Submitted to Ind.Eng.Chem.Res

The paper addresses the selection of controlled variables, that is, “what should we control”. The concept of self-optimizing control provides a systematic tool for this, and in the paper we show how it may be applied to the Tennessee Eastman process which has a very large number of candidate variables. In the paper we present a systematic procedure for reducing the number of alternatives. One step is to eliminate variables which with constant setpoints result in large losses or infeasibility when there are disturbances (with the remaining degrees of freedom reoptimized). The following controlled variables are recommended for this process: Optimally constrained variables: Reactor level (minimum), reactor pressure (maximum), compressor recy-cle valve (closed), stripper steam valve (closed) and agitator speed (maximum). Unconstrained variables with good self-optimizing properties: 1) Reactor temperature, 2) composition of C in purge and 3) recycle flow or compressor work. The feasibility of this choice is confirmed by simulations. A common suggestion is to control the composition of inerts. However, this seems to be a poor choice for this process because disturbances or implementation error may give infeasibility