Page 54 - Distillation theory

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

28 Basic Concepts of Distillation

2

a) α N r +
23

S
r
Figure 2.5. Trajectory bundles under finite reflux of ace-
1 N + α 23 13 3 tone(1)-benzene(2)-chloroform(3) azeotropic mixture for
x ≡ N − r (a) rectifying and (b) stripping section. Solid lines with ar-
D r
rows, trajectories; solid line, α-line; dotty line, separatrix un-
2
der infinite reflux; big circles, stationary points under infinite
reflux; little circles, stationary points under finite reflux.
b) x B
S s
N + s

1 3
13

The concentration profile of the rectifying section under reflux R and an over-
head product composition x iD will be represented by broken lines, the lengths of
which come through points x ij , which are found by means of solving Eqs. (2.3)
and (2.5).
In a similar way, with the help of the Eq. (2.3) and the equation of the material
balance:

x j+1 = [S/(S + 1)]y j + [1/(S + 1)]x B (2.7)
we create a trajectory for a stripping section.
Just as in the case of the infinite reflux, the broken lines can be replaced by
the continuous curves. The distillation trajectories under the finite reflux, first, are
different for two column sections and, second, have the composition points of the
corresponding product (x iD or x iB ) as parameters as well as reflux ratio or reboil
ratio (R or S).
The trajectory bundles of the rectifying and stripping sections for the azeotrope
mixture: acetone(1)-benzene(2)-chloroform(3) under R = 2.5, S = 1.4 are illus-
trated in Figs. 2.5a and 2.5b, respectively, while the product compositions are
x 1D = 1, x 2D = 0, x 3D = 0, and x 1B = 0, x 2B = 0.85, x 3B = 0.15.
The trajectories of Figs. 2.5a and 2.5b were constructed in the following way:
in the case of fixed R and x D or S and x B , an arbitrary point in the triangle x was
chosen and the calculation was performed from this point to bottom in accordance
with Eqs. (2.3) and (2.5) for the rectifying section and from this point to top in
accordance with the Eqs. (2.3) and (2.7) for the stripping section.
The trajectory starting in product point x D or x B and ending in the point cor-
responding to the feed tray is the only one of the whole bundle. It is the profile of
concentrations of the column section.