Page 328 - Distillation theory

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302 Synthesis of Separation Flowsheets

exist. This rule excludes, as far as possible, the necessity of expensive methods of
separation with application of entrainers and two levels of pressure. The third rule:
sequences with recycle flows are less preferable than sequences without them. For
example, if it is possible to manage without columns of autoextractive distillation,
splits in columns with two feedings have to be excluded.
The rest of heuristic rules are less obvious. The fourth rule: sequences with
minimum number of columns are preferable. Hence, it follows that splits with
distributed components have to be excluded if there are ones without them. Cap-
ital costs at minimum number of columns are the smallest. However, energy ex-
penditures in this case are not always the smallest. For zeotropic mixtures, the
situation is the same as for azeotropic: flowsheets with prefractionator are more
profitable sometimes than without it (see Section 8.2). Therefore, the designer has
to decide him- or herself if he or she should use this rule at the stage of automatic
sequences.
The fifth rule: sharp splits with a preliminary recycle for change of feed compo-
sition are more preferable than semisharp splits without preliminary recycle. Fig-
ure 8.22 shows two possible separation flowsheets of mixture acetone(1)-benzene
(2)-chloroform(3) into pure components in the case of the bottom product point

2 x B ) 2 (
a)
x ) 2 (
1 D 3
) 0 (
x x ) 1 ( = x ) 2 (
F B F
x ) 1 ( x ) 1 ( x ) 2 ( x ) 3 (
F x F ) 0 ( F F F
x ) 1 (
B
2 x ) 3 (
B
1 3
13
x D ) 1 ( x B ) 3 ( x D ) 2 ( = x F ) 3 ( x D ) 3 (
2 x ) 2 (
b) B 1 3
x B ) 1 ( = x F ) 2 (
) 1 (
x
F ) 1 ( ) 2 (
) 0 ( x x
x x ) 0 ( F F
F F
x ) 1 (
B
2
1 3
13
x D ) 1 ( x D ) 2 (
Figure 8.22. Sequences for distillation of the acetone(1)-benzene(2)-chloro-
form(3) mixture: (a) with best semisharp distillation in the first column, (b) with
primary recycle in order that bottom of first column belong to possible bottom
1
(2)
product segment Reg (thick line).
B
2,3