APPLICATION OF ADVANCED PH CONTROL ALGORITHMS TO CSTR SYSTEMS

Abstract

In this study, several advanced pH control algorithms developed recently were evaluated using a computer-controlled laboratory scale continuous stirred tank reactor (CSTR). The reference of performance was a digital PI controller tuned optimally. This performance was compared, in terms of peak error, settling time and integral of absolute error (IAE), with control based on strong acid equivalent, gain scheduling, nonlinear internal model control (NIMC) and robust nonlinear control law (RNCL). Strong acid equivalent Y, which forms a new linear control objective, was found to perform better than the control objective based on pH. This signifies the strong acid equivalent to be a more direct measure of the acidity of the system. A gain scheduling technique resulting in a nonlinear PI controller was applied to both Y-based and pH-based control to further improve the perfomance. The associated variable gain controller was developed based on a set of first order-dead time models of the process in such a way that the controller gain could be varied nonlinearly with change in magnitude of the controlled variable (i.e. pH or Y). NIMC was found to be too sensitive to modeling errors and under the conditions tested, led to a steady state offset. The adaptive mechanism in RNCL was able to correct this problem, and this control algorithm yielded the best performance relative to all others under the same specified operating conditions. RNCL may be system-specific but it provides the essential framework for developing adaptive, nonlinear model-based controllers for other nonlinear processes as well.