Three-component mixtures distillation region

Figure 3.9. Subregions of distillation under infinite reflux Regsa for some structures of three-component mixtures and product regions for given xp, 3,4a,4b... — classification according to Gurikov (1958). Product regions are shaded bottom regions are darker shaded.

Figure 3.16. Regions Reg°° and subregions Regsa of distillation and product simplexes B.egsimp of some structures of three-component mixtures. 1,3.4a,..., classification according to Gurikov (1958). Dotty lines with arrows, boundaries of distillation regions thin lines, boundaries of distillation subregions dotted lines, boundaries of product simplexes.

The location of trajectory bundles and possible product composition segments at reversible distillation of three-component mixtures determines the location of trajectory bundles, and of possible product composition regions of multicomponent mixtures and the locations of trajectory bundles of real adiabatic columns. 

For azeotropic mixtures, the main difficulty of the solution of the task of synthesis consists not in the multiplicity of feasible sequences of columns and complexes but in the necessity for the determination of feasible splits in each potential column or in the complex. The questions of synthesis of separation flowsheets for azeotropic mixtures were investigated in a great number of works. But these works mainly concern three-component mixtures and splits at infinite reflux. In a small number of works, mixtures with a larger number of components are considered however, in these works, the discussion is limited to the identification of splits at infinite reflux and linear boundaries between distillation regions Reg° . Yet, it is important to identify all feasible splits, not only the spUts feasible in simple columns at infinite reflux and at linear boundaries between distillation regions. It is important, in particular, to identify the spUts feasible in simple columns at finite reflux and curvilinear boundaries between distillation regions and also the splits feasible only in three-section columns of extractive distillation. 

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... 

These three azeotropic mixtures represent the corners of a nonreactive distillation boundary plane and divides the composition simplex into a MTBE-rich and a MeOH-rich regions (Giittinger, 1998). As depicted in figure 5.3, all the residue curves originate at the iC4-MeOH azeotrope and end either at the MeOH or MTBE pure component. 

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>). 

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 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. 

Determination of possible composition regions Reg Reg of top and bottom products at reversible distillation of all three-component constituents of initial mixture. 

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