A short note on control structures for thermally coupled distillation sequences for four-component mixtures

Thermally coupled distillation represents an alternative to conventional distillation to separate multicomponent mixtures. Industrial implementation of those systems can be achieved by using the divided-wall distillation column. It has been proven that a savings of up to 30% in the total annual costs can be obtained with these complex distillation sequences. In a previous work by Cardenas et al. (Cardenas, J. C.; Hernandez, S.; Gudino-Mares, I. R.; Esparza-Hernandez, F.; Irianda-Araraujo, C. Y.; Dominguez-Lira, L. M. Analysis of Control Properties of Thermally Coupled Distillation Sequences for Four-Component Mixtures. Ind. Eng. Chem. Res. 2005, 44, 391-399), it was found that thermally coupled distillation sequences not only can provide savings in the total annual costs but also can present good dynamic responses under closed-loop operation in contrast to distillation sequences based on conventional columns. In the previous study, pairings in the control loops were established according to practical considerations, i.e., distillate composition-reflux rate, bottoms composition-reboiler heat duty, and side-stream purity-flow rate. As a result, in this work some new pairings in the control loops, obtained by using the relative gain array, were tested in order to improve the dynamic responses of the thermally coupled distillation sequences for the separation of quaternary mixtures of hydrocarbons. The results showed that the dynamic responses were improved significantly for the case of changes in the set points. As a result, savings in total annual costs can be achieved without introducing control problems.