Page 215 - Distillation theory

P1: FCH/FFX P2: FCH/FFX QC: FCH/FFX T1: FCH
0521820928c06 CB644-Petlyuk-v1 June 11, 2004 20:17

6.5 Conditions of Separability in Extractive Distillation Columns 189

x '
D

N − 1
m 2
x E
x B
x
x f-1 s S
F+E
x
2 e-1
+
N = x m S x e
m e
x e−1 x
x f-1
F
x f
x N =
+
F s f x
1 3 2,3
13
x
D
Figure 6.10. Joining of the stripping, intermediate, and rectifying sec-
tion trajectories of extractive distillation of the acetone(1)-water(2)-
methanol(3) azeotropic mixture at minimum reflux (bottom feed is the
control one); x F+E , total composition of inital feed F and entrainer E,
3
region extractive section Reg R (shaded).
w,e
1,2
Therefore, the joining of trajectories of the bottom and the intermediate sec-
tions in the mode of minimum reflux for the case when the bottom feed is the
control one is similar to that of section trajectories of two-section column at direct
split (see Section 5.6). In this mode, zones of constant concentration arise in the
bottom and in the intermediate sections. The column trajectory may be put in
brief as follows:

(2) (2) (3) (3) (3) (2) (2) (1)
x → S s → x ⇐⇓ x → S m → x e ⇐⇓ x → x
B f f −1 e−1 D .
Reg B Reg t s N s + Reg min,R Reg min,R Reg t e Reg 1 att Reg D
sep,e
sep,e
If the top feed is the control one, two variants are feasible in the mode of
minimum reflux. For the first of them, point x e−1 (composition at the tray higher
than input of entrainer) should coincide with point N and point x e (composi-
+
r
tion at the tray of the intermediate section lower than input of entrainer) as at
any mode should coincide with point N . For this variant, a zone of constant
+
m
concentration arises in the top section in the cross-section of the input of the en-
trainer, and a pseudozone of constant concentrations, caused by the sharpness of
separation but not by the value of parameter (L/V) m , arises in the intermediate
section.
For the second variant of minimum reflux mode with top control feed −
(L/V) m = K j ; that is, not a pseudozone arises in the intermediate section, but
a true zone of constant concentrations, caused by the value of the parameter
(L/V) m . In the top section, for this variant, no zone of constant concentrations