Page 129 - Distillation theory

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4.8 Boundaries of Nonsharp Reversible Distillation 103

a) 2

D x
x t
rev
E x
1 4
y
x
' x
D
3

b) 2

' D x x
y
4
1
D x x rev
t
E x
3
Figure 4.23. Reversible intermediate section trajectories
for extractive distillation of four-component mixtures: (a)
mixture 1,2 is overhead product, and component 4 is en-
trainer; (b) component 1 is overhead product, and mixture
3,4 is entrainer. Short segments with arrows, liquid–vapor
tie-lines in arbitrary cross-sections.

the product point does not get into this segment, semisharp distillation, rather than
sharp distillation, is feasible. In this example, the boundary of semisharp reversible
distillation joining point x max at edge 2-3 with point of azeotrope 13 should exist.
B3
Reversible distillation is possible when both the bottom product point x B and
the feed point x F are located to the left of this boundary, but it is impossible
when the feed point is located to the left and the product point is located to
the right of it. Therefore, the boundary of possible separation at distillation is
not the boundary between the distillation regions at the infinite reflux joining
vertex 2 with point of azeotrope 13, but the boundary of sharp and semisharp
reversible distillation, including segment [2, x max ] at side 2-3 and boundary of
B3
semisharp distillation from point x max to point 13. Similarly, as for point x max , for
B3 B3
any point x best of the boundary of reversible distillation, the trajectory of reversible
B
distillation goes from this point to point of branching x branch , from which three
rev
branches of the trajectory go to three bottom section nodes − points 1, 3, and 13
↑ → N 1
rev
(x best → x branch → N 2 ).
B rev rev
↓ → N 13
rev