Particle size measurement elutriation

The terminal velocity in the case of fine particles is approached so quickly that in practical engineering calculations the settling is taken as a constant velocity motion and the acceleration period is neglected. Equation 7 can also be appHed to nonspherical particles if the particle size x is the equivalent Stokes diameter as deterrnined by sedimentation or elutriation methods of particle-size measurement. 

There is a wide variety of methods for particle size measurement which measure different types of particle size. When selecting a method, it is best to take one that measures the type of size which is most relevant to the property or the process which is under study. Thus, for example, in powder elutriation, pneumatic conveying or gas cleaning, it is most relevant to use one of the sedimentation methods which measure the Stokes diameter, i.e. the diameter of a sphere of the same density as the particle itself, which would fall in the gas at the same velocity as the real particle (assuming Stokes law). In flow through packed or fluidized beds, on the other hand, it is the surface-volume diameter (or diameter... 

The use of the effective (aerodynamic) particle density is largely restricted to fluidization and pneumatic conveying applications at the moment. It is, however, potentially useful in other areas like particle size measurement by sedimentation or elutriation, or flow through packed beds where it has not yet been fully accepted. 

In fluidized bed experiments, most authors assume that all attrition products are elutriated. Consequently, they measure either the decrease in bed mass and use Eq. (2) (e.g.,Kono, 1981 Kokkoris etal., 1991, 1995) or the elutriated mass (e.g., Seville et al., 1992, Werther and Xi, 1993). It should be noted that all these authors used a certain particle size as a threshold below which all particles are assigned to be attrition products provided that all initial particles are clearly larger. Breakage events, which lead to particle sizes above the threshold level are, therefore, not considered. The choice of this threshold is very arbitrary and differs between the various research groups. 

The prototype cotton-dust analyzer used in the initial study was designed to measure dust smaller than 100 pm, whereas a vertical elutriator in a card room measures only the dust that is smaller than about 15 pm. Differences in particle size distributions of dust from various types of cotton would likely affect the relationship between the two dust measurements. Therefore, we deemed it necessary to investigate the use of sizing screens with smaller openings i.e., openings whose size approximated the maximum size of particles collected by a vertical elutriator. The purpose of this report is to describe additional modifications to the cotton-dust analyzer and to present data on the performance of the machine when 17-, 50-, and 100-pm sizing screens were used. 

It is not always easy to determine what procedure to follow in making a particle-size distribution. Obviously, if the particles are soluble in water or any other fluid, sedimentation procedures must be applied with caution. It is likewise clear that any sedimentation technique is markedly affected by the shape of the particles used, and that results are subject to interpretation. In other words, determinations depending upon sedimentation (and elutriation) are merely equivalent measures of spheres having the same rate of settling. Greatest reliance naturally applies to that size range whose motion is known to be specified by Stokes law. 

Two instruments have been developed for on-line measurement of flowing powders coarser than 100 pm in size [23-26]. In the first instrument a side stream of solid particles from a process line is fed into an air elutriator that separates it into an oversize and undersize stream. The particle flow rate into the elutriator is measured and the cut size for the elutriator adjusted so that the flow of oversize particles out equals 50% of the inlet flow. The elutriator cut size is then equal to the average size of the powder. In the second instrument the flow rate is varied and the signal ratio of the two... 

Crystal Size Distribution. The measured crystal size distribution followed a log-normal form, suggested as characteristic for most small particles by Herdan (6). Figure 1 shows results obtained with the 4A crystal powder and with the 2 types of pellets formed from it. Here and below, the Linde crystal powder as received, the pelletized powder, and the pellets formed from the elutriated powder will be referred to as CPR, CPS, and ECPS, respectively. Clearly, the pelletizing process did not alfect the size distribution of the original material. Furthermore, the elutriated particles do have a somewhat larger average size. 

Design of fluidized bed crystallizers requires estimates of the required seed bed volume and the quality of fluidization. Fluidization behavior of seed was measured in the laboratory for monosized cuts. For a given volumetric flow rate through any given fluidized bed crystallizer, there is a minimum and maximum particle size which will result. The minimum is that below which the particles will elutriate out the top of the column. The maximum is determined by the size at which removal or controlled attrition (see below) takes place at the bottom. 

Sedimentation methods are also useful in determining particle size distribution. Webb (28) describes the techniques employed in liquid sedimentation and gives data comparing the results with those obtained in mechanical screening. Sedimentation methods are particularly useful in measuring sub-sieve sizes. Air-elutriation methods (Roller, 29) are also useful, especially when used in connection with microscopic examination (Wiley, Deloney, and Denton, 30 Matheson, 31). 

Several methods are available for the measurement of the size of particles, e.g. toicroscopic inspection, sieving, elutriation, sedimentation. Generally, the results obtained will depend on the method of measurement since different fimdamental dimensions are involved in the different methods. For irregularly ped particles it is usual to use a particle size technique which duplicates the process of interest, e.g. to use projected diameter (microscopic) or area in paint or pigment studies diameters based on surface area determinations would have relevance in chemical reactions involving solids, such as catalysis and adsorption. 

Andrews Elutriator. A device for particle-size analysis. It consists of a feed vessel or tube a large hydraulic classifier an intermediate classifier a graduated measuring vessel. (L. 

The catalyst sample is confined to a small cup, into which gas is tangentially added at a high velocity (about 150 m/s). After a test run over a period of an hour, the so-called Davison Index (DI) is determined by measuring the increase in the weight fraction of particles below 20 microns. The increase is determined from both the fraction of elutriated fine material and a particle size analysis of the remaining fraction in the cup. 

In practical terms the cut size fractionation of an elutriator is never a clean cut fractionation and the oversize fraction often contains some of the particles smaller than the cut size and vice-versa. In industrial situations the efficiency of an elutriative fractionation is measured by what is known as a coarse grade-efficiency curve. This curve is often referred to as Tromp s curve after its originator [3]. To be able to calcu-... 

The dynamic methods involve the placement of particles to be measured in an environment which is subsequently disturbed. The members belonging to the particle set react differently to these imposed environmental impulses. These different reactions are observed and therefrom deductions are made as regards the size characteristics. As examples of dynamic methods mention may be made of sieving, streaming, elutriation, and sedimentation. 

Catalyst manufacturing processes usually make particles in a distribution of sizes, although special shapes such as Raschig rings or cylinders made by extrusion and other shapes made by stamping may be quite uniform. Size distribution is measured by sieving or elutriation. A mean diameter is a convenient quantity. The kind of mean value that is applicable when surface is the main property of interest is the volume-surface mean that is applied in P7.01.09. 

In elutriation methods, particles are classified in a column by a rising fluid stream. A series of cyclones are used to separate particles into different size ranges. Gas adsorption of a gas on a powder is another method for determining surface area. Measurements are usually interpreted by using the Braunauer, Emmett, and Teller (BET) theory. 

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