Page 56 - Design of Simple and Robust Process Plants

40 Chapter 3 Design Philosophies
2. What will be the most economical way to transport it? Like mode of
transportation, volume and frequency of shipment, this requires supply
chain optimization
Note: In principle, one has to see the transport containment as a moving inventory.
On the subject of sequential thinking, consider the selection of a sequence of dis-
tillation columns. Let us assume that distillation has been selected on good grounds
as the optimal separation technique, and that the engineer has studied and selected
the optimal sequence of distillation separations, including energy optimization. The
result is a sequence of subjacent distillation columns, but we still have to answer
questions such as:

. Do we need to separate in a sequential way?
. Can't we combine the separation in a unit? (Think about separation of three
or four components in one column either through the application of a simple
side stream/stripper or with a divided wall column).
. Can't we apply a dephlegmator? (for details, see Chapter 5)
Another example can be seen in a sequence of distillation separations, where we
may separate the first component A, followed by B and C. The overhead of the first
column is condensed, collected in a reflux drum, and then pumped to the next col-
umn as a default design (see Figure 5.32 in Chapter 5). However, the solution
might be not to condense the total overhead stream but only the reflux (avoiding the
need for a reflux drum) and to accommodate the vapor feed to the next column (Fig-
ure 5.33 in Chapter 5). This situation is an example of what is called:

ªdo, undo, redoº
or ªclimbing a hill, descending the hill, and climbing it againº.

In the above-mentioned, we brought a stream to a higher exergy level, decreased it
to a lower level, and brought it back again at a higher exergy level. Typical situations
to be avoided are: heating±cooling±reheating, pressurizing±depressurizing±pressur-

Do Undo Redo

Pressurizing - depressurizing - pressurizing
Heating cooling re-hearing and Separation - mixing - separation
Fig. 3.2. Avoid a repetition of process steps.