Decentralized control system design is evaluated for a basic and an internally heat-integrated generic, ideal, double-feed, two-product reactive distillation system. The internally heat-integrated reactive distillation column (HiRDC) is obtained via catalyst redistribution into the rectifying zone and feed tray relocation. For basic regulatory control, three two-point control structures with a fixed reflux rate or a fixed reflux ratio operation policy are synthesized and evaluated for their closed-loop disturbance rejection characteristics using rigorous dynamic simulations. To eliminate the purity offsets for on-target product purities, product purity composition controllers manipulating the reflux ratio and stripping tray temperature set point are implemented. Reactive tray section composition control is necessary for on-target product purity operation of HiRDC.
In this paper, the interaction of reaction and separation in an optimal design of an ideal, hypothetical, exothermic, quaternary (A + B↔C + D) reactive distillation (RD) system is explored. The relative volatilities of the components are αC:αA:αB:αD = 8:4:2:1. The closed loop dynamics of the column are evaluated for five different decentralized control structures. The prevention of the accumulation of light reactant A is crucial for successful control. A drastic improvement in the controllability is observed for a modified design of the RD system having a greater number of reactive trays as the accumulation of A is effectively prevented. The controllability improvement is observed to be control structure specific. A comprehensive understanding of the interaction of reaction and separation is very helpful for the design of RD columns of improved controllability. Application of the plant‐wide control principles in the control of RD columns is highlighted.
A feed-forward neural network (FNN) and a layered recurrent neural network (LRNN) based two composition estimators, respectively, are designed for the purpose of tight product purity control for an ideal, quaternary, hypothetical, kinetically controlled, reactive distillation (RD) column. The output variables of the considered control structure i.e. the compositions, are estimated using the chosen tray temperatures as inputs to the estimators. The performances of the estimators in the control of the column for the servo, regulatory, feed impurity disturbances and catalyst deactivation are studied. The estimator based control is found to be effective for the on-spec product purity control. One-to-one relation between the number of tray temperature measurements and their sensitivity to the accuracy of estimation is observed. Overall, the performance of LRNN is found to be superior over the FNN for the throughput manipulations tested for the more number of inputs to estimator.
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