Evaluation of model-based control techniques for a buffered acid-based reaction system : Process operation and control

TLDR: In this article, the authors investigate the ability to control the neutralisation of an acid stream by a strong alkali stream using a model-based nonlinear transformation control technique to augment a PI controller.

Abstract: We investigate, by simulation, the ability to control the neutralisation of an acid stream by a strong alkali stream using a model-based nonlinear transformation control technique to augment a PI controller. This approach is compared with a standard PI controller and a Generic Model Control (GMC) Controller. Tuned for the same acid stream flow rate disturbances, both the GMC controller and nonlinear PI controller were able to return the effluent pH to its setpoint with an integral absolute error (IAE) approximately six times less than for a PI controller. Comparing the two model-based controllers over a range of acid stream flow disturbances the nonlinear transformation technique was able to significantly outperform the GMC controller, especially when the errors involved in estimating inputs to the controllers were considered. When comparing the nonlinear PI and GMC controllers with a buffer added to the acid stream it was shown that the performance of the controllers was intimately connected with the ability to accurately measure the inputs and outputs of the pH process; these being the buffer concentrations, strong acid/base concentrations, and flow rates of the effluent and influent streams. Of the two controllers the GMC controller was most sensitive to measurement errors. Without accurate knowledge of the inputs and outputs to the process the performance of the GMC controller declined considerably for positive flow rate disturbances, and was unstable for most negative flow rates. The nonlinear PI controller, however, was able to successfully control the process for most disturbances with only a moderate decline in performance. These results have shown that it is indeed possible to successfully control an acid-base neutralisation process with a simple model-based nonlinear transformation applied to a PI controller, and that the more complex model-based control techniques, as represented by GMC control may not, in practice, be appropriate for this problem. The next stage will be to evaluate the performance of the nonlinear transformation technique on more complex chemical engineering control problems