2.1 Introduction
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. The
structural decision include the selection/placement of manipulators and measurements as
well as the decomposition of the overall problem into smaller subproblems (the control configuration).
In practice, the control system is usually divided into several layers. Typically, layers
include scheduling (weeks), site-wide optimization (day), local optimization (hour), supervisory/
predictive control (minutes) and regulatory control (seconds)