In an industrial environment, a plant's control strategy should be simple
enough, at least conceptually, so that everyone from the operator
to the plant manager can understand how it works. Our governing
philosophy is it is always best to utilize the simplest control system that
will achieve the desired objectives. The more complex the process, the
more desirable it is to have a simple control strategy. This view differs
radically from much of the current academic thinking about process
control, which suggests that a complex process demands complex control.
Our viewpoint is a result of many years of working on practical
plant control problems, where it is important to be able to identify
whether an operating problem has its source in the process or in the
control system.
The goals for an effective plantwide process control system include
(1) safe and smooth process operation; (2) tight control of product quality
in the face of disturbances; (3) avoidance of unsafe process conditions;
(4) a control system run in automatic, not manual, requiring minimal
operator attention; (5) rapid rate and product quality transitions; and
(6) zero unexpected environmental releases.
As illustrated in the previous chapter, the need for a plantwide control
perspective arises from three important features of integrated processes:
the effects of material recycle, of chemical component inventories,
and of energy integration. We have shown several control strategies
that highlight important general issues. However, we did not
describe how we arrived at these strategies, and many of our choices
may seem mysterious at this point. Why, for instance, did we choose