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In this article we explore the application of linear PI cascade control schemes to improve the performance of industrial PI/PID controllers for controlling outlet reactor concentration. By departing from simple I/O first-order dynamical models obtained from step responses, it is shown that the incorporation of a secondary loop for regulating the reactor temperature at a given interior position significantly improves the control performance in the face of feed composition and temperature disturbances. The effects of the temperature sensor location and the usage of multiple temperature measurements are also evaluated.

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Internal model control (IMC) tuning rules have proven to yield acceptable performance and robustness properties when used in the control of typical processes (e.g., distillation columns, chemical reactors). In general, analytical IMC tuning rules are derived for proportional-integral (PI)/proportional-integral-derivative compensators by matching an approximate process model to a low-dimensional reference model. In the case of time-delay processes, an approximate model is obtained by taking a finite-dimensional approximation to the delay operator by means of Pade or Taylor expansions. For some typical cases arising commonly in process control, including first-order plus time-delay plants, this paper studies the optimality of PI-IMC tuning rules to match the prescribed closed-loop behavior (i.e., the reference model response). To this end, optimal PI settings are computed by means of numerical optimization based on random search algorithms. Small deviations of IMC tuning from optimality are found for moderate time delays. However, significant deviations are displayed for large time delays, which motivate the use of tuning techniques based on numerical optimization to refine IMC settings.

The intent of this note is to show that the incorporation of a temperature measurement in the stripping section improves the performance of distillate cascade controller. The proposed controller regulates the distillate composition by manipulating the reflux flow rate, based on a primary component, which is driven by the distillate composition measurement, and a secondary component, which is driven by two temperature measurements located at the most sensitive trays of the stripping and rectifying sections. Compared to the standard single-temperature cascade control scheme, the proposed two-temperature cascade controller has better behavior, because of the improvement of the feed-forward-like disturbance rejection capability of the secondary control component. Numerical simulations are used to illustrate the performance of the control scheme in the face of feed flow and composition disturbances.