Robustness of Nonlinear Decoupling in a Polypropene Reactor

Abstract

The paper discusses the robustness of internal nonlinear decoupling in order to control the reactor in an industrial polypropene plant. The present control strategy consists of single loop Pi-controllers, and the basic reactor variables which are controlled are: level in the reactor, level in the reflux accumulator and pressure in the reactor. Of these, the pressure determines the conversion. Chain length is manually regulated by changing the feed rate of hydrogen, which terminates the growing chains. It is assumed that a perfect state observer is attainable. The robustness of the decoupling strategy is compared to that of the present control strategy, as well as with that of a multivariable Pi-controller.The uncertainty description which is used accounts for possible errors in the control valves. The structured singular value gives a conservative requirement for robust stability, and indicates that 8 % static error is tolerated in each of the control valves. At the bandwidth of the controlled system, the uncertainty is ca. 40 %. This result is rather insensitive to changes in the operating point. Nonlinear decoupling maintains its desirable properties over a wide range of pressure set points.