In this work, we consider the problem of implementing fault-tolerant control on a gas-phase polyethylene reactor operated around an open-loop unstable operating point. While operation at an open-loop unstable equilibrium point is carried out to meet performance requirements, it also makes the process vulnerable to failures in the control loop, which can result in instability and loss of product/equipment. In a simplified model for the process [1,3], the process state variables available as measurements are the monomer concentration, the inert concentration and the reactor temperature. The manipulated inputs available are the catalyst feed rate, the inlet cooling water temperature to the heat exchanger, and the reactor feed temperature. Common disturbances include the coolant feed temperature and the catalyst feed rate. In addition, the reactor feed temperature, catalyst feed rate and the coolant temperature are constrained between physical bounds, giving rise to manipulated input constraints.

We implemented a hybrid systems approach for fault-tolerant control in [2], where upon occurrence of a fault, stability region-based reconfiguration is done to achieve fault-tolerant control. We further extended the work in [4] where we focussed on single actuator faults and proposed an integrated framework for fault-detection and fault-tolerant control. An application of these fault-tolerant control methods will be carried out on the polyethylene reactor to demonstrate the ability of the fault-tolerant control control designs to (1) preserve closed-loop stability in the presence of faults, (2) account for uncertainty and external disturbances, and (3) respect state constraints (such as keeping the reactor temperature above the dew points of the gases and below the melting point of the polymer) in the presence of infrequent measurements.

[1] S. A. Dadebo, M. L. Bell, P. J. Mclellan, and K. McAuley, Temperature control of industrial gas-phase polyethylene reactors, J. Proc. Contr., 7:83-95, 1997.

[2] N. H. El-Farra, A. Gani, and P. D. Christofides, Fault-tolerant control of process systems using communication networks, AIChE J., 51:1665-1682, 2005.

[3] K. B. McAuley, D. A. Macdonald, and P. J. McLellan, Effects of operating conditions on stability of gas-phase polyethylene reactors, AIChE J., 41:868-879, 1995.

[4] P. Mhaskar, A. Gani, N. H. El-Farra, and P. D. Christofides, Integrated fault detection and fault-tolerant control of process systems, In Proceedings of 16th International Federation of Automatic Control World Congress, to appear, Prague, Czech Republic, 2005.