Particle direct techniques

The removal of solid particles from gas/vapor or liquid streams can be accomplished by several techniques, some handling the flow dry, others wetting the stream to settle/agglomerate the solids (or even dissolve) and remove the liquid phase from the system with the solid particles. Some techniques are more adaptable to certain industries than others. Figure 4-54 illustrates typical ranges of particle size removal of various types of common equipment or technique. All of these will not be covered in this chapter. Attention will be directed to the usual equipment associated with the chemical/petrochemical industries. 

Nevertheless, direct test particle calculations have been of great conceptual importance, particularly in cases where there is a consensus on the relevance of simplified model solutes [2-4, 6, 9, 10,41 15]. The related particle insertion techniques are used for simulating phase equilibria, as discussed in Chap. 10. 

The main objective of the present section, however, is to begin with a very standard technique such as optical microscopy and to use it to illustrate why colloids are difficult to see and what modern developments have emerged in recent years to allow us to see and do things that were considered impossible until a decade ago. We also use this opportunity to review briefly some new techniques that are currently available to measure interaction forces between particles directly. We appeal to some of these techniques in other chapters when we discuss colloidal forces. 

Coulter Counter To avoid the tedium of direct microscopic counting, a Coulter counter can be employed. By using this technique, not only the cell number, but the cell size can be measured. The disadvantage of this technique is that it cannot distinguish between cells and any impure particles. The technique is also difficult to use with organisms in chains and is useless with mycelial organisms. 

Suspensions contain micronized drug for proper delivery to and absorption in the respiratory system. Typical particle size of the micronized drug is from 2 to 5 microns [5], Aerodynamic mean particle size as measured by cascade impactor or direct method of microscopic analysis is usually from 0.5 to 4 microns [5], Additional particle-sizing techniques such as light scattering can be used [6],... 

None of these indirect techniques relate the breakage of an individual particle to its mechanical properties. To achieve this, direct techniques are required. These are described later, and their capabilities and limitations discussed. Direct techniques also allow more sophisticated mathematical modelling to be undertaken, which is particularly valuable when the particles are not homogeneous, for example, cells with walls and membranes surrounding cytoplasm, or a liquid-filled microcapsule. 

The direct determination of some major elements (Ca, K, Mg, Na, and P) and Zn by ICP-AES was performed in powdered milk [14]. Samples were diluted with a 5 or 10 percent (v/v) water-soluble, mixed tertiary amine reagent at pH 8. This reagent mixture dissociated casein micelles and stabilized liquid phase cations. No decrease in analyte emission intensities was observed. Reference solutions were prepared in 10 percent (v/v) mixed amine solution, and no internal reference element was needed for ICP-AES. The direct technique is as fast as slurry approaches, without particle size effects or sensitivity losses. 

The MC method is a powerful technique for investigating complicated phenomena that are difficult to solve by the conventional differential equation approach. In the MC approach, all one needs are the individual probabilities of various kinetic events. It is easy to understand the advantages of applying the MC method to emulsion polymerization if we note that it is possible to simulate the formation processes of all polymer molecules in each polymer particle directly because the volume of the reaction locus is very small. One... 

There are three methods commonly used to ascertain the quantity of radioactive atoms in a given sample film exposure, Geiger-Miiller counting, and scintillation spectrometry. None of these techniques measures the number of /3 particles directly, but rather monitors the results of collisions between the j3 particles emitted from the radioactive atoms and some component of the assay system. 

One of the major uses of transmission electron microscopy in the area of catalysts is the measurement of particle size distributions for supported metals. Chemical techniques can only effectively be used to obtain a global value for the dispersion. By observing the particles directly in transmission electron microscopy, it is possible to check the heterogeneity, and detect the existence of bimodal size distributions. Figure 9.8 shows an example of metallic particles distributed in a zeolite. 

Microscopy is a widely used particle sizing technique in which individual particles are observed and measured. Optical microscopy is used for examination of particles from about 150—0.8 microns. For smaller particles an electron microscope is needed. A single particle has an infinite number of linear dimensions and it is only when they are averaged that a meaningful value is yielded. When a linear dimension is measured parallel to some fixed direction, the size distribution of these measurements reflects the size distribution of the projected areas of the particles [3]. ... 

Summary. In conclusion, some suggestions are made on how to model the problem of radiative heat transfer in porous media. First, we must choose between a direct simulation and a continuum treatment. Wherever possible, continuum treatment should be used because of the lower cost of computation. However, the volume-averaged radiative properties may not be available in which case continuum treatment cannot be used. Except for the Monte Carlo techniques for large particles, direct simulation techniques have not been developed to solve but the simplest of problems. However, direct simulation techniques should be used in case the number of particles is too small to justify the use of a continuum treatment and as a tool to verify dependent scattering models. 

Particle size characterization techniques can be broadly classified as direct and indirect techniques. In the direct techniques, the characterization is performed directly on the actual particle, whereas in the indirect techniques, the characterization relies on relationships between physical properties and particle size or shape. 

The direct techniques of particle size characterization include sieving and various microscopic methods, each discussed briefly. Details can be... 

Sampling is by far the most important part of particle size analysis by microscopy (and probably all particle size techniques). A kilogram of drug substance will contain many millions of particles. Since, at most, the particle size analysis samples a few thousand particles, the measured particles must be selected with care. Allen [20] presents an extensive discussion of bulk sampling issues relevant to all particle size analysis, irrespective of the particular technique. Our interest, though, is primarily directed toward sampling as it relates to the specimen used for particle size analysis by microscopy. We will assume that the 50 mg or so of sample dehvered to the laboratory is truly representative of the bulk powder. 

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