Four-Component Azeotropic Mixtures

To define the frame of possible product composition regions Reg, or RegB at the edges of concentration simplex, it is enough to determine values of phase equilibrium coefficients in the points of edges for the components present and for the component, which is absent in the product. 

Define the main notions (1) region of reversible distillation of top Reg g bottom Reg -e, and intermediate sections Reg g (2) regions of trajectory tear-off Reg and Reg (3) region of possible product points of sharp reversible distillation Reg, Reg, and Reg and (4) node of trajectory bundle of reversible distillation Nrev- 

Can trajectory bundle of reversible distillation go outside the boundaries of distillation region Reg°°  

Let K2 Ki in the feed point. In which regions of components order Reg can be located (1) top reversible section trajectory, (2) bottom reversible section trajectory, (3) intermediate reversible section trajectory  

Answer the same question as in item 3 for a four-component mixture, in the feed point of which K3 K2 K4 Ki (in extractive distillation column, the top product is component 3 and the entrainer is mixture 1,4). 

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Let s examine four-component azeotropic mixture (Fig. 3.12) with one region t 2 I... 

Figure 3.12. Examples of possible xdq.) xdq) -xbo)) and impossible (xop) xb(2)) splits of four-component azeotropic mixture. Thick lines with arrows, bond and c-lines dotted crossed line, absence of bond dotty Mne, sep-aratrix impossible product points are striked out.

Nodes and Ng are defined by means of calculation of line of conjugated liquid-vapor tie-lines from point xd or line of conjugated vapor-liquid tie-lines from point xb- In Fig. 3.12, line of conjugated vapor-liquid tie-lines from point Xb(3) is shown for four-component azeotropic mixture at separation according to split 1 2,3,4. In this case, Ng coincides with vertex 3. 

Figure 3.15. An example of product simplex Regsimp of four-component azeotropic mixture (shaded).

Fill up structural matrix for four-component azeotropic mixture the structure of concentration space of which is shown at Fig. 3.15. 

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. ... 

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). 

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... 

The structure of residue curve bundles of four-component mixtures is significantly more complex and diverse than that of three-component mixtures. This is due to the fact that each four-component mixture consists of four three-component constituents. Therefore, the number of types of four-component mixtures is enormous. In addition to that, four-component mixtures can have four-component node and saddle azeotropes. In contrast to three-component mixtures, the enormous... 

Figure 1.10. a-lines and a-surfaces (shaded) caused by ternary azeotropes for (a) three-component and (b) four-component mixtures. Arrows, direction of residium curves 213,123,132,312,321,231, regions... 

In Safrit Westerberg (1997), a heuristic algorithm is based on the information about local characteristics of stationary points, and checked by the authors at large amounts of three-component mixtures and at some four-component mixtures. This algorithm takes into consideration azeotropes formed by any number of components. 

The previously enumerated methods of calculation of the minimum reflux mode for nonideal zeotropic and azeotropic mixtures have considerable defects (1) they presuppose preliminary setting of possible separation product compositions, which is a comphcated independent task for azeotropic mixtures (2) they embrace only three- and four-component mixtures or only special splits and (3) they do not take into consideration the leap of concentrations in feed cross-section. 

Deviations from the described evolution for nonideal zeotropic and azeotropic mixtures are analogous to those that were discussed before for three-component mixtures. As an example of such deviation, let s examine separation of four-component mixture, the top product of which is component 1 and inside concentration tetrahedron there is a23-surface, that divides it into component-order... 

The example of tangential pinch for four-component mixture is quasisharp separation of azeotropic mixture acetone (l)-benzene (2)-chloroform (3)-toluol (4) of composition Xf (0,350 0,250 0,150 0,250) at intermediate split 1,3(2) 2,4(3) (admixture components heavy and light key are in brackets correspondingly) at the following composition the products xd (0,699 0,001 0,300,0) and xb (0 0,500 10 0,500). The same top product composition, as in the previous example (Fig. 5.18b) of separation of three-component mixture in the top section, is accepted for convenience of analysis. In this case, the boundary elements of top section trajectory bundle, located in face 1-2-3, completely coincides with top section trajectory bundle at separation of previously mentioned three-component mixture. 

To illustrate the algorithm of presynthesis, we examine two examples of homogenous azeotropic mixtures. The first example is a four-component mixture... 

To analyze variants of autoextractive distillation with one-component entrainer and one-component top product, it is sufficient to examine edges of concentration pentahedron one of the components of the edge should be the entrainer, and the other one should be the top product. The rest of the components absent at the edge should have intermediate volatilities. The segments Re of trajectory tear-off of intermediate section at separation of three- and four-component constituents of five-component mixture are marked out in Fig. 8.20 at edges that do not contain binary azeotropes. As one can see in this figure, one can separate all three-component constituents and some of the four-component constituents of five-component mixture under consideration by means of autoextractive distillation with one-component entrainer and top product, but it is impossible to separate five-component mixtures itself this way. 

Figure 3.10 shows typical RCM for nonideal mixtures involving azeotropes. For the mixture ace tone/heptane /benzene (plot a) there is only one distillation field. The problem seems similar to a zeotropic system, except for the fact that the minimum boiler is a binary azeotrope and not a pure component. With the mixture acetone/chloroform/toluene (plot b) there is one distillation boundary linking the high-boiler with the low-boiler azeotrope. Consequently, there are two distillation regions. Similar behavior shows the plot c, with two azeotropes. The mixture acetone/chloroform/methanol (plotd) has four azeotropes (3 binaries and 1 ternary) displaying a behavior with four distillation regions. 

Despite the obviously tedious calculations involved, the VSM has been very successful in predicting binary isotherms from single component data. Figure 3.12 shows how closely the model conforms to experimentally determined data for four pairs of gases adsorbed on an activated carbon. Because interaction between adsorbed species and between adsorbates and adsorbent is taken into account in the VSM, the model is capable of predicting azeotrope formation. Figure 3.13 is an example where azeotrope formation is predicted for the adsorption of a mixture of i - CtHio and C2H4 on zeolite 13X. 

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