Page 172 - Design of Simple and Robust Process Plants

5.3 Combination of Process Functions 157

Vent

CW

Fig. 5.9. Reactor with reflux condenser and dephlegmator for inert removal.

given separation, while the sub-cooling of the returning condensate is minimal. The
combination of functions in this apparatus means that rectification, cooling and the
use of a reflux pump might be avoided. The disadvantage is that the separation is
mostly limited to only three theoretical stages, with application of a rectifying col-
umn this limitation is avoided.
The application of side draws, divided wall columns and dephlegmators can lead
to large savings in costs. The evolution of a distillation train which has taken
advantage of divided wall columns and dephlegmator techniques is shown in
Figures 4.16±4.20 in Chapter 4, which illustrate the replacement of five distillation
columns by two divided wall columns and a dephlegmator. The options for distilla-
tion of one to three purified streams in one unit is summarized in Table 5.2. (For
ranking of distillation separations with increasing complexity up to four streams,
see Table 5.9.)

Table 5.2. Distillation for one, two and three purified streams in one-column configuration.

Distillation No. of purified streams
Flash 0
Stripper 1
Absorber 1
Dephlegmator 1
Simple distillation 2
Distillation with side draw 2 pure, 1 impure
Distillation with purified side draw, 3
(side stripper or dephlegmator/rectifier)
Divided wall column 3
Direct coupled distillation 3