Classification, suspensions

A classification by chemical type is given ia Table 1. It does not attempt to be either rigorous or complete. Clearly, some materials could appear ia more than one of these classifications, eg, polyethylene waxes [9002-88 ] can be classified ia both synthetic waxes and polyolefins, and fiuorosihcones ia sihcones and fiuoropolymers. The broad classes of release materials available are given ia the chemical class column, the principal types ia the chemical subdivision column, and one or two important selections ia the specific examples column. Many commercial products are difficult to place ia any classification scheme. Some are of proprietary composition and many are mixtures. For example, metallic soaps are often used ia combination with hydrocarbon waxes to produce finely dispersed suspensions. Many products also contain formulating aids such as solvents, emulsifiers, and biocides. 

In sohd—sohd separation, the soHds are separated iato fractions according to size, density, shape, or other particle property (see Size reduction). Sedimentation is also used for size separation, ie, classification of soHds (see Separation, size separation). One of the simplest ways to remove the coarse or dense soHds from a feed suspension is by sedimentation. Successive decantation ia a batch system produces closely controUed size fractions of the product. Generally, however, particle classification by sedimentation does not give sharp separation (see Size MEASUREMENT OF PARTICLES). 

The liquid bulk flow limits the upward flow of small particles from the internal side and has a significant influence on the separating effect. Hydroclones are applied successfully for classification, clarification and thickening of suspensions containing particles from 5 to 150 tm in size. 

The product withdrawal pipe is located in the annulus halfway up the draft tube where the vector of the circulation velocity of the suspension and that of the withdrawal velocity of the product point in the same direction. Using this arrangement ensures low particle classification effects. Furthermore, the feed tubes and withdrawal tube can be exactly positioned in the reactor and scaled... 

Two-phase suspension systems produce beaded products with broader particle-size distribution (e.g., 1-50 /rm). The microspherical particles usually need to be classified repeatedly to reduce the particle-size distribution in order to improve the resolution and efficiency in the separation for use in chromatography. The actual classification process depends on the size range involved, the nature of the beaded product, and its intended applications. Relatively large (>50 /rm) and mechanically stable particles can be sieved easily in the dry state, whereas small particles are processed more conveniently in the wet state. For very fine particles (<20 /rm), classification is accomplished by wet sedimentation, countflow setting, countflow centrifugation, or air classification. 

Each of the PLgel individual pore sizes is produced hy suspension polymerization, which yields a fairly diverse range of particle sizes. For optimum performance in a chromatographic column the particle size distribution of the beads should be narrow this is achieved by air classification after the cross-linked beads have been washed and dried thoroughly. Similarly, for consistent column performance, the particle size distribution is critical and is another quality control aspect where both the median particle size and the width of the distribution are specified. The efficiency of the packed column is extremely sensitive to the median particle size, as predicted by the van Deemter equation (4), whereas the width of the particle size distribution can affect column operating pressure and packed bed stability. 

Classification of Processes and Reactors. Most styrene polymers are produced by batch suspension or continuous mass processes. Some are produced by batch mass processes. Mass in this sense includes bulk polymerization of the polymer... 

Disperse systems can be classified in various ways. Classification based on the physical state of the two constituent phases is presented in Table 1. The dispersed phase and the dispersion medium can be either solids, liquids, or gases. Pharmaceutically most important are suspensions, emulsions, and aerosols. (Suspensions and emulsions are described in detail in Secs. IV and V pharmaceutical aerosols are treated in Chapter 14.) A suspension is a solid/liquid dispersion, e.g., a solid drug that is dispersed within a liquid that is a poor solvent for the drug. An emulsion is a li-quid/liquid dispersion in which the two phases are either completely immiscible or saturated with each other. In the case of aerosols, either a liquid (e.g., drug solution) or a solid (e.g., fine drug particles) is dispersed within a gaseous phase. There is no disperse system in which both phases are gases. 

The air classification method separates dry solid particles according to weight and/or size by suspension in and settling from an air stream of appropriate velocity. Centrifugal fractionation separates particles suspended in a so-... 

The different theoretical models for analyzing particle deposition kinetics from suspensions can be classified as either deterministic or stochastic. The deterministic methods are based on the formulation and solution of the equations arising from the application of Newton s second law to a particle whose trajectory is followed in time, until it makes contact with the collector or leaves the system. In the stochastic methods, forces are freed of their classic duty of determining directly the motion of particles and instead the probability of finding a particle in a certain place at a certain time is determined. A more detailed classification scheme can be found in an overview article [72]. 

The prime difficulty of modeling two-phase gas-solid flow is the interphase coupling, which deals with the effects of gas flow on the motion of solids and vice versa. Elgobashi (1991) proposed a classification for gas-solid suspensions based on the solid volume fraction es, which is shown in Fig. 2. When the solid volume fraction is very low, say es< 10-6, the presence of particles has a negligible effect on the gas flow, but their motion is influenced by the gas flow for sufficiently small inertia. This is called one-way coupling. In this case, the gas flow is treated as a pure fluid and the motion of particle phase is mainly controlled by the hydrodynamical forces (e.g., drag force, buoyancy force, and so... 

This type has particular application for dry grinding since the material remains in suspension and is conveyed pneumatically thruout its residence in the mill. The Whizzer classifer is a type particularly applicable to internal use with the Ring-Roller mill... 

The primary antacids can be classified as aluminum-containing, magnesium-containing, calcium carbonate-containing, sodium bicarbonate-containing, or a combination of any of these classifications. These drugs are typically taken orally, either as tablets or as a liquid oral suspension. 

A solution is a homogeneous mixture of a solute and solvent. In a classification of matter, matter is divided into mixtures and pure substances. Mixtures are divided into homogenous mixtures, solutions, and heterogeneous mixtures such as suspensions. Pure substances are divided into elements made up of atoms and compounds made up of molecules. 

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