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8.5 Multicomponent Azeotropic Mixtures: Automatic Sequencing and Selection 303

not getting into the possible bottom product composition segment at side 2-3
(flowsheet with preliminary recycle (b) and flowsheet with semisharp separation
in the first column (a)).
The flowsheet (a) has considerably heavier energy expenditures because of the
big necessary reflux number in the third column and big secondary recycle flow
rates of azeotrop 13 and components 1 and 2. Besides that, this flowsheet is bigger
by one column.
The sixth rule: if a product in the synthesized sequence is the bottom prod-
uct of one column and the top product of another one, then these two columns
have to be made into one four-section column that is the same as the main col-
umn in flowsheet with prefractionator (Fig. 6.12d). This situation arises if the split
with a distributed component was chosen in one of the previous columns. Uni-
fication of two columns leads to decrease of energy and capital expenditures on
separation.

8.5.2. Examples of Sequencing and Selection

8.5.2.1. Example 1
We discuss two examples, for which presynthesis was made in the previous sec-
tion. We accept equimolar feed composition for mixture acetone(1)-benzene(2)-
chloroform(3)-toluene(4). One of the unstable nodes N − is component 1. At
the opposite face 2-3-4 of concentration tetrahedron, there is possible bottom
1
product region Reg B , but the potential product point does not get into this region
2,3,4
1
x B /∈Reg B (x B,2 = x B,3 = x B,4 = 0,333). Therefore, direct split in the first column 1 :
2,3,4
2,3,4 is feasible only with preliminary recycle. Equimolar mixture of components
2,4 can be accepted as a recycle. In this case, the necessary recycle rate is close
to minimum possible. Such a split is one of the most preferable. In the first col-
umn, we obtain pure component 1 and zeotropic mixture 2,3,4 that can be easily
separated in the second and third columns (sequences in Fig. 8.23a,b).
The following split in the first column (indirect) 1,2,3 : 4 is feasible without any
recycle because component 4 is stable node N + and face 1-2-3 is entirely filled
4
up with possible top product points Reg D . However, this face contains binary
1,2,3
azeotrope 13. Therefore, direct split 1: 2,3 with preliminary recycle of component
2 can be applied in the second column. That separates mixture 1,2,3,4 into pure
componentsinthreecolumnswithonepreliminaryrecycle(sequenceinFig.8.23c).
The last of potential splits in the first column (intermediate) 1,3 : 2,4 is feasible
2,4
because edge 1-3 is filled up with top product points Reg D and edge 2-4 is filled
1,3 1,3
up with bottom product points Reg B . However, this split is not expedient because
2,4
top product is binary mixture with azeotrope.
Splits 1 : 2,3,4 (entrainer is component 4, sequences in Fig. 8.23d,e), 1 : 2,3,4 (en-
trainer is mixture 2,4, sequences in Fig. 8.23f,g), and 1 : 2,3,4 (entrainer is mixture