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6.9 Complexes of Heteroazeotropic and Heteroextractive Distillation 211

the role of extractive section. So that the section above the feed cross-section can
function as extractive one, the pseudoproduct point of this section should be
located outside the concentration triangle (see Section 6.4). For that, it is necessary
to bring it into the top cross-section of the column, besides that phase from the
decanter which is entrainer, an additional amount of this entrainer. Trajectory
tear-off of the section under consideration from the side of the concentration
t
triangle in the points x , where the component absent at this side is average in
e
volatility, is feasible only in this case. Such a process was described in the works
(Polyakova et al., 1977) for the mixture vinyl acetate(1)-methanol(2)-water(3)
(Fig. 6.17a) and for the mixture cloroform(1)-acetone(2)-water(3) (Fig. 6.17b) and
(Wahnschafft, 1997) for the mixture methanol(1)-isopropyl acetate(2)-water(3).
In these examples, water is the entrainer. The mixture chloroform(1)-acetone(2)-
water(3) with ternary saddle heteroazeotrope was investigated for the first time
in Reinders & De Minjers (1940).
Types of the mixtures at Fig. 6.17a,b are different, but for heteroextractive
distillation it is substantial that in both cases there is binary heteroazeotrope
and that at the side, where this binary heteroazeotrope is located (Reg L1−L2 ), is
t
intermediate section trajectory tear-off segment (Reg ) (i.e., the segment where
e
absent at this side component is average in volatility). For sharp heteroextractive
distillation it is also necessary that at the side where the bottom product point
t
is located there is the bottom section trajectory tear-off segment (Reg ) (i.e.,
s
the segment where it is the lightest component that is absent at this side). The
distillation trajectory looks as follows:
x B → S s → x f ⇐⇓ x f −1 → S e →y D ≈ N Haz ⇔ x L1
Reg Reg t Reg sh,R Reg sh,R Reg t Reg Reg bound
B s sep,s sep,e e vap L1−L2
We note that at heteroextractive distillation, the intermediate section trajectory
t
tear-off segment Reg can also be located outside the segment of the existence of
e
two liquid phases Reg L1−L2 . In this case, one should use a three-section column,
into top cross-section of which the reflux is brought in (i.e., the condensate of
vapor from the column that is close by composition to binary heteroazeotrope).
One of the phases from the decanter that plays the role of entrainer and additional
entrainer is brought in below in cross-section between top and extractive sections.
At such arrangement of the column, top section trajectory passes along the side
t
from the intermediate section trajectory tear-off point x to the region of the
e
existence of two liquid phases Reg L1−L2 .
The above-examined examples of various sequences and splits of three-
component mixtures with the help of heteroazeotropic and heteroextractive dis-
tillation allow to make a general conclusion about the usage of these distillation
complexes not only for three-component mixtures, but also for mixtures with big-
ger number of components.
Heteroazeotropic distillation can be effectively used in the following cases: (1)
−
inside concentration simplex there is a heteroazeotrope–unstable node x Haz = N ,
and the top product point y D is close to it (the examples are Fig. 6.16a,b,c); (2) at