Control Performance of Thermally Coupled Distillation Sequences with Unidirectional Flows

The Petlyuk distillation system has been considered with special interest in the field of separation processes because of the high energy savings that it can provide with respect to the operation of sequences based on conventional columns. The original design of the Petlyuk structure, however, shows two interconnections that seem to affect its operational and controllability properties. For one thing, the arrangement is more complex than the common structure of the conventional distillation systems of one feed and two products with no recycle streams. Furthermore, the interconnections show a bidirectional flow of the vapor streams between the two columns of the Petlyuk arrangement, posing a major control challenge for its operation. To overcome this problem, two new structures have been suggested that use unidirectional flows of the vapor or liquid interconnecting streams. The new options have been conceptually developed to improve the controllability properties of the Petlyuk system, but no formal analysis has been conducted on this matter. In this work, a comparative analysis of the control properties of the Petlyuk column and the new arrangements with unidirectional interconnecting flows is presented. Through a singular value decomposition analysis, it is shown that the new schemes provide better theoretical controllability properties than the Petlyuk system. Closed loop tests using proportional-integral controllers were also carried out, and the results showed that, in most of the cases considered, the new arrangements improved the dynamic responses of the Petlyuk column. Such arrangements, therefore, show promising perspectives for its practical consideration.