Page 214 - Design of Simple and Robust Process Plants

5.7 Simplification and Ranking per Unit Operation 199
cost. Another advantage is that this technique is able to provide sharp separations,
as long as there is a reasonable difference in volatility. The risk concerning the
design of distillation systems is rather low, and also impacts upon the development
track of new processes. The disadvantages include the high capital cost involved,
and also the high related energy costs.
Despite distillation and absorption being available for more than a century, only
during the past few decades have considerable improvements been made in this
process. The drive behind these developments has, in the main, been the energy
crises, in addition to new-found capabilities of modeling complex systems, both sta-
tically and dynamically. These improvements have resulted in:

. the large-scale application of packed towers;
. improved packing performance, and the wide application of gauze and sheet
packing;
. reactive distillation applications; and
. divided wall column applications (three-component separation) (Kaibel,
1987; Triantafafyllou and Smith, 1992; Lestak and Smith, 1993; Mutalib,
1995).
Despite such rapid advances, only the first two developments mentioned have been
applied more widely. Reactive distillation has been introduced to a degree, but is
limited to only a few specific applications, while divided wall columns are only just
beginning to be implemented in some broader area.
The development of flowsheets (see Figures 4.16±4.20 in Chapter 4) taught us
how much can be saved by combining the different distillation set-ups. Nonetheless,
it might be considered remarkable how seldom designers deviate from sequential
distillations of two-component separations, and this concept has formed the basis
behind the listing of these techniques as:
. One- and two-component separation
. Three-component separation
. Four-component separation
One- and two-components separations are ranked in order of their increasing com-
plexity (Figures 5.35 and 5.36):
1. A flash is the most simple form of vapor liquid separation, where the vapor is
enriched with the most volatile component.
2. A stripper column takes care of the purification of a liquid stream by removing
volatile components by a vapor stream.
3. A stripper column is applied for the purification of a liquid stream where the
overhead forms an heterogeneous azeotrope, which after condensing sepa-
rates into two liquid phases. The component to be removed from the feed
leaves the system, while the remaining stream is applied as reflux. Water pur-
ification from dissolved hydrocarbons is a very common application for this
type of stripper. The method can be applied to minimum boiling heteroge-
neous azeotrope removal.