Page 333 - Distillation theory

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8.6 Binary and Three-Component Azeotropic Mixtures 307

i, j,k
(2)
of segments Reg on edges of concentration simplex [Fig. 8.12] not of component
ord
names).
In the discussed examples, synthesis of separation flowsheets is carried out
automatically on the basis of formal rules. In more complicated cases, in the pro-
cess of synthesis there can be no other variants other than variants of obtaining
azeotropes as some products. These cases are discussed in the following section.
After identification of several preferable sequences, choosing among the opti-
mum sequences, taking into consideration possible thermodinamic improvements
and thermal integration of columns, arises. This task is similar to the synthesis of
separation flowsheets of zeotropic mixtures (see Section 8.3), and it should be
solved by the same methods (i.e., by means of comparative estimation of expen-
ditures on separation). The methods of design calculation, described in Chapters
5 ÷ 7 for the modes of minimum reflux and reflux bigger than minimum, have to
be used for this purpose. In contrast to zeotropic mixtures, the set of alternative
preferable sequences for azeotropic mixtures that sharply decreases the volume
of necessary calculation is much smaller.

8.6. Binary and Three-Component Azeotropic Mixtures

If automatic sequencing does not lead to obtaining the prescribed set of target
products and the obtained set of products contains azeotropes, then it is necessary
to use the special methods to separate the obtained binary and three-component
mixtures. These methods were intensively investigated in recent decades. Using
curvature of separatrix lines in concentration triangle, and application of entrain-
ers and of two levels of pressure for shift of azeotropes points, belong to these
methods. At this stage of synthesis of sequence, the complete automation is not
expedient because visualization of separatrix lines, α-lines, binodal lines, and lines
liquid–liquid in the concentration triangle can be more profitable.
Some examples of separation of three-component mixtures using curvature
of separarix lines are described in Chapter 3. Examples of the application of
entrainers forming heteroazeotropes and of columns with decanters for het-
eroazeotropic and heteroextractive distillation are given in Chapter 6.
We next discuss the examples of application of semisharp extractive distillation
(Petlyuk & Danilov, 2000b) for separation of ternary mixture with two binary
azeotropes, examples of application of two levels of pressure, and choice of en-
trainers that do not form heteroazeotrpes.

8.6.1. Application of Semisharp Extractive Distillation

Figure 8.25 shows separation of ternary mixture with two binary azeotropes. The
sharp autoextractive distillation at split 2 : 1,3 is possible in the first column
(entrainer – component 1) because at side 1-2 there is trajectory tear-off seg-
3
t
ment Reg of intermediate section [1,α 23 ], the whole side 1-3 is possible bottom
e
1,2