H∞ control of free-radical polymerisation reactors

Abstract

This paper examines the problem of controlling a continuous stirred tank reactor (CSTR) in which free-radical polymerisation of styrene takes place. The continuous polymerisation reactor under consideration is governed by a 6th order nonlinear differential equation which has three equilibrium states. These equilibrium states are corresponded to low, medium and high monomer conversion rates. However, for economical and practical reasons, it is desirable to operate the reactor at the unstable medium monomer conversion rate. In general, the problem of designing a robust nonlinear controller for a highly nonlinear system such as chemical reactors is still an ongoing research area. In this paper, it is shown that by combining a nonlinear observer design procedure, the linear H ∞ control theory and the feedback I/O linearisation technique, a robust nonlinear controller can be developed for the continuous polymerisation reactor. The performances of this controller have been evaluated through computer simulations and compared to that of a decentralised PID controller structure.