Page 274 - Distillation theory

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248 Trajectories of the Finite Columns and Their Design Calculation

x D

x E
x
E 2
x F
x B

α
13
1 3
13 x
x F
D
Figure 7.14. Calculated section trajectories for ace-
tone (1)-water(2)-methanol(3) extractive distillation
at (L/V) m /(L/V) min = 1.3 and different E/D.The
m
little circles are compositions at trajectories tear-off
points and at feeds cross-section.

distributed component. This means that instead of segment [x min , x ] there is only
∞
f f
point x f (Fig. 7.13c). The distillation trajectory for the column under consideration
may be presented as follows:
1
x B → qS → x f ⇐⇓ x f −1 → x e ⇐⇓ x e−1 → x D
s .
Reg Reg t Reg qsh,R Reg qsh,R Reg t Reg Reg
B s sep,s sep,e e att D
The task of designing of extractive distillation columns, besides calculation of
section trajectories, includes a number of subtasks. These are the same subtasks
as for two-section columns and additional subtasks of determination of mini-
mum entrainer flow rate and of choice of design entrainer flow rate. Optimal
designing of extractive or autoextractive distillation includes optimization by two
parameters – by entrainer flow rate and by reflux number. Figure 7.14 shows in-
fluence of entrainer flow rate on section trajectories at fixed value of parameter
t
t
σ = (L/V) m /K (as is shown in Section 6.4 (L/V) min = K ).
j m j
The entrainer flow rate influences expenditures for separation not only in ex-
tractive distillation column itself, but also in the column of the entrainer recovery.
In the case of separation of a multicomponent azeotropic mixture in an autoextrac-
tive distillation column (see Chapter 8), the intermediate columns can be located
between this column and the column of autoentrainer recovery. In this case, the
flow rate of the entrainer also influences expenditures for separation in the inter-
mediate columns. In connection with the aforesaid, the necessity arises to carry