Page 56 - Distillation theory

P1: FCH/FFX P2: FCH/FFX QC: VINOD/IYP T1: FCH
0521820928c02 CB644-Petlyuk-v1 June 11, 2004 17:58

30 Basic Concepts of Distillation

2
a)

x t
r )3(
x
R 1 x B(3)
x B(2)
R 2 B(1)
R = R x F
1
lim 3 x y F
x D(1)
x D(2)
D(3)
1 3
2
x t Figure 2.7. A location of product points
s )3(
b) R = R R 4 and trajectories under minimum reflux
1
lim 3 2
R = R 5 for given three-component feed x F : (a)
lim
x = = x t first class of fractionation, (b) second class
t
t
x
r )3( r 4( ) r )5(
x
x B(5) of fractionation, (c) third class of fraction-
x B(3) B(4) ation. R 1 < R 2 < R 3 < R 4 < R 5 < R 6 <
x F R 7 =∞; splits x D(1) : x B(1) at R 1 , x D(2) :
x 1
D(3)
x D(4) x B(2) at R 2 , x D(3) : x B(3) at R 3 = R , x D(4) :
lim
x D(5) x B(4) at R 4 , x D(5) : x B(5) at R 5 = R ,
2
lim
t
t
at R 6 and R 7 =∞, x and x − tear-off
s
r
points of rectifying and stripping section
1 3 trajectories.
x = x t t
t
r 7( ) s 7( ) 2 x s )6(
R = ∞ x t
c) 7 R 6 s )5(
x t
r 6( ) R = R
2
x t lim 5
x = = x
r )5(
x
B ) 5 ( B ) 6 ( B ) 7 (
x F
x = = x
x
D ) 5 ( D ) 6 ( D ) 7 (
1 3
mode, the second area of constant concentrations appears in the rectifying section
(column overhead at x 1 = 1).
With further increase of R, we immediately pass to the third class of fraction-
ation. For binary mixtures, the second class of fractionation is unavailable. The
third class of fractionation is characterized by the fact that, in the case of R in-
crease, the compositions of the separation products are not changed and the areas
of constant concentrations in feed cross-section disappear (Fig. 2.6b). In the case
of R changing, the compositions on the trays will change as well (in Fig. 2.6b,
R 6 =∞, R 6 > R 5 > R 4 > R 3 ).
We have come to an important result: the product compositions under infinite
reflux and under a significantly large finite reflux (the third class of fractionation)
are identical.