Page 163 - Distillation theory

P1: JPJ/FFX P2: FCH/FFX QC: FCH/FFX T1: FCH
0521820928c05 CB644-Petlyuk-v1 June 11, 2004 20:15

5.4 Evolution of Section Trajectory Bundles for Three-Component Mixture 137

(L/V)
rev
0.78

L/
( V ) rev = 2 K
0.77

0.76

I

0.75

II

0.74 x 2
0 0.1 0.2 0.3 0.4 + 0.5 + 0.6
SN qsh N qsh N
sh
Figure 5.19. (L/V) rev as functions x 2 on the reversible-distillation trajectories in rectifying sec-
tion for the split 1,3 : 2 (for given x D in the tangential-pinch region Reg tang ) of the acetone(1)-
N
t
benzene(2)-chloroform(3) mixture: I, sharp separation ((L/V) t rev ≡ K = K ); II, quasisharp
2
2
N
SN
separation ((L/V) rev = (L/V) rev ).
Previously, we discussed sharp splits in the column section, which are important
theoretical abstraction useful at the solution of the main questions of optimal
designing of distillation units.
If sharp separation is feasible, then product points at real separation can be
located as much as one wants close to vertexes or sides of concentration triangle.
We call such separation “quasisharp” one. If sharp separation is not feasible (i.e.,
at supposed sharp separation product point does not get into possible product
(2) (2)
segment Reg D or Reg ), neither is “quasisharp” separation. In this case, only
B
nonsharp separation is feasible, for that there is some minimum feasible content of
admixture component or components. We call the separation at minimum feasible
content of admixture component or components “the best non-sharp” one.
Let’s discuss the structure and location of section trajectory bundles at qua-
sisharp separation (Fig. 5.20). Let product point at quasisharp separation x D be
located in the vicinity of some product point at sharp separation that lies at the
side of concentration triangle. At the set value of the parameter (L/V) 1 , distilla-
tion trajectories at sharp and quasisharp separation are close to each other and,
in particular, the stable nodes are close to each other. However, saddle point at
quasisharp separation is located outside the concentration triangle in the vicinity
of its side and inside the concentration triangle there is “quasisaddle” point qS,
in the vicinity of which the change of concentration at neighboring plates is very