Application of a model reference adaptive control system to pH control. Effects of lag and delay time

A model reference adaptive control system was used for the pH neutralization of acidic wastewater containing acetic and propionic acids with sodium hydroxide. Buffering changes were made by including sulfuric and common-ion salts. The experimental setup consists of a continuously stirred tank reactor implemented with a pH electrode, control valve, and on-line computer. A numerical simulation was made to study the influence of the model parameters and the effects of factors such as lag, delay time, valve dead band, dead zones, short cuts, and presence of dissolved CO2. The tuning methodology and the effects of lag and delay were validated experimentally. For short delays (<4 s) and moderate lags (<5 s), the controller can be tuned easily by adjusting two model parameters (a delay, R, and a gain, beta(A)). Once tuned, the controller has sufficient robustness and adaptive performance against buffering changes and other perturbations. Nevertheless, for long delays (>10 s), the controller has important limitations.