. II 49, Til-251. 

A clarifier is designed by keeping fhe particles from flowing out of the clarifier. In order to do so, the liquid rising rate QJA must be less than the particle settling rate u QJA < u,). Therefore, the area of a clarifier can be calculafed by... 

Sedimentation analysis is commonly used and rather simple method of determining the size and size distribution function, based on the difference in particle settling rates in a gravity field. In this method a pan is placed into a homogeneously mixed disperse system, and the weight of particles, P, that accumulate on the pan, is monitored as a function of time, t (Fig. V-30). 

These data are presented in Table 3. What is immediately evident from these data is that the closest correlation occurred when particle settling rates were compared to the apparent viscosity measured at shear rates of 1 sec . The xanthan polymer solution showed a greater ability to suspend the solids and had a higher apparent viscosity in the low shear rate range. In contrast the HEC solution had higher apparent viscosities at shear rates above 10 sec but exhibited lower particle suspension properties. The dependence on low shear rate apparent viscosity is not entirely unexpected. Roodhart has shown emperically that the zero shear viscosity must be factored into a Stokes law type calculation before settling velocities can be calculated for HPG solutions. Thus, reliance on viscosity measurements at the customary shear rates would not have selected the more efficient fluid for particle suspension. 

In general, filter aid filtration should be used only for systems that meet two key requirements. First, the desired product is the filtrate, not the cake. Second, the filter aid is acceptable in the filter cake or the filter cake can be easily repulped and refiltered to remove the filter aid. In addition, the particle-settling rate should be less than 0.012 m/min and the particle concentration lower than 0.1 wt% (< 0.3% for rotary drum precoat). 

At finite concentration, tire settling rate is influenced by hydrodynamic interactions between tire particles. For purely repulsive particle interactions, settling is hindered. Attractive interactions encourage particles to settle as a group, which increases tire settling rate. For hard spheres, tire first-order correction to tire Stokes settling rate is given by [33]... 

Given tire a size dependence of tire settling rate, sedimentation can be used for particle size analysis. Indeed, a quick... 

As shown in Figure 2, adsorption of dispersants on particle surfaces can increase 2eta potential further, enhancing electrostatic repulsion. Increased repulsion between particles is evidenced by lower viscosity in concentrated slurries, or decreased settling rates in dilute suspensions. The effect of added dispersants on settling of (anhydrous) iron oxide particles is shown in Figure 3. 

Shoiild the particles have a tendency to cohere slightly during sedimentation, each sampling time, representing a different nominal detention time in the clarifier, will produce different suspended-sohds concentrations at similar rates. These data can be plotted as sets of cui ves of concentration versus settling rate for each detention time by the means just described. Scale-up will be similar, except that detention time will be a factor, and both depth and area of the clarifier will influence the results. In most cases, more than one combination of diameter and depth will be capable of producing the same clarification result. 

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