Controllability analysis for the liquid-phase catalytic oxidation of toluene to benzoic acid

Abstract

The liquid-phase catalytic oxidation of toluene is the main industrial commercial process for producing benzoic acid. It is a strongly exothermic and highly nonlinear process that exhibits poor controllability characteristics with input/output multiplicity and non-minimum phase behavior. Focusing on inherent safety, this study explores the open and closed-loop controllability of this process. The framework contains two parts. First, an approach for analyzing the stability of zero dynamics of the system is used to investigate the phase behavior of the process and is selected as an open-loop indicator for controllability. Second, based on the model predictive controller, closed-loop dynamic simulation, including set-point tracking and disturbances rejection, is performed to illustrate the dynamic performance in various sub-regions with different controllability characteristics. The results of the dynamic simulation confirm everything predicted by the open-loop controllability analysis. The outcomes are expected to guide realistic industrial operation and process control system design. An attempt is made with the help of this liquid-phase oxidation process to show how to clarify and deal with the causes of the complex phenomena that arise in the operation and control of the chemical processes.