Page 192 - Design of Simple and Robust Process Plants

5.6 Overall Process Simplification 177
5.6.1
Overall Process Design Improvements

In order to achieve simplification of the overall process, a careful examination must
be made of the overall process design improvements. These simplifications may be
found not only in the overall process flow diagram, but also in the finer details
which might, in fact, have a greater impact. The techniques mentioned in the pre-
vious sections which have been directed at specific units clearly contribute to an
improved design. However, they also need to be judged from an overall process per-
spective, and therefore they will be referred to here in a wider context.
Process synthesis requires the efficient selection, sequencing, and integration of
the different reaction and separation units, as was described in Chapter 4. This is a
very difficult task, as can be concluded from the options for simplification described
in this chapter. It is important to understand that the alternatives shown represent
only the ªtip of the icebergº. Here, an improvement in a steam system serves as an
illustration of how simplification can still be effective in a highly conventional pro-
cess. A conventional system (Figure 5.28) has a de-aerator and a slightly over-pres-
sured steam drum/tank which supplies the steam for de-aeration, followed by
chemical dosing to remove remaining oxygen. The system loses some water through
the vent, as well as an amount for process steam injection and condensate usage.
This flowsheet represents an accumulation of effects resulting in water usage,
energy losses, chemical dosing, and water pretreatment for water losses.
The alternative (Figure 5.29) is based on the prevention of steam or condensate
injection for process reasons. A low-pressure condensate flash drum is provided
where the flash steam is used by a process consumer. With these modifications, the
make-up is absolutely minimized, and so is the introduction of oxygen. As a result,
the de-aerator and condensate pump can be removed, and the chemical dosing
required is minimal as no oxygen enters the system. The ultimate result is less
energy losses, lower capital and operational costs, and minimal water treatment
costs. This is a typical overall simple and robust design improvement.
The overall process design might be examined from different perspectives in
order to identify opportunities for improvements. Indeed, the following pathways
might be explored to improve the overall design:
. Operation at 100% conversion
. Additives and supplies
. Prevention of waste streams
. Adiabatic operation of reactors
. Combination and coupling of functions (see Section 5.3.2 for coupled distilla-
tion separations)
. Energy integration.