Separation of homogeneous fluid mixtures

As pointed out previously, the separation of homogeneous fluid mixtures requires the creation or addition of another phase. The most common method is by repeated vaporization and condensation— distillation. The three principal advantages of distillation are 

The ability to handle a wide range of throughput. Many of the alternatives to distillation can only handle low throughput. 

The principal cases which are not suited to distillation include the following  

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Distillation is by far the most commonly used method for the separation of homogeneous fluid mixtures. The cost of distillation varies with operating pressure, which, in turn, is mainly determined by the molecular weight of the materials being separated. Its widespread use can be attributed to its ability to... 

The most common method for the separation of homogeneous fluid mixtures with fluid products is distillation. Distillation allows virtually complete separation of most homogeneous fluid mixtures. It is no accident that distillation is the most common method used for the separation... 

Separation of Homogeneous Fluid Mixtures by Distillation - Summary 177... 

SEPARATION OF HOMOGENEOUS FLUID MIXTURES BY DISTILLATION - SUMMARY... 

Choice of Separator for Homogeneous Fluid Mixtures I - Distillation... 

As pointed out in the previous chapter, the separation of a homogeneous fluid mixture requires the creation of another phase or the addition of a mass separation agent. Consider a homogeneous liquid mixture. If this liquid mixture is partially vaporized, then another phase is created, and the vapor becomes richer in the more volatile components (i.e. those with the lower boiling points) than the liquid phase. The liquid becomes richer in the less-volatile components (i.e. those with the higher boiling points). If the system is allowed to come to equilibrium conditions, then the distribution of the components between the vapor and liquid phases is dictated by vapor-liquid equilibrium considerations (see Chapter 4). All components can appear in both phases. 

Consider now the particular case in which a homogeneous multicomponent fluid mixture needs to be separated into a number of products, rather than just two products. As noted previously, distillation is the most common method of separating homogeneous fluid mixtures and in this chapter, the choice of separation will be restricted such that all separations are carried out using distillation only. If this is the case, generally there is a choice of order in which the products are separated that is, the choice of distillation sequence. 

In Chap. 6 we treated the thermodynamic properties of constant-composition fluids. However, many applications of chemical-engineering thermodynamics are to systems wherein multicomponent mixtures of gases or liquids undergo composition changes as the result of mixing or separation processes, the transfer of species from one phase to another, or chemical reaction. The properties of such systems depend on composition as well as on temperature and pressure. Our first task in this chapter is therefore to develop a fundamental property relation for homogeneous fluid mixtures of variable composition. We then derive equations applicable to mixtures of ideal gases and ideal solutions. Finally, we treat in detail a particularly simple description of multicomponent vapor/liquid equilibrium known as Raoult s law. 

For simplicity, we consider here only a binary mixtures (A, B) and do not discuss the complications posed by extensions to multicomponent systems. Figure 1 shows a schematic phase diagram in the plane of variables temperature T and concentration c of species B. Kinetics of phase separation in bulk fluid mixtures is triggered by a rapid quench (at time t = 0) from the one-phase region into the miscibility gap. The initial equilibrium state (f < 0) is spatially homogeneous, apart from small-scale concentration inhomogeneities. The final equilibrium state towards which the system ultimately evolves (t oo) consists of... 

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