Automatic Process Control

Abstract

Automatic control has long been a significant feature of the process industries, but only recently has there been any attempt to treat process control problems quantitatively. This chapter discusses automatic process control in the chemical engineering picture. It identifies and evaluates the possibilities of intelligent development of the field. The important broad concepts are presented in a general description of the nature of control. These concepts are applied to the solution of typical process control problems in an elucidation of available techniques. The theoretical developments in the field of servomechanisms and governors are generally applicable to process control problems. A great variety of measuring devices for determining the chemical composition of process flow streams is now available, and many of these instruments are being incorporated in automatic control systems. The developments that can treat control problems are dynamic characterization of processes from normal operating methods of attacking problems involving nonlinear systems; and proved techniques for sampled data systems. The analytic control problem deals with the appraisal of existing control systems. On the other hand, the synthetic problem is the design of the whole control system, including in its broadest implication the design of process, as well as the specification of, control instruments. Before the synthetic problem can be tackled intelligently, the criteria of satisfactory control must be identified. While doing process control, only a few types of control action, that is, control modes are important, namely: on-off or two-position control; proportional control; integral control or automatic reset; and derivative or rate action.