Page 283 - Distillation theory

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7.7 Conclusion 257

y x x B 2
D L 1
x 23
L 2
x
F
x x x
B F + E F
12

' x
D
123
1 3
13
Figure 7.17. Calculated section trajectories for ethyl
acetate(1)-ethanol(2)-water(3) heteroazeotropic dis-
tillation (ethyl acetate is entrainer).

the before-chosen liquid–liquid tie-line or until the achievement of constant
+
concentrations zone in point N . In the first case, it is accepted that the preferable
s
conditions (one section and one phase of refluxing) can be preserved, and itera-
tions by product compositions are carried out. In the second case, one proceeds to
calculation of the second section accepting x f = N − x, x is set beforehand
+
s
difference of concentrations of one of the components. If at some tray, vapor com-
position corresponding to chosen before liquid–liquid tie-line is obtained then the
number of sections should be equal to two and refluxing should be carried out by
one phase. In the opposite case, one gradually increases vapor ratio in the bottom
section proceeding to refluxing by two phases and determining optimal vapor ra-
tio and corresponding to it fraction of the second phase in the reflux. In all cases,
product compositions are defined more exactly in the process of iterations and
the optimal number of trays in each section (optimal x f ) is determined by modi-
fying the value of x (see Section 7.3). Figure 7.17 shows calculation trajectories
obtained by means of usage of this algorithm.
After design calculation of the main column and of decantor, additional
columns entering into the unit are calculated and comparison of different se-
quences is carried out.

7.7. Conclusion
The knowledge of the trajectory bundles structure at finite reflux and of their
location in the concentration simplex allowed a new class of design calculation
algorithms to be developed that guarantees a full optimal solution of the task
without the participation of the user.
At reflux bigger than minimum and at separation without distributed compo-
nents for all feasible quasisharp splits – the direct, the indirect, and the intermedi-
ateones–possiblecompositionsatthetraysadjacenttothefeedcross-sectionfrom
above x f−1 and below x f fill some segments [x f−1 ] and [x f ] located in the vicinity of