Page 228 - Design of Simple and Robust Process Plants

5.8 Contradiction between Simplification and Integrated Designs? 213
5.8
Contradiction between Simplification and Integrated Designs?

The consideration of whether combining functions into a single unit is in fact a con-
tradiction of the term ªsimplificationº, has been the focal point of debate on more
than one occasion. However, there are two elements which might be added to this
argument and help to solve this apparent contradiction:
1. Understanding of the characteristics of the unit.
2. Are the number of DOFs for operation really increasing?
Nobody would argue against a need to understand the characteristics of a unit in terms
of its operation, but to put this into perspective it may help to examine some well-
known units and focus on their problem areas.
Although a pump ± as every operator knows ± can fail as a result of cavitation and
dead heading, it is not always realized that the pump's characteristics are influenced
by variations in its speed of operation. Likewise, for an adsorption unit, it is known
that there is an adsorption isotherm applicable to the specific components to be
removed, and that this is dependent upon the componentsº concentration(s). So,
does the operator realize the consequences when the inlet concentration falls to a
much lower concentration! And will a new equilibrium situation develop in the bed,
perhaps resulting in a spontaneous breakthrough?
Our global understanding of a distillation column may be more obvious, but is it
always realized that a temperature deviation occurring in the column might be
caused by the introduction of components in the feed, and which might accumulate
in the column? Initially, such an effect is not visible because the temperature is often
not pressure corrected. Usually, the temperature is likely to be controlled, and so the
column must first go off-specification before a problem is noticed. These are just
some examples of problems that might occur among well-known unit operations.
For new unit operations, it is essential that the operation is fully understood,
especially when there have been deviations from stationary conditions. The only
effective way to achieve this is through dynamic simulation of the unit. Simple and
robust process plants must be fully understood in order to permit automation,
hands-off control, and adequate operational supervision. The modeling accuracy is
not essential, although simplified reaction kinetics are often a requirement for this
purpose, as long as all trends are representative of the operation.
If we examine reverse-flow reactors for catalytic combustion, the frequency of the
reverse-flow actions has an effect on the reactor's performance, so must be under-
stood. In addition, when the inlet concentration falls or rises, the reactor may either
starve or over-heat ± conditions which, again, require close examination and under-
standing. Although the design must accommodate for these situations, the operator
must be aware of these potential problems and know how to react to their occurrence.
The same applies for units such as reactive distillation or divided wall columns.
Gone are the days when the operator's experience of a unit was built up ªon the jobº-
that is, when the process was itself in operation. Today, it is vital that we have a
thorough understanding of a unit before start-up, and this knowledge needs to be