Page 182 - Distillation theory

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

156 Distillation Trajectories and Conditions of Mixture Separability

Figure 5.33. The joining of section trajectories under min-
imum reflux (thick lines with arrows) for the intermedi-
ate split of the ideal mixture. A progressive change in
3,4
2
+
position of rectifying Reg min,R ≡ S → N and stripping
sep,r
r
r
1,2
1,2
2
Reg min,R ≡ S → N lines (thin lines) as L/V increases.
+
sep,s
s
s
3,4
To make the calculation of (L/V) min accurate, one must take into considera-
r
tion the concentration of these components when determining the coordinates of
2
3
3
2
+
+
points S , S ,... N , S , S ,... N . At high product purities, one can determine
r
r
r
s
s
s
the coordinates of all stationary points for sharp separation. The column trajec-
tory at nonsharp separation, the calculation direction taken into consideration,
may be presented in the following brief form:
(k+1) 1(k+1) 2(k+1) (n) (n) 2(n−k+1)
x → qS → S → x ⇓⇒ x ← S
D r r f −1 f s
Reg Reg t Reg t min,R min,R Reg t
D r r Reg Reg s
sep,r sep,s
← qS 1(n−k+1) ← x (n−k+1)
s B
Reg t Reg
s B
3,4 1,2
2
+
Figure 5.33 shows movement of the lines Reg min,R ≡ S − N and Reg min,R ≡
sep,r r r sep,s
1,2 3,4
2
S − N at the increase of the parameter (L/V) r for separation of the equimolar
+
s s
mixture pentane(1)-hexane(2)- heptane(3)-octane(4) at intermediate split 1,2 :
3,4. Found values of (L/V) min are after the first stage 0,468, after the second stage
r
0,471 (for comparison at “criterion of the smallest angle” [Koehler et al., 1991] it
is 0,463).
The algorithm of calculation of minimum reflux mode at tangential pinch has
some peculiarities. At tangential pinch in top section (L/V) min pinch =
r = (L/V) r
max(L/V) rev along reversible distillation trajectory (see Fig. 5.19), if along with
2
2
that there is intersection of separatrix bundles S − N + and x F − S − N + or
r r s s
2 + 2 + pinch
S − N and x F − S − N . If there is no such intersection at (L/V) r , then
s s r r
general algorithm, described above, is used. In this case, the phenomenon of