Description of separation in a closed system

Separation is a major activity of chemical engineers and chemists. To separate a mixture of two or more substances, various operations called separation processes are utilized. Before we understand how a mhmire can he separated using a given separation process, we should he able to describe the amount of separation obtained in any given operation. This chapter and Chapter 2 therefore deal with qualitative and quantitative descriptions of separation. Chapter 2 covers open systems this chapter describes separations in a closed system. 

In Section 1.1, we briefly Illustrate the meaning of separation between two regions for a system of two components in a closed vessel. Section 1.2 extends this to a multicomponent system. In Section 1.3, various definitions of compositions and concentrations are given for a two-component system. In Section 1.4, we are concerned with describing the various indices of separation and their interrelationships for a two-region, two-component separation system. A number of such indices are compared with regard to their capacity to describe separation in Section 1.5 for a binary system. Next, Section 1.6 briefly considers the definitions of compositions and indices of separation for the description of separation in a multicomponent system between two regions in a closed vessel. Finally, Section 1.7 briefly describes some terms that are frequently encountered. 

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The description of the extent of separation achieved in a closed vessel for a mixture of molecules is treated in Chapter 1. Chapter 2 illustrates how to describe the separation of molecules in open separators under steady and unsteady state operation a description of separation for a size-distributed system of particles is also included. Chapter 3 introduces various forces developing species-specific veiocities, fluxes and mass-transfer coefficients, and illustrates how the spatial variation of the potential of the force field can develop multicomponent separation ability. The criteria for chemical equilibrium are then specified for different types of multiphase separation systems, followed by an illustration of integrated flux expressions for two-phase and membrane based systems. 

An open system is one which can undergo all the changes allowed for a closed system and in addition it can lose and gain matter across its boundaries. An open system might be one phase in an extraction system, or it might be a small-volume element in an electrophoretic channel. Such systems, which allow for the transport of matter both in and out, are key elements in the description of separation processes. 

The molecular bulkiness de.scriptors may be related to the ability of an analyte to take pan in nonspecific intermolecular interactions (dispersive interactions or London interactions) with the components of a chromatographic system. These descriptors are the most often found to be significant in QSRR analysis. The bulkiness parameters are decisive in the description of separations of closely congeneric analytes. For example, carbon number normally suffices to differentiate the members of homologous series. On... 

It is useful now to illustrate how the descriptive treatment of a particular separation process, e.g. distillation, has been implemented in an evolutionary fashion via the different chapters as identified in row 7 of Table 1. In Section 1.1, Example I of Figure 1.1.2 illustrates the result of heat addition to an equimolar liquid mixture of benzene-toulene a benzene-rich vapor phase and a toluene-rich liquid phase. Using definitions of compositions etc. introduced in Section 1.3, separation indices such as the separation factor (also the equilibrium ratio Ki) describe the separation achieved in a closed vessel for the benzene-toluene system and a methanol-water system for various liquid-phase compositions. Section 1.5 illustrates via Example 1.5.1 and the values of various separation indices, i2 and f, the... 

In Section 1.2, we introduced a brief description of separation for multicomponent systems. Although we learnt there that perfect separation in such a system requires as many regions as there are components, we will restrict ourselves here to separation systems with only two regions in a closed vessel. Thus perfect separation is, in general, ruled out from our considerations. 

The preceding chapters introduced first the notion of separation and then a variety of indices to describe separation. These indices were used to characterize quantitatively the amount of separation achieved in a closed or an open separation vessel. The quantitative description included systems at steady or unsteady state involving chemical or particulate systems. Systems studied were either binary or multicomponent or a continuous mixture. Not considered in these two chapters was the fundamental physicochemical basis for these separations appropriately, this is the focus of our attention in this chapter. 

The main handicap of MD is the knowledge of the function [/( ). There are some systems where reliable approximations to the true (7( r, ) are available. This is, for example, the case of ionic oxides. (7( rJ) is in such a case made of coulombic (pairwise) interactions and short-range terms. A second example is a closed-shell molecular system. In this case the interaction potentials are separated into intraatomic and interatomic parts. A third type of physical system for which suitable approaches to [/( r, ) exist are the transition metals and their alloys. To this class of models belong the glue model and the embedded atom method. Systems where chemical bonds of molecules are broken or created are much more difficult to describe, since the only way to get a proper description of a reaction all the way between reactant and products would be to solve the quantum-mechanical problem at each step of the reaction. 

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