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Stabilizing Autothermal Chemical Reactor Systems

A two-bed catalytic reactor with a free-product heat exchanger was observed to be unstable at its optimum operating point. Investigations were made of several stabilizing control systems comprising feedback manipulation of the flowrates of two streams drawn from the process feed and reinserted at downstream locations. One stream was injected at the entrance to the first reactor bed, bypassing the heat exchanger, and the other was injected as a quench between the two beds. Both single-loop output feedback and multivariable state feedback configurations incorporating integral action were investigated. Designs were effected through the Linear Quadratic approach and through analysis of characteristic Nyquist loci. The most successful configurations regulated the first bed inlet temperature by manipulating only the bypass stream. Use of the quench stream was found to be ineffectual. Both types of controllers stabilized the process, but dynamic simulations revealed a propensity of the state controller to allow temperature waves to repeatedly propagate through the system. For a sustained feed composition disturbance, the controlled process became unstable owing to a change in the number of open-loop unstable process poles. It was possible to restabilize the process by retuning the state controller, but not by retuning the output controller. However, the restabilized process was not at its optimal operating point for the new feed condition. When the controller set point was adjusted to reoptimize the process, both of the original controller designs were able to stabilize the process.