Page 164 - Distillation theory

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138 Distillation Trajectories and Conditions of Mixture Separability

2
x D
(L/V) 3
(L/V)
x F
2
(L/V)
1
x B
qS
+
N
1
x D
+
N
2
1 3
+
N 3
Figure 5.20. The trajectories of rectifying section for
quasisharp separation of the ideal mixture. (L/V) 3 =
1, (L/V) 3 > (L/V) 2 > (L/V) 1 , the region between the
reversible-distillation trajectory and the distillation tra-
jectory under infinite reflux is shaded. qS, quasistationary
point.

small. In point qS, trajectory of quasisharp distillation changes its direction: be-
tween points x D and qS it is almost parallel to the side of concentration triangle
(3) (3) +(3)
+
and between points qS and N it goes inside (x → qS r → N r ).
D
Let’s note, that distillation trajectory of three-component mixture for the set
product point is located between trajectory at infinite reflux (i.e., at L/V = 1, and
reversible distillation trajectory) (Kiva, 1976; Petlyuk & Serafimov, 1983; Wahn-
schafft et al., 1992; Castillo, Thong, & Towler, 1998).
The structure of the trajectory bundle has interesting peculiarities for qua-
sisharp distillation at non-monotonous change of the parameter L/V along re-
versible distillation trajectory (i.e., at “tangential pinch”). Figure 5.19 shows for
the mixture acetone(1)-benzene(2)-chloroform(3) dependence of the parameter
(L/V) rev on concentration of component 2 along the trajectory of reversible qua-
sisharp distillation (composition in the point x D : 0,699; 0,001; 0,300). This depen-
dence has two extremums – minimum and maximum. The point of maximum
(L/V = 0,758) corresponds to stationary saddle-node point SN inside concentra-
tion triangle (Fig. 5.18b). This stationary saddle-node point SN is a node point for
the trajectories, located closer than this point to the side of triangle, and a saddle
point for the rest of trajectories. The saddle-node point exists for the set product
composition x D at the unique value of the parameter L/V: (L/V) pinch = 0,758. At
the smaller value, there is no trajectory tear-off from the side and at the bigger
value trajectory bundle has only node point N . The section’s trajectory can be
+
(3) (3) +(3)
put in brief as follows: x → SN r → N r .
D
The structure of trajectory bundle also has interesting peculiarities at the best
nonsharp separation. In this case, trajectory bundle also has saddle-node and
node points (Shafir et al., 1984). Figure 5.21a shows for the azeotropic mixture
acetone(1)-benzene(2)-chloroform(3) the line of best bottom product (Poellmann
1
& Blass, 1994), connecting the end of possible segment Reg at side 2-3 at sharp
B
2,3