Page 204 - Distillation theory

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178 Distillation Trajectories in Infinite Complex Columns and Complexes

D x '
D

2
F 1
F 2

S m
B
+
N m

1 3
Figure 6.5. Intermediate section trajectories of ideal ternary mixtures (K 1 > K 2 >
+

K 3 ) for two-feed column. x , pseudoproduct point, S m and N , stationary points of
D
m
intermediate section regions Reg R .
int
tie-lines liquid-vapor) and the points of top x D or bottom product x B , along with
that, in accordance with Eq. (6.3) can be located only in the vicinity of sides 1-2 or
2-3 in Fig. 6.3 or facets 1-2-3 or 2-3-4 in Fig. 6.4. Hence, it follows that feasible splits
for columns with one or two feeds are the same (i.e., if the flows of several feeds
are mixed before separation, we can only get the same products as in a column
with several feeds, but the energy consumption for separation will be bigger).
Figure 6.5 shows stationary points S m and N of trajectory bundles of inter-
+
m
mediate section Reg R and separatrixes of saddle stationary point S m obtained by
int
means of calculation for ideal mixture pentane(1)-hexane(2)-heptane(3) at the
composition of pseudoproduct x =−1.0; x = 1.5; x = 0.5 and at the value
D,1 D,2 D,3
of L/V = 1.2.
In contrast to nonsharp separation in the top and bottom sections, the interme-
diate section has at reversible distillation trajectory not just one node stationary
point, but there are saddle point S m and node point N m . Separatrixes of the saddle
points S m divide concentration triangle into four regions Reg R filled trajectory
int
bundles of intermediate section, one of which is the working one Reg R .
w,int
6.3.3. Control Feed at Minimum Reflux Mode

The trajectory of the intermediate section in a three-section column connects
the trajectories of the top and bottom sections, and should join them in the cross-
sections of the top and the bottom feeds (i.e., at trays above and below these cross-
sections, the material balance should be valid). In the mode of minimum reflux,
only two of three sections adjacent to one of the feeds x F1 or x F2 , called “control”