Page 229 - Design of Simple and Robust Process Plants

214 Chapter 5 Process Simplification and Intensification Techniques
used in the design of the automation, the control software, and for operation sup-
port. New designs are often not more complicated, neither are ªHigeeº devices espe-
cially complex, and only need to be understood in their specific situation.
A second point for debate is whether, in integrated design, the number of DOFs for
operation are really increasing. For most integrated units, the DOFs are less than the
summation of the separate units. In addition, the complexity factor (as defined in
Section 2.2 in Chapter 2) is less for an integrated unit compared with summation
for the separate units.
If we consider the reverse-flow reactor, a separate reactor with its heat train would
certainly have a complex heat exchanger network in order to obey plug flow. It
would also require similar provisions for starving the reactor, or to protect against its
overheating. However, examination of a standard heat exchanger network with its
control functions for such a system would increase the DOFs to some extent. A reac-
tive distillation and a divided wall column also has fewer DOFs and a lower complex-
ity than the sum of the separate units.
What will differ is the design of the control configuration. Knowledge concerning
the unit, as captured in dynamic models, needs to be used to design a robust control
configuration.
Unfortunately, the above do not always lead initially to simplicity. When we re-
examine the design of the tubular reactor for oxidation (Figure 5.26), it becomes
clear that the introduction of more injection points adds both to the DOFs and to
the complexity. It should be recalled from the complexity term that this can be
impacted by automation. The increased complexity of the reactor system had to be
compensated for by improved control (Turunen et al., 1999).

Summary
The question of there is a contradiction between simplification and further integra-
tion of units can be discussed from two different viewpoints:
Understanding of the units is, and will always be, essential for adequate opera-
tion. From this perspective there is no great difference between new and older tech-
niques, and each unit has its specific characteristics which need to be understood.
The difference is that new designs must be initiated and operated based on knowl-
edge ± failure is no longer acceptable. In the past, older unit operations were often
started and operated based on learning experiences, not today the only way to gain
sufficient operational insight of a unit is through dynamic simulation. The model-
ing for this purpose does not require a high level of accuracy, but it does need to
include simplified reaction kinetics.
The number of operational DOFs for integrated units is not, in general, increas-
ing with respect to nonintegrated units. Indeed, most applications now show a
reduction both in DOFs and complexity of operation. Some applications show an
increase in those situations, an increased level of automation, and control in these
cases is the requirement to cope with simple and robust process operational require-
ments. The higher level of automation and control must be supported by dynamic
simulations and control design.