Page 166 - Distillation theory

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140 Distillation Trajectories and Conditions of Mixture Separability

The region of two-directed distillation (pitchfork region Reg pitch ) is located be-
tween the line of the best product compositions (pitchfork distillation boundary)
and boundary of distillation regions at infinite reflux Reg ∞ (Castillo et al., 1998;
Davydyan, Malone, & Doherty, 1997; Wahnschafft et al., 1992).
For bottom product points x B , located in this region, distillation trajectory at
some values of the parameter V/L is directed to vertex 1 and at other values, bigger
than them, it is directed to vertex 3 (Fig. 5.21b). It is connected with the fact that
for these product points reversible distillation trajectory has two branches, one of
the branches goes to vertex 1 and another branch goes to vertex 3. The point N +
small values of the parameter V/L is located at one of the branches, and at bigger
values it is located at other branches.

5.4.4. The Product Is Azeotrope

In some cases, at separation of azeotropic mixtures one of the product is bi-
nary or ternary azeotrope. In these cases, as in the case of the product being
R R
pure component, section working region Reg w,r ≡ Reg att or Reg w,s ≡ Reg att is
two dimensional. The saddle and node points are located at different trajecto-
ries of reversible distillation inside concentration triangle. Figure 5.22a shows
R
rectifying section region Reg w,r ≡ Reg 23 for binary azeotrope 23 of the mixture
att
acetone(1)-chloroform(2)-methanol(3). Figure 5.22b shows rectifying section
region Reg R ≡ Reg 123 for ternary azeotrope 123 of the mixture hexane(1)-
w,r att
methanol(2)-methylacetate(3). Azeotrope can be a separation product of the

2 S 1
x D
x t
+ rev
N
x F −
N
a)
12
x B 23
x
D
S 2
123
Figure 5.22. The rectifying section reg-
ion Reg R ≡ Reg 23 or Reg 123 (shaded)
att
att
1 3 w,r
13 for (a) the acetone(1)-chloroform(2)-
methanol(3) mixture (azeotrope 23 is
2
x D + the product), (b) the hexane(1)-metha-
N
1 nol(2)-methylacetate (3) mixture (azeo-
x F trope 123 is the product).
b)
x B
12 −
N
23
x D
S 1
+
N 2 123
S 2
1 3
13