Three-Component Azeotropic Mixtures

at reversible distillation of three-component azeotropic mixtures, the product segment constitutes part of a side of the concentration triangle (e.g. Fig. 4.24) and 

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Figure 1.5 shows mainly physically valuable types of three-component azeotropic mixtures deduced by Gurikov (1958) by means of systematic apphcation of Eq. (1.12). In Fig. 1.5, one and the same structure cover a certain type of mixture and an antipodal type in which stable nodes are replaced by unstable ones and vice versa (i.e., the direction of residue curves is opposite). Besides that, the separatrixes are shown by the straight lines. Let s note that the later classifications of three-component mixture types (Matsuyama Nishimura, 1977 Doherty Caldarola, 1985) contain considerably greater number of types, but many of these types are not different in principle because these classifications assume light, medium, and heavy volatile components to be the fixed vertexes of the concentration triangle. 

In Fig. 2.4b, another example of the trajectory bundles is shown (let s call the picture of trajectory bundles a distillation diagram), but already for a three-component azeotropic mixture acetone(l)-benzene(2)-chloroform(3). 

Let s examine three-component azeotropic mixtures with one binary azeotrope and with two regions of distillation at infinite reflux Reg°° (Fig. 3.6a). There is some region (triangle to the right of separatrix) where two points of the bottom product corresponding to one top product point exist. This fact is explained by the 5-shape of c-hnes in this region (Fig. 3.6b, points xb(2) and xb(3>). 

In Figs. 3.10a, b, distillation trajectories at R = oo and A = oo for two types of three-component azeotrope mixtures are shown. 

Azeotropic mixtures can almost never be separated completely into pure components in the sequence of columns without recycles at R = oo and N = oo. The set of products of such a system of columns almost always contains not only pure components, but also azeotropes (pseudocomponents). Mixtures, for which concentration simplex contains only one distillation region, are an exception. For three-component azeotropic mixtures, the only phase diagrams of such type are the diagram shown at Fig. 3.10b and antipodal it. Such a mixture can be separated into two columns and into pure components. Two variants of flowsheet with direct 1 2,3 or indirect 1,2 3 split in the first column are feasible. 

The analysis of the thermodynamically reversible process of distillation for multicomponent azeotropic mixtures was made considerably later. Restrictions at sharp reversible distillation were revealed (Petlyuk, 1978), and trajectory bundles at sharp and nonsharp reversible distillation of three-component azeotropic mixtures were investigated (Petlyuk, Serafimov, Avet yan, Vinogradova, 1981a, 1981b). 

Locations of trajectories bundles Regr. of node points of these bundles Nrev, and of possible product segments Reg Wd Regf can be shown in diagrams of three-component azeotropic mixtures sharp reversible distillation for various types of such mixtures (Fig. 4.11). 

The diagrams of reversible distillation were constructed for some types of three-component azeotropic mixtures. It is interesting that some types of mixtures with one binary azeotrope and with two distillation regions [types 3 and 5 according to classification (Gurikov, 1958)] permit sharp separation into component and binary zeotropic mixture at some feed compositions. The mixture acetone(l)-benzene(2)-chloroform(3) is an example of such mixture. 

Residue curve maps exist for mixtures having more than three components but cannot be visualized when there are more than four components. However, many mixtures of industrial importance contain only three or four key components and can thus be treated as pseudo-temary or quaternary mixtures. Quaternary residue curve maps are more compHcated than thek ternary counterparts but it is stiU possible to understand these maps using the boiling point temperatures of the pure components and azeotropes (31). 

Absolute (100%) ethanol is often made by adding benzene to the ethanol -water binary azeotrope (two components), to make a ternary azeotrope (three components). This ternary alcohol-water-benzene (18.5 7.4 74.1) azeotrope comes over until all the water is gone, followed by a benzene-ethanol mixture. Finally, absolute ethanol gets its chance to appear, marred only slightly by traces of benzene. 

ABS Three-component copolymer of acrylonitrile, butadiene, and styrene, alloy Rubber-toughened materials in which the matrix can be a mixture of polymer tyrpes. alternation copolymer Ordered copolymer in which every other building is a different mer. azeotropic copolymer Copolymer in which the feet and composition of the copolymer are the same, blends Mixtures of different polymers on a molecular level may exist in one or two phases, block copolymer Copolymer that contains long sequences or runs of one mer or both mers. 

Nitromethane shows the simplest residue curve map with one unstable curved separatrix dividing the triangle in two basic distillation regions. Methanol and acetonitrile give rise two binary azeotropic mixtures and three distillation regions that are bounded by two unstable curved separatrices. Water shows the most complicated residue curve maps, due to the presence of a ternary azeotrope and a miscibility gap with both the n-hexane and the ethyl acetate component. In all four cases, the heteroazeotrope (binary or ternary) has the lowest boiling temperature of the system. As it can be seen in Table 3, all entrainers except water provide the n-hexane-rich phase Zw as distillate product with a purity better than 0.91. Water is not a desirable entrainer because of the existence of ternary azeotrope whose n-hexane-rich phase has a water purity much lower (0.70). Considering in Table 3 the split... 

As a consequence of these restrictions, separation of binaiy mixtures by extractive distillation corresponds to onfy two possible three-component distillation region diagrams, depending on whether the binary mixture is pinched or close-boiling (DRD 001), or forms a minimumboiling azeotrope (DRD 003). The addition of high-boiling solvents... 

Keyes process. A distillation process involving the addition of benzene to a constant-boiling 95% alcohol-water solution to obtain absolute (100%) alcohol. On distillation, a ternary azeotropic mixture containing all three components leaves the top of the column while anhydrous alcohol leaves the bottom. The azeotrope (which separates into two layers) is redistilled separately for recovery and reuse of the benzene and alcohol. 

According to Gibbs phase rule a completely soluble binary mixture is enriched in both phases, whilst an immiscible binary mixture, with its three phases, cannot be enriched (see Fig. 29, a—d). It wiU be recognized, on the other hand, that three-component systems having a miscibility gap, f.e. showing two liquid phases and one vapour phase, are separable by countercurrent distillation [1]. A typical example is the preparation of absolute alcohol by azeotropic distillation with benzene. 

Exceptions to these general rules about behaviour in the critical region occur at special points on a binary mixture critical line - examples are extrema in pressure or temperature, points where azeotrope lines join critical lines, or double points (intersections of two critical lines). Moreover, in the special higher-order critical points ( tricritical points ) found in systems with a greater number of variables (three- and four-component fluid mixtures He + He), different exponents may be found. ... 

The transesterification of dimethyl carbonate (DMC) to diethyl carbonate (DEC) represents a complex reaction system including five components and three binary azeotropes shown in Table 10.5. The characteristics of some binary mixtures of the substances involved in the reaction system have been investigated by Franchesconi and coworkers [114-116], Luo et al. [117-119] and Rodriguez et al. [120, 121], resulting in a good description of the required thermodynamic properties. 

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