Page 218 - Distillation theory

P1: FCH/FFX P2: FCH/FFX QC: FCH/FFX T1: FCH
0521820928c06 CB644-Petlyuk-v1 June 11, 2004 20:17

192 Distillation Trajectories in Infinite Complex Columns and Complexes

The dimensionality of the separatrix sharp split bundle of the top section
1
2
Reg sh,R (S − S − N ) is equal to m m − m r and that of the separatrix min-reflux
+
sep,r r r r
2
bundle Reg min,R (S − N ) is equal, correspondingly, to m m − m r − 1 (if m r = 1
+
sep,r r r
1
−
then S ≡ N ,if m r = m m − 1 then the separatrix bundle degenerates into point
r r
+
N ).
r
Therefore, the condition of joining of the top section separatrix min-reflux
bundle Reg min,R with the trajectory of the intermediate section is m m − m r − 1 =
sep,r
m m − 2, that is,
m r = 1 (6.11)
and the condition of joining of the top section separatrix sharp split bundle Reg sh,R
sep,r
with the trajectory of the intermediate section is m m − m r = m m − 2, that is,
m r = 2 (6.12)
We have a considerable limitation of sharp extractive distillation process in
the column with two feeds: the process is feasible if the top product components
number is equal to one or two. This limitation arises because, in the boundary
element formed by the components of the top product and the entrainer, there
is only one point, namely, point N , that belongs to the trajectory bundle of the
+
m
intermediate section. If Eq. (6.11) is valid, then the joining of the trajectories of
the intermediate and top sections takes place as at direct split in two-section
columns in the mode of minimum reflux. If Eq. (6.12) is valid then joining goes
on as at split with one distributed component.
Finally, if m r > 2, then joining is similar to joining in two-section columns with
several distributed components (i.e., composition of pseudoproduct should meet
some limitations).
Therefore, extractive distillation at three and more components in the top prod-
uct is feasible in principle, but requires a search for an allowed composition of the
pseudoproduct. If it is necessary to design a sharp extractive distillation column
at m r = 3, then it is not allowed to set the rate of the entrainer arbitrarily, but it
is necessary to determine it from the conditions of joining of trajectories of the
top and intermediate sections. The parameter E/D ensures an additional degree
of freedom, which allows to increase by one the number of product components
in the top product m r .
The analysis carried out before for three- and four-component mixtures com-
pletely corresponds to the above-formulated general conditions of trajectories
joining for multicomponent mixtures.
The general algorithm of calculation of the minimum reflux mode for columns
of extractive distillation with two feeds requires the check-up of the conditions of
trajectories joining for the cases of bottom and top control feed and requires the
determination of the values of (L/V) min and (L/V) min . The bigger of these two
m2 m1
values corresponds to the mode of minimum reflux, and the corresponding feed
is the control one.