This paper presents a new approach to construct controlled variables(CVs)for chemical processes.The necessary conditions of optimality(NCOs)are selected as CVs

thus the process is maintained at the optimum operation point under the actions of feedback controllers.However

some NCOs are online immeasurable

so the nonlinear function relationships between measured variables and unmeasured NCOs are found via neural network

and select the functions as CVs.An exothermic reactor example illustrates that the proposed method is able to achieve approximate optimal control.