SUDDEN DESTABILIZATION OF CONTROLLED CHEMICAL PROCESSES

Abstract

Input multiplicity occurs when more than one set of inputs can produce the same set of outputs. Input multiple steady states are divided into compatible steady states having process gains of similar sign, and opposed steady stales with process gains of opposite sign. For a system controlled with reset action, only the compatible steady states satisfy the necessary condition for stability. Any disturbance which drives the controlled system from the designed steady state to a less stable or unstable compatible steady state can cause sudden destabilization of the process. Several examples are given of the possible types of behavior resulting from this phenomenon.

Multiple steady states also occur for systems with proportional controllers. For single-input-single-output systems with continuous process characteristics, whether or not reset action is used, two steady states positioned next to each other cannot both be stable under closed-loop control. However, under proportional control, opposed steady states for which 1 + K_cK_p is positive can be stable.

KEYWORDS:

• Input multiplicity
• Compatible and opposed steady states
• Attainability

Additional information

Notes on contributors

SACHINDRA K. DASH

To whom all correspondence must be made. Present address: ICOTRON, 17602 North Black Canyon Highway, Suite 106, Phoenix, Arizona 85023.

LOWELL B. KOPPEL

Present address: SETPOINT, Inc., 14701 St. Mary's Lane, Houston, Texas 77079.