Particle-impeller impacts

The first of these mechanisms is only important when there is a close clearance between the impeller and a stationary surface, e.g. between the impeller and a draught tube. Such a constriction should be avoided, especially in crystallization. Without constrictions, the second mechanism dominates at low concentrations of solids since then particle-impeller impactions are the most energetic and most frequent. 

Three particle collision mechanisms can occur in an agitated vessel. These are (a) particle-vessel, (b) particle-impeller and (c) particle-particle. Most of the work on collisions has been related to secondary nucleation, but there are other systems where mechanical abrasion following impact may occur and may be undesirable, e.g. breakdown of friable catalysts, or in mammalian cell culture on microcarriers or desirable, e.g. removal of an impervious outer skin which forms on ore particles during some leaching processes. 

If the solids are present in high concentration, then particle-particle impaction occurs so much more frequently than particle-impeller that, provided the impacts are above the threshold level, this mechanism becomes dominant. In that case ... 

Scale-up of the apparatus is predicted to decrease the total number of attrition particles and reduce the proportion of fines contributed by impeller impact relative to turbulent fluid erosion of parent particles, but it was concluded more work is still necessary to quantify and experimentally confirm those aspects. The implication, however, is that mean crystal size increases with scale up - a fortuitous result often observed in practice. 

A dependence of both crystal and impeller material properties as well as the probability of crystal-impeller collision on fine particle generation rate has also been demonstrated. Thus the relative effects of impact, drag and shear forces responsible for crystal attrition have been identified. The contribution of shear forces to the turbulent component is predicted to be most significant when the parent particle size is smaller than a 200 pm while drag forces mainly affect larger crystals, the latter being consistent with the observations of Synowiec etal. (1993). 

The stress acting on particles is due to a relative velocity between the particles and the fluid. If their mean velocities also differ, contact between the particles or between a particle and the tank wall or the impeller elements leads to impact stress. However, this impact stress is negligible if the density differences and the particle concentrations are low. 

On the reverse, how does the presence of particles affect local and global flow features in the vessel such as the vortex structure in the vicinity of the impeller, power consumption, circulation and mixing times, and the spatial distribution of turbulence quantities more specifically colliding particles have an impact on the liquid s turbulence (Ten Cate et al., 2004) while local particle concentrations affect the effective (slurry) viscosity which may be useful in the macroflow simulations ... 

Substantial improvements in LB techniques have been elfected—in terms of immersed or embedded boundary methods for dealing with moving and curved boundaries (impeller blades, solid particles) and of grid refinement techniques— which have had a positive impact on the fast proliferation of dedicated CFD tools. Here, too, the details of the computational techniques do matter. 

Treatability studies will be required prior to treatment. Metal oxides have variable solubilities in glass. Systems are designed for an organic content under 20%. Materials for melter construction must be selected for their compatibility with the wastes to be treated. The mass and size of feed particles are limited by their impact on the impeller. 

Although the motor power consumption is strongly correlated with the torque on the impeller,it is less sensitive to high frequency oscillations caused by direct impact of particles on the blades as evidenced by Fast Fourier Transform (FFT) technique. ... 

Wear can be described as an undesirable deformation of the surface of an object, moving or fixed part of the equipment by way of removal of small particles due to mechanical action such as abrasion, impact, friction, etc. In addition to the mechanical reasons, deformation could arise due to chemical action such as in the case of impellers in the fertiliser industries which are exposed to the reacting phosphoric acid fumes containing dust particles of phosphates. 

Attrition. Attrition causes small particles of crystalline material that have already been formed to be broken from identifiable crystals, and thereby, to be added to the body of solids that are present as individual or discrete particles. Their presence increases as the level of mechanical energy input in the system increases. They can be formed by contact of the crystals with a pump impeller or due to impact of the slurry on tube sheets, piping, or vessel walls. In some crystallizer designs, the attrition is sufficiently large so that if the crystallization rate is reduced to a very low value, the product size progressively decreases as particles separate from the existing solids and become new particles. Attrition can occur whether or not supersaturation is present. 

The high impaction imparted by the impeller can result in the comminution of particles but more importantly from the mixing point of view can break down... 

Nucleation includes primary as well as secondary nucleation. In primary nucleation, there is spontaneous generation of new particles in the absence of existing particles. Secondary nucleation includes the formation of nuclei in the neighborhood of an existing particle, and those microparticles that are formed by breakage by impact with the impeller or the container walls (in other words, first-order processes), or by particle-particle collisions (second-order processes). Denoting the supersaturation at time t by cr(t), we may represent the primary nucleation rate by the particle growth... 

Especially during a fast precipitation process, a short blend time is of an outstanding importance, as nudeation and growth processes compared to cooling crystallization are very fast. Here, insufficient blending together with dead zones may have a dramatic and disadvantageous impact on product quality. The objective for such a process must be to find a suitable impeller system to prevent these effects and therefore to prevent uncontrolled particle formation. 

The most important impact on the power input necessary for suspension comes from the impeller type. Axially working impellers are the most economic tools for suspending. A comparison of a Viscoprop impeller with a Rushton turbine shows that the Viscoprop impeller is able to reach the same result with less than 1/3 of the energy consumption. It has to be considered that for shear-sensitive particles such as needle- or plate-shaped crystals the power input is of less importance than particle exposure to the local specific energy dissipation rate. 

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