A methodology for screening level control structures in plantwide control systems

Abstract

In developing plantwide control structures, the very lowest loops, such as flow and level loops, are typically implemented using PID controllers. Here a general methodology for assessing the viability of plantwide level control strategies is presented. The methodology can be used as a screening tool to help direct a designer to plantwide control architectures that are promising. The approach is based on using steady-state gain matrices that include the rate of change of integrating variables, such as liquid levels. The relative gain and Niederlinski index are used to assess potential level control strategies. The methodology is applied to the Tennessee Eastman process, and it is shown that the vast majority of potential level control strategies are problematic in this plant. It is also shown that only two of the three levels in the process are integrating in nature. Insight into why the Tennessee Eastman process is so difficult to control is gained through the methodology.