Page 61 - Distillation theory

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2.8 Separation of Azeotropic Mixtures by Distillation Under Two Pressures 35

x = x ( p ) x = x ( p )
) 2 (
) 1 (
D Az 1 D Az 2
Figure 2.12. A sequence with recycle for separation
p 1 p 2
x of bynary azeotropic mixtures under two pressures. p 1
F
and p 2 , pressures in columns.

x ) 1 ( x B ) 2 (
B

cost-effective schemes of adiabatic distillation is the analysis of scheme of the
vapor and liquid flows between sections of the reversible distillation column.

2.8. Separation of Azeotropic Mixtures by Distillation Under Two
Pressures or Heteroazeotropic and Extractive Distillation
On many occasions, general geometric theory of distillation allows development
of flowsheets of multicomponent azeotropic mixture separation without using
such special methods as distillation under two pressures or heteroazeotropic and
extractive distillation with entrainers (i.e., with additional components injected
into the unit).
In some other cases, these methods have to be used. For example, if a binary
mixturecontainsanazeotropewhosecompositionishighlydependentonpressure,
it is possible to separate this mixture into two columns, operating under different
pressures, according to the flowsheet with recycle (Fig. 2.12).
In separating the azeotropic mixtures, we are most frequently obliged to apply
an entrainer (i.e., to introduce an additional component into the separable mixture

a) x L 1 b)
y
D
x 2
B
x x
L 2 F
x F+ E 23
x F 12
y
D
x
L 2
x
E
x B 1 x L 1 13 123 3
Figure 2.13. (a) A heteroazeotropic column with decanter, (b) the distil-
lation trajectory of heteroazeotropic column for separation of benzene(1)-
isopropil alcohol(2)-water(3) mixture (benzene-entrainer). x L1 and x L2 , two
liquid phases; x F , initial feed; x F+E , total feed into column; region of two liquid
phases is shaded.