Page 97 - Distillation theory

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3.6 Separation of Azeotropic Mixtures in Sequence of Columns 71

(bonds chain 12 → 1 → 3 → 4 → 24). Here are feasible sets of products at such
feeding: 12; 1; 3 and 4 or 12; 3; 4 and 24 or 1; 3; 4 and 24.
Let’s return to the example of industrial mixture that is a by-product of wood
pyrolysis. It is seen from the examination of product simplex Reg simp ≡ 1 → 234 →
23 → 38 → 568 → 58 → 78 → 89 → 8 that in a column sequence consisting of
eight columns this mixture can be separated into nine products. However, only
component 1(acetaldehyde) and component 8(water) can be isolated purely. For
all this, water can be isolated only partially because it is a constituent of azeotropes
38, 568, 58, 78, and 89. The rest of the seven components – 2, 3, 4, 5, 6, 7, 9 – cannot
be isolated at all by means of distillation without recycles at R =∞.

3.6. Separation of Azeotropic Mixtures in Sequence of Columns
with Recycles at R =∞ and N =∞
Azeotropic mixtures can almost never be separated completely into pure com-
ponents in the sequence of columns without recycles at R =∞ and N =∞.The
set of products of such a system of columns almost always contains not only pure
components, but also azeotropes (pseudocomponents). Mixtures, for which con-
centration simplex contains only one distillation region, are an exception. For
three-component azeotropic mixtures, the only phase diagrams of such type are
the diagram shown at Fig. 3.10b and antipodal it. Such a mixture can be separated
into two columns and into pure components. Two variants of flowsheet with direct
1 : 2,3 or indirect 1,2 : 3 split in the first column are feasible.
Other types of azeotropic mixtures can be separated into pure components
only in the sequence of columns with recycles using mode of R =∞ and N =∞.
Such possibility was for the first time shown in Balashov et al. (1970), Balashov &
Serafimov (1984) and Balashov et al. (1984) at the example of the mixture shown
in Fig. 3.6. This possibility is caused by curvature of separatrix 2-13 and by location
of feed point x F at concave side from this separatrix. Usage in the first column of
the best semisharp indirect separation: 2,13 : 1 (point x D1 lies on separatrix 2-13),
usage in the second column of direct separation 2 : 1,13, and usage in the third
column of separation 13 : 3 with recycles of azeotrope 13 in feeding of the first
column (Figs. 3.22a, b) were proposed in the above-mentioned works. Later, it was
shown (Wahnschfft, Le Redulier, & Westerberg, 1993) that in order to completely
separate the mixture it is necessary to use the recycle of component 2 and/or of
component 1. If we limit ourselves only by recycle of azeotrope 13, then point of
total feeding of the first column will move to the point 13 (Fig. 3.22a) that in its
turn leads to the necessity of increasing of the recycle, etc. As a result, complete
separation of the mixture at recycle of only azeotrope 13 cannot be achieved.
The smaller the curvature of the separatrix 2-13 and the smaller the concentra-
tion of component 2 in the point x F , the bigger is the necessary value of recycles.
Usually, the curvature of the separatrix between distillation regions is not big
for azeotropic mixtures. It leads to the necessity of bigger recycles and corre-
spondingly of bigger energetic and capital expenses. This way for separation of