Economic Plantwide Control: Control Structure Design for Complete Processing Plants

A chemical plant may have thousands of measurements and control loops. By the term plantwide control it is not meant the tuning and behavior of each of these loops, but rather the control philosophy of the overall plant with emphasis on the structural decisions. In practice, the control system is usually divided into several layers, separated by time scale: scheduling (weeks) , site-wide optimization (day), local optimization (hour), supervisory and economic control (minutes) and regulatory control (seconds). Such a hierchical (cascade) decomposition with layers operating on different time scale is used in the control of all real (complex) systems including biological systems and airplanes, so the issues in this section are not limited to process control. In the talk the most important issues are discussed, especially related to the choice of ”self-optimizing” variables that provide the link the control layers. Examples are given for optimal operation of a runner and distillation columns.