Nonlinear pH Control in a CSTR

pH control problems are charcterized by the severe nonlinearity arising from the titration curve. Given the general structure of the nonlinear dynamic model, consisting of material balances and chemical equilibria equations, a novel approach has been developed for the design of nonlinear controllers for pH processes. It consists of defining a new equivalent control objective which is linear in the states and using a linear control law in terms of this new control objective. The new control objective is interpreted physically as the Strong Acid Equivalent of the system. This approach has been developed for systems consisting of any number of acids and bases. Several methods for estimating or measuring the Strong Acid Equivalent on line are discussed. Computer simulations compare the performance of our control methodology to linear PI control for a weak acid/strong base system. The results show a dramatic improvement in control.