Mechanical classifiers, separation processe

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

The mechanics and applications of multiphase flow has been an area of continuing interest to chemical, environmental, and civil engineers (23,77). The multiphase flow patterns may be classified as bubble flow, plug flow, stratified flow, wave flow, slug flow, annular flow, spray flow, and froth flow. Typical sketches of these various flow patterns are shown in Fig. 3. They are self-explanatory. In the field of absorptive bubble separation processes, only multiphase bubble flow and froth flow are of interest to the process engineer. 

Membranes are classified by whether the thin permselective layer is porous or dense, and by the type of material (organic, polymeric, inorganic, metal, etc.) this membrane film is made from. The choice of a porous vs. a dense film, and of the type of material used for manufacturing depends on the desired separation process, operating temperature and driving force used for the separation the choice of material depends on the desired permeance and selectivity, and on thermal and mechanical stability requirements. For membrane reactor applications, where the reaction is coupled with the separation process, the thin film has also to be stable under the reaction conditions. 

Separation processes may be classified by sizes of entities to be separated, mechanisms of separation, types of membranes, and driving forces. Classification by mechanisms is as follows ... 

Size separation is a process which often follows size reduction its purpose is to separate and classify the solid particles according to their size. The main methods are screening, magnetic separation, and mechanical classifying. Magnetic separation is limited to iron and similar metals, and is often used to separate iron impurities from other materials to eliminate damage to chemical plant or discoloration of the products. 

This is essentially a process to separate the particles according to their sinking rates in water. Wet classifiers are used with grinding and may include rubber-lined or ceramic cyclones (called hydrocyclones) and spiral mechanical classifiers. 

The term membrane in general refers to a barrier through which molecules can traverse by one mechanism or another. Membranes as used is separation processes may be broadly classified as porous or nonporous. 

Figure 10 12 shows the interaction between the HOMO of one ethylene molecule and the LUMO of another In particular notice that two of the carbons that are to become ct bonded to each other m the product experience an antibondmg interaction during the cycloaddition process This raises the activation energy for cycloaddition and leads the reaction to be classified as a symmetry forbidden reaction Reaction were it to occur would take place slowly and by a mechanism m which the two new ct bonds are formed m separate steps rather than by way of a concerted process involving a sm gle transition state... 

Preliminary Selections Assembling background information permits tentative selection of promising equipment and rules out clearly unsuitable types. If the material to be processed is a shiny or pumpable suspension of sohds in a hquid, several methods of mechanical separation may be suitable, and these are classified into setfiiug and filtration... 

The standard methods of drying can be classified as deposition of the moisture as either water or ice decomposition of the water chemical precipitation absorption adsorption mechanical separation and vaporization. The completeness with which dryness can be accomplished by any process depends upon the factors controlling the equilibrium conditions achieved in the operation. A brief discussion of each method is first given. 

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. 

The various flow instabilities are classified in Table 6.1. An instability is compound when several elementary mechanisms interact in the process and cannot be studied separately. It is simple (or fundamental) in the opposite sense. A secondary phenomenon is a phenomenon that occurs after the primary one. The term secondary phenomenon is used only in the very important particular case when the occurrence of the primary phenomenon is a necessary condition for the occurrence of the secondary one. 

For electric multipolar interactions, the energy transfer mechanism can be classified into several types, according to the character of the involved transitions of the donor (D) and acceptor (A) centers. Electric dipole-dipole (d-d) interactions occur when the transitions in D and A are both of electric dipole character. These processes correspond, in general, to the longest range order and the transfer probability varies with l/R, where R is the separation between D and A. Other electric multipolar interactions are only relevant at shorter distances dipole-quadrupole (d-q) interaction varies as l/R, while quadrupole-quadrupole interaction varies as l/R °. 

In the present paper the various processes which have been proposed for separating the oil from the sands have been classified according to the key physical and chemical factors involved. This represents an application of concepts described previously by the author on the interfacial properties of the oil sands (2). In addition, the theories of K. A. Clark on the mechanism of the hot water process are updated in the light of new data on the flotation of oil. 

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