The Research On PH Advance Control Based On Strong Acid Equivalent

pH control has been a difficult and challenging problem due to its high nonlinearity, time-varying characteristics, and its sensitivity to disturbance. On the other hand, pH control has found many applications in different industries. It's very important to control pH in a definite range in these processes.As for the high nonlinearity of neutralization process, Strong Acid Equivalent (SAE) which is a weighted sum of ionic concentrations was introduced and changed the pH control model into a good approximation of linear control model. Based on this linear model, two advance process control strategies, feedforward-adaptive control and multiple models adaptive control, were adopted to control the pH of CSTR (Continuous Stirred Tank Reaction) .Feedforward-adaptive control strategy used feedforward controller to compensate the disturbances resulting from the changes of influent stream, and used feedback controller to reduce the impact of other disturbances. An in-line mixer was embedded, in which the titrating stream and influent stream are mixed before they enter into the CSTR. The output of this in-line mixer was used for the feedforward controller to compensate influent stream changes effectively. It's also used to excite persistently for the reliable estimation of process parameters. Feedforward controller was designed according to the nonlinear equation based on neutralization titration curve, while feedback controller was designed according to the linear model of pH based on SAE.Considering the frequent disturbances and pH value's susceptibility in neutralization processes, the multiple models adaptive control(MMAC) based on hybrid estimation algorithm was presented. Multiple models were used to adapt to the changes of titration curves. Fixed models were used to accelerate the transient performance, while adaptive model was used to eliminate the stable error. As for the identification of the model, hybrid estimation algorithm that combines the recursive least squares algorithm and projection algorithm was used, which can accelerates the convergence rate and improves the performance of MMAC. The model switching rule based on this parameters estimation algorithm was given in this paper.The simulation research based on the two proposed approach was done to control the pH of CSTR. Simulation results showed that the proposed methods have well performance in set-point tracking, disturbance rejection and can solve the time variant and nonlinear problems. Finally, this paper introduced the batch production which is suitable for small scale and multi-production manufacture. The implementation of recipe and phase based on DeltaV system was illuminated, and the realization of pH control based on SAE in batch process was given in this paper.