Page 337 - Distillation theory

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8.6 Binary and Three-Component Azeotropic Mixtures 311

x
E
2 x = x ) 2 ( 1
E B
) 1 ( ) 2 (
x = x
) 1 ( B F
x 3
F + E
) 2 (
x
x ) 1 ( F
F
x D ) 1 ( x D ) 2 (
) 1 ( ) 2 (
x B x B
1 3
13 ) 1 (
x F
Figure 8.28. Phase equilibria map and sequence for extractive distilla-
tion of a binary azeotropic mixture (1,3) with a high boiling entrainer
(2); split 1 : 2,3 in the first column.

an azeotrope with a minimum of bubble temperature or the entrainer is heavy
component − component 2 and the azeotropic mixture 1,3 under separation
has an azeotrope 13 with a maximum of bubble temperature (e.g., acetone(1)-
benzene(2)-chloroform(3)) then the ternary mixture has a residue curve map like
that shown in Fig. 8.22b or its antipode, and the sequence of two columns with a
recycle has to be used for separation.
The main problem consists of the choice of a suitable entrainer that would form
a separatrix of big curvature and create a respectively large possible composition
product segment at the side of the concentration triangle. If the separatrix is
rectilinear or has a small curvature, then it is impossible or uneconomical to apply
the sequence in Fig. 8.22b.
If the entrainer − component 2 forms azeotropes with one or two components
of the azeotropic mixture 1,3 under separation (Fig. 8.29), then sequence of two
columns with recycle of binary azeotrope 12 of entrainer with one of the compo-
nents can be used for separation.

2
x D ) 1 (
x ) 2 ( = x 1
B E
12 ) 1 ( ) 2 (
x F + ) 1 ( E x ) 1 ( x F + E x F
x ) 1 ( = x ) 2 ( F
D F
x E
) 1 (
x
) 2 (
x B ) 2 (
D 3 x B
1 3
13
) 1 (
x
F
Figure 8.29. Phase equilibria map and sequence for extractive distil-
lation of a binary azeotropic mixture (1,3) with a recycle of azeotropic
entrainer (azeotrope 12); split 1,2:3inthe first column.