(545d) Control of Reactive Distillation Columns Separating Reacting Mixtures with Somewhat Unfavorable Relative Volatilities

For the separation of reacting mixtures with somewhat unfavorable relative volatilities (i.e., the reactants are the light and heaviest components and the products are the lightest and heavy ones), arranging an external recycle from the bottom to a certain stage of the reactive section onto the reactive distillation columns (RDC-ERBR) can highly improve the system steady state performance. In this article, the dynamics and control of these novel process designs separating a hypothetical reversible reaction, A + B ↔ C + D (α_C > α_A > α_D > α_B), were studied. With the arrangement of the external recycle, the severe asymmetry between the two product compositions and the most lightest reactant feed flow rate can be suppressed as compared with conventional reactive distillation columns, thereby presenting favorable influences to process dynamics and controllability. Three decentralized control systems with different manipulating modes of the external recycle flow rate (i.e., it is set to be a constant value, ratioed to B feed flow rate, and used to manipulate the B composition in the sensitive stage) were developed for RDC-ERBR to evaluate its influences to the process control. The results showed that the decentralized control system of the RDC-ERBR with the external recycle flow rate ratioed to B feed flow rate has the best closed loop control performance as compared to the other decentralized control systems of the RDC-ERBR and the CRDC.