Page 125 - Distillation theory

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0521832772c04 CB644-Petlyuk-v1 June 11, 2004 17:49

4.6 Diagrams of Extractive Reversible Distillation for Three-Component Mixtures 99

In the first case, it follows from Eqs. (4.22) to (4.24), that d > 0, d < 0, x <
1 3 D1
0, x > 1 (i.e., the pseudoproduct point is located without the concentration tri-
D3
angle at the continuation of side 1-3 beyond vertex 3) (Fig. 4.19a). In the sec-

ond case, d > 0, d < 0, x > 1, x < 0 (i.e., the pseudoproduct point is located

1 3 D1 D3
without the concentration triangle at the continuation of side 1-3 beyond vertex 1)
(Fig. 4.19b).
In both cases, the trajectory tear-off point of sharp reversible distillation in
the intermediate extractive section should lie at side 1-3 and the trajectory of
intermediatesectionisaline,whichisageometriclocusofpointswherethestraight
lines passing through a given point of pseudoproduct are tangent to residue curves.
t
This trajectory reaches side 1-3 at the tear-off point x , and vertex 2 is the node
rev
N rev of the trajectory bundle at different pseudoproduct points. The location of
point x t rev and of the whole trajectory of extractive reversible distillation depends
on that of the pseudoproduct point x (i.e., on the ratio E/F between the flow

D
rates of the entrainer and the main feeding). Changing the parameter E/F, we get
the trajectory bundle of extractive reversible distillation that, for an ideal mixture,
fills up the whole concentration triangle.
We got an important result: at reversible distillation in the intermediate sec-
tion of the column of extractive or autoextractive distillation the component, which
is intermediate in the value of phase equilibrium coefficient between the compo-
nent separated as top product and the component brought in as an entrainer, is
exhausted.

4.6.2. Azeotropic Mixtures
This result also remains valid for azeotropic mixtures. A necessary condition for
exhausting of the some component in the intermediate (extractive) section at re-
versible distillation consists of the fact that the whole trajectory of intermediate
(extractive) section should be located in the region where this component is in-
termediate in phase equilibrium coefficient (in the region of reversible distillation
m
of the intermediate section Reg rev,e ). The segment of the side containing only the
component separated as top product and component brought in as an entrainer
(k)
is a boundary element of this region Reg D,E .
The same result we get if we use the process of reverse autoextractive (reex-
tractive) distillation (Kiva et al., 1983; Petlyuk, 1984; Petlyuk & Danilov, 1999),
that is, entering the lightest component in the form of vapor lower than the point
of main feeding and withdrawing pure heaviest component as bottom product
(Fig. 4.18b).
Theapplicationofextractivedistillationisofgreatpracticalimportancebecause
it ensures the possibility of sharp separation of some types of azeotropic mixtures
into zeotropic products, which is impossible in a column with one feeding. The
mixture acetone(1)-water(2)-methanol(3) is an example of this type of mixture.
Trajectories of reversible distillation of three sections of extractive distillation
column, the feeding of which is binary azeotrope acetone-methanol, the extractive