Page 271 - Distillation theory

P1: JPJ/FFX P2: JMT/FFX QC: FCH/FFX T1: FCH
0521820928c07 CB644-Petlyuk-v1 June 11, 2004 20:18

7.4 Design Calculation of Extractive Distillation Columns 245

x D
x '
D
x E
x F
−
N
m 2
x B x E
x F+E

+
x e ≡ N m x min x m S
f − 1 f − 1 s S
∞ + max
x ≡ N x f − 1
f s min
x − x f x f
e 1
1 3
13
x F
x D
Figure 7.12. Calculated section trajectories for acetone
(1)-water(2)-methanol(3) extractive distillation. E/F =
2.03, R r = 7, x F (0.6, 0.0, 0.4), the region of intermediate
3
R
section trajectories Reg w,e is shaded.
1,2
7.4.1. Three-Component Azeotropic Mixtures
In Fig. 7.12, x ∞ = N s + and coordinates of point x min tied with it by the con-
f
f −1
ditions of material balance in the feed cross-section are determined with the
1
help of Eq. (5.18). Point x min is located at line S s − N (Reg qsh,R ), and point
+
f s sep,s
3 2,3
x max is located at line S m − N (Reg qsh,R ). This determines coordinates of points
+
f −1 m sep,e
1,2
(x max sh min sh sh can be located out-
) . Along with that segment, [x f −1 ]
) and (x
f −1 lin f lin lin
side the working trajectory bundle of the intermediate section N − S m − N +
−
m
m
in Reg att of point N . The same way as for two-section columns at segments
+
m
sh
sh
[x f −1 ] and [x f ] , there are points x opt and x opt interconnected by conditions of
lin lin f −1 f
material balance for which summary number of trays in the bottom and interme-
diate sections is minimum.
As far as the joining of trajectories of the top and the intermediate sections
is concerned, the composition point at the first tray below the cross-section of
the entrainer input x e should be located quite close to the boundary element
of the concentration simplex that contains components of the top product and
of the entrainer (in Fig. 7.12 – to side 1-2). Allowable concentration of impurity
components in point x e is determined by the requirements to the purity of the top
product. Therefore, the composition in point x e is not an optimized parameter and
the composition in point x e−1 at the first tray above the cross-section of entrainer
input is determined by the conditions of material balance in this cross-section.