15.1 Background

This textbook has emphasized controller design, primarily for SISO control loops. Much attention has been focused on control algorithms (such as PID or IMC) and tuning [adjusting the PID parameters, or the IMC "filter factor" (l)]. Some attention has also been paid to control-loop interaction, particularly for two input–two output systems. Much of what a control or process engineer does, however, is not related to specific control algorithm and tuning details. A process engineer must be concerned about the operation of an entire process plant or operating unit, and not just individual control loops or even unit operations. The issues addressed by a process engineer are similar to those developed in Chapter 1, but on a much larger scale. At this point, the plant operating and control objectives are really inseparable, so we repeat from Chapter 1 the following topics on the development of a control strategy:

• What are the major operational objectives of the plant?

• What sensors should be paired with what manipulated inputs to form a control structure to achieve the major objectives?

• For any control valve or sensor failure, what steps will a process operator (and control system) need to take?

Much of this type of discussion occurs during the design [or "retrofit" (redesign)] stage of a process. Here, process engineers develop and review process flow sheets and process and instrumentation diagrams (P&ID). Typically, a detailed operational description of the proposed process and control strategy is developed, with consideration to all possible operating modes that can be selected by an operator, as well as to all equipment failure modes.

In this chapter, we cannot do justice to the scope of work that is needed to develop a control strategy and get an automation and control (including the plant!) strategy "up and running." We attempt to show some of the major issues in "plantwide control" by using some illustrative examples. In addition to showing examples of the many control loops required by even a simple process, we provide an overview of the various levels (hierarchy) involved in a chemical manufacturing organization.

We should note that the term plantwide control is somewhat of a misnomer. Process unit control is really a more proper term, since an entire chemical process manufacturing plant is usually composed of a large number of process units. Many of these process units contain reactors and separators with recycle. In addition, there are recycle flows between many of the process units, and the units draw from and contribute to many utility streams, such as steam and natural gas. Although process unit control is a better term, we will continue the tradition of using plantwide control as the term for regulation of a process unit.