NONLINEAR CONTROL OF PH PROCESSES USING THE STRONG ACID EQUIVALENT

pH control problems are characterized by their severe nonlinearity as reflected in the titration curve of the process stream. Given the general structure of the nonlinear dynamic model, consisting of material balances and chemical equilibria equations, a new approach has been developed for the design of nonlinear controllers for pH processes. It consists of defining an alternative 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. A minimal order realization of the full-order model has been rigorously derived, in which the titration curve of the inlet stream appears explicitly. The strong acid equivalent is the state in the reduced model and can be calculated on line from pH measurements given a nominal titration curve of the process stream. Computer simulations evaluate the performance of this control methodology in the presence of disturbances and model uncertainty.