Modelling and Self-Tuning Control of a Multivariable pH Neutralization Process Part I: Modelling and Multiloop Control

This paper describes the design, modelling and control of a multivariable pH neutralization process. The experimental facility consists of two stirred tank reactors with an acid-base neutralization taking place in each tank. The process has been designed as a demonstration unit for the evaluation of advanced Control strategies such as adaptive and multivariable control. The neutralization process has four controlled variables, pH and liquid level in each of the two tanks. This strong acid, strong base neutralization poses a difficult multivariable problem because each manipulated variable has a significant effect on each controlled output. This process is also highly nonlinear and time varying due to the inherent nonlinearity associated with pH control and the shifts in the titration curve that occur when the amount of buffering agent changes in an unpredictable fashion. A physical model of the neutralization process has been developed which is in good agreement with experimental step response data over a wide range of experimental conditions. A multiloop control system consisting of four PID controllers was tedious to tune and had difficult coping with changes in the amount of buffering. In a companion paper (Hall and Seborg, 1989), self-tuning control resulted in improved control especially when only the process gain was estimated on-line, assuming that the dynamics were not changing significantly.