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
to use fresh liquid reactant feed streams in the control ofliquid inventories?
What prompted us to have a reactor composition analyzer? Why
were we concerned with a single direct handle to set production rate?
In this chapter we outline the nine basic steps of a general heuristic
plantwide control design procedure (Luyben et aL, 1997). After some
preliminary discussion of the fundamentals on which this procedure
is based, we outline each step in general terms. We also summarize
our justification for the sequence of steps. The method is illustrated in
applications to four industrial process examples in Part 3.
The procedure essentially decomposes the plantwide control problem
into various levels. It forces us to focus on the unique features and
issues associated with a control strategy for an entire plant. We highlighted
some of these questions in Chap. 1 in discussing the HDA
process. How do we manage energy? How is production rate controlled?
How do we control product quality? How do we determine the amounts
of fresh reactants to add?
Our plantwide control design procedure (Fig. 3.1) satisfies the two
fundamental chemical engineering principles, namely the overall conservation
of energy and mass. Additionally, the procedure accounts for
nonconserved entities within a plant such as chemical components
(produced and consumed) and entropy (produced). In fact, five of the
nine steps deal with plantwide control issues that would not be addressed
by simply combining the control systems from all ofthe individual
unit operations.
Steps 1 and 2 establish the objectives of the control system and the
available degrees of freedom. Step 3 ensures that any production of
heat (entropy) within the process is properly dissipated and that the
propagation of thermal disturbances is prevented. In Steps 4 and 5 we