Page 74 - Distillation theory

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48 Trajectories of Distillation in Infinite Columns Under Infinite Reflux

2 2
a) b)
x D(2)
2
x D(1)
5
x F
3
x B(1)
6

4 x B(2)
x B(3)
1 3 1 3
13 13
1
Figure 3.6. (a) Product regions (shaded) under infinite reflux for given x F and different
N and D/F for ternary azeotropic mixture: line 1 − x B at N =∞ and x F2 < D/F, line 2 −
x D at N =∞ and x F2 < D/F, line 3 − x D at D/F < x F2 , line 4 − x B at D/F < x F2 + x F3 .
(b) Some product points for given x F (x D(1) and x B(1) , x D(2) and x B(2) , x D(2) and x B(3) ).

elements of concentration simplex at an arbitrary location of feed point the region
of possible product composition at sharp distillation Reg D and Reg B . This term is
widely used in sequencing.

3.3.5. Product Composition Regions for Azeotropic Three-Component Mixtures

Let’s examine three-component azeotropic mixtures with one binary azeotrope
and with two regions of distillation at infinite reflux Reg ∞ (Fig. 3.6a). There is
some region (triangle to the right of separatrix) where two points of the bottom
product corresponding to one top product point exist. This fact is explained by
the S-shape of c-lines in this region (Fig. 3.6b, points x B(2) and x B(3) ).
The main difference between the azeotropic mixtures (and also nonideal zeo-
tropic mixtures) and the ideal ones are that, to determine possible splits of an
azeotropicmixture,specialanalysisisrequired.Theavailabilityofafewdistillation
regions under the infinite reflux Reg ∞ can result in sharp separation becoming
completely impossible or in a decrease in sharp splits number. Let’s note that for
ideal mixtures the line of possible products compositions at R =∞ and N =∞ and
set feed composition goes partially inside the concentration simplex and partially
along its boundary elements. For azeotropic mixtures, this line can go along the
boundary elements of the distillation region (Fig. 3.6a, line 2).
The question about feasible splits is one of the principal questions in the distil-
lation theory. The understanding of this question was gradually transformed and
became more precise.
The original oversimplified view on feasible azeotropic mixtures splits consists
ofthefollowing:thefeedpointandproductpointshavetobelongtoonedistillation
region (x D ∈ Reg ∞ and x B ∈ Reg ∞ if x F ∈ Reg ). This view is quite accurate if
∞
the separatrix of distillation regions is linear. In a general case, at curvilinear
separatrixes, the feed point can lie in one distillation region at infinite reflux and