Plantwide control of a benchmark bleach plant

This paper presents a study on the plantwide control of a benchmark pulping process. Control structure determination was based on Relative Gain Array (RGA) and Relative Disturbance Gain (RGD) analysis. RGA and RDG were calculated based process a multiple-input multiple-output (MIMO) process transfer model identified from open-loop tests. Both static and dynamic RGA and RDG were calculated and analysed. These analyses confirmed the setting provided by the authors of the Benchmark, based on static RGA. Controllers in the individual control loops were tuned using Internal Model Control (IMC) tuning methodologies and the best set of controller parameters was chosen by evaluating the control system performance for set-point tracking and disturbance rejection in terms of the Integral of Absolute Error (IAE), setting time, time constant and percentage of overshoot. The Kappa factor, a key quality variable, was controlled by PI controllers combined with Smith-predictors and tuned with IMC. PI-only controllers tuned using IMC were used to control the secondary variables. With Smith predictors larger controller gains and smaller reset times can be used leading to faster control response. The proposed control strategy is successfully implemented on the benchmark simulation.