Classification of Particles

Centrifuges are used for the classification of particles in size ranges below 10 /im. Two types are used solid bowl centrifuges, usually with a cylindrical, conical bowl, rotated about a horizontal axis and nozzle bowl machines, fitted with discs. 

Most studies of hydrocyclone performance for particle classification have been carried out at particle concentrations of about 1 per cent by volume. The simplest theory for the classification of particles is based on the concept that particles will tend to orbit at the radius at which the centrifugal force is exactly balanced by the fluid friction force on the particles. Thus, the orbits will be of increasing radius as the particle size increases. Unfortunately, there is scant information on how the radial velocity component varies with location. In general, a particle will be conveyed in the secondary vortex to the overflow, if its orbital radius is less than the radius of that vortex. Alternatively, if the orbital radius would have been greater than the diameter of the shell at a particular height, the particle will be deposited on the walls and will be drawn downwards to the bottom outlet. 

The major limitation of microscopic receptor models is that the analytical method, the classification of particles possessing a defined set of properties, has not been separated from the source apportionment of those particles. Equation 2 has never been used in this application. The source identification takes place by recognition of the particle bv the microscopist and thus is difficult to standardize. 

The model allows applying more general classification functions, describing non-ideal classification of particles. Since, however, the parameters of classification were not taken on as design variables, this ideal classification appeared to be satisfactory for simulation purposes. 

Another important classification of particle dark matter rests upon its production mechanism. Particles that were in thermal equilibrium in the early Universe, like neutrinos, neutralinos, and most other WIMPs (weakly interacting massive particles), are called thermal relics. Particles which were produced by a non-thermal mechanism and that never had the chance of reaching thermal equilibrium in the early Universe are called non-thermal relics. There are several examples of non-thermal relics axions emitted by cosmic strings, solitons produced in phase transitions, WIMPZILLAs produced gravitationally at the end of inflation, etc. 

Cascade impactors and cyclones have been used in order to determine the aerodynamic size distribution. Impactors allow the classification of particles with an aerodynamic diameter Dp between 0.1 pm < Dp < 5 pm, while cyclones work in the 2 to 20 pm range. 

FIGURE 12.2 The Geldart classification of particles for air at ambient conditions. Region A Range of properties for well-behaved FCC catalyst (from Kunii and Levenspiel, 1991). 

Care must also be taken to ensure that particle size distributions are adequate as selected particle size distributions will achieve greater homogeneity in reference materials especially when very low sample intakes are used for measurements. This is extremely important for environmental reference materials such as soils, sediments, etc., and is optimally achieved by jet-milling with ultra-fine classification of particles. With this technique, which is non-contaminating, fast and well-controlled, the sieved fraction below 2 mm of the material is ground by... 

Jet-milling with ultrafine classification of particles can be achieved using a Fluidized Bed Opposed Jet Mill 100 AFG (Alpine, Augsburg, Germany). It is a... 

A candidate CRM for trace elements in Antarctic coastal marine sediment was prepared by coarse sieving, drying, jet-milling with ultrafine classification of particles and Turbula mixing. The material was found to be of appropriate particle size distribution, dryness and homogeneity to be certified as a stable and homogeneous CRM. 

A microscopical examination of the material was carried out in order to establish a classification of particles (types and sizes), showing that the large majority of organisms (>98%) were cladocerans mainly of the species Daphnia magna with the occasional presence of a few copepods and rotifers [25]. 

FIGURE 11.32 Geidart s classification of particles with respect to their fluidizability. 

Figure 84 Greldhart classification of particles for fluidization by air at ambient temperature. Region A corresponds to the properties of well-behaved fluid cracking catalysts. [After D. Kunii and O. Levenspiel, Fluidization Engineering, with permission of Butterworth-Heinemann, Boston, MA, (1991).]...

Agarwala JP (1994) Inclined settler classification of particles and transport of ions through liquid membranes with an electric field. Ph.D. Thesis, University of Colorado... 

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