Page 206 - Distillation theory

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180 Distillation Trajectories in Infinite Complex Columns and Complexes

D

2
F 1
F 2
x F2 N + s
x F1+ F2
B S s
S r
x B

S
x m
D

1 3
x F1
Figure 6.6. Section trajectories of ideal ternary mixtures (K 1 > K 2 > K 3 )
for two-feed column with direct split 1 : 2,3.

coordinates of the stationary points that are located inside and at the boundary
elements of the concentration simplex. Such necessity exists for the intermedi-
ate section because, at nonsharp separation, part of the stationary points located
outside the concentration simplex and their coordinates cannot be determined.
The rest of the algorithm is similar to that for columns with one feed. Conditions
of joining of trajectories in cross-sections of both feeds are checked at various
2
values of the parameter (L/V) r and the value (L/V) , at which there is a joining in
r
the cross-section of control feed, is found. Both feasible cases of joining described
above (see Subsection 6.3.3) are checked. The first case corresponds to direct or
indirect split in the column with one feed, and the second case corresponds to
intermediate split.
It was shown in the work (Glanz & Stichlmair, 1997) that in some cases expenses
on separation are smaller if feed with a higher bubble temperature is brought into
the higher feed cross-section of the column. Figure 6.6 features such a case. In this
figure part of the trajectory of the intermediate section is directed into the side,
oppositethepartofthetopsectiontrajectory(controlfeedisbottom,thetrajectory
passes through point S m ). At this part of the trajectory of the intermediate section,
there is an increase of temperature at the trays of the column in the upward
direction, which is indicative of the process inverse to distillation (see Chapter 2).
The trajectory in Fig. 6.6 may be briefly described as follows:

x (1) → S r (2) → x (3) ⇓⇒ x (3) ↔ S m (3) ↔ x (3) ⇓⇒x (3) ← S s (2) ← x (2)
f −1,2
B
f,2
D
f,1
f −1,1
.
min,R
min,R
min,R
Reg D Reg r t Reg t att Reg sep,int Reg sep,int Reg sep,int N 3 + Reg t s Reg B
Therefore, the conceptual design calculation of columns with several feeds
includes the determination of the best succession of bringing in these feeds along