Assessment and Future Directions of Nonlinear Model Predictive ...

388 K. Naidoo et al.1. It greatly simplifies model maintenance. If a new product grade, or a new CVneeds to be added, or the process changes in some way, it is not necessaryto overhaul the composition controller.2. It allows the top tier controller to be taken off-line while still running thelower-tier controller.3. It facilitates a gas composition overshoot strategy, which significantly reducesthe grade transition time.4. It improves disturbance handling. If any of the feed composition variablesexperience a major process disturbance, the gas composition controller willimmediately take action to reject it before the polymer product quality metricsare affected. If this were not done, the controller would only respondafter the quality has moved, and usually this would only become apparentafter a laboratory result is available. The cascaded structure, similar to cascadedPID loops, effectively gives the control solution an extra degree offreedom for disturbance rejection.One potential problem with a cascaded strategy is that there is no guaranteethat the lower tier slave controller will meet the targets required by the mastercontroller. This is similar to a cascaded PID control scheme where the slave loopwill saturate when the control valve goes fully open. The implication for thepolymer APC system is that the master quality controller will continue to ramp(causing so-called wind-up), unless proper anti-windup detection is provided.The solution for the cascaded MPC system is to configure two sets of productquality predictions at the upper tier. The first set of quality predictions are basedon the concentration ratio targets (the gas concentration set-points), while thesecond set of predictions are based on the actual gas concentration process values- using the same models in each case. The latter are used to estimate any bias (oroffset) between the upper tier property models, and the actual product qualityfeedback provided by the laboratory analysis. This bias is added to both sets ofproperty predictions. The bias-updated target- based predictions (based on thegas composition set-point) are then used as feedback for the property controller.The controller will see any such bias as an unmeasured disturbance and willrespond accordingly. In addition to this, anti-windup flags are used. If the steadystate value of the lower tier controller’s CV (the gas concentration ratio) is notequal (within tolerance limits) to the steady state target value provided by theproperty controller, a wind-up flag is set to the corresponding MV in the uppertier controller. If there are other handles available, the controller will then usethose until the wind-up condition has passed.4.3 Overshoot StrategyThe cascaded control structure facilitates a safe method for implementing anovershoot strategy on the plant. Polymers qualities, especially in the case of aUNIPOL gas phase reactor (this is effectively a continuously stirred tank reactorwith a large inventory), have very slow process dynamics. During product qualitytransitions, it is essential that the closed loop responses be faster than these