(596b) Energy-Saving Design of a Dividing-Wall Heterogeneous Azeotropic Column for the Separation of Pyridine and Water

Design and control of a heterogeneous azeotropic distillation system for the separation of pyridine and water have been studied in Wu and Chien (2009). By adding an entrainer (toluene) into the system, two additional azeotropes are formed. Because of the minimum-boiling heterogenous azeotrope of toluene and water, a simple two-column heterogeneous azeotropic column system is devised and can be used to other separation systems with the same residue curve map (RCM) type. In this paper, a further intensified dividing-wall column is proposed to thermally-coupled the two columns to reduce the overall reboiler duty. The design of the dividing-wall column is more difficult than a conventional two-column sequence because a decanter exists in the system and also because one of the original columns is served as pre-concentrator column for the fresh feed and also served as recovery column for the aqueous outlet.

This complex two-column heterogeneous azeotropic distillation system can be thermally-coupled into a single lower dividing-wall column. A decanter is present at the top of the dividing-wall column with the organic phase totally refluxed back to the column and the aqueous outlet stream combining with fresh feed is entered into the right-bottom section of the dividing-wall column. To clearly demonstrate the energy-saving potential of the proposed dividing-wall design, the total stages of the original two columns are preserved in the new design. Only liquid splits into left-bottom or right-bottom section of the dividing-wall column and also the feed location are optimized. The result shows that a significant 29.44% reduction of the total reboiler duty can be obtained. By further considering to change the feed location to the decanter, an additional 5.55% energy reduction can be realized.

Operation and control of this process intensified design is also investigated in this paper. Previous study (Wu, et al., 2013) showed that the control performance of an extractive dividing-wall column is hamperd because of lossing one important control degree-of-freedom. However, because of this proposed design having a dividing wall at bottom part of the column, the two important control degree-of-freedom (two reboiler duties) are still present. Dynamic simulations of the proposed control strategy under feed flow rate and feed composition disturbances will be shown in this paper. 

References:

Wu, Y. C.; Chien, I. L. Design and Control of Heterogeneous Azeotropic Column System for the Separation of Pyridine and Water. Ind. Eng. Chem. Res. 2009, 48, 10564-10576.

Wu, Y. C.; Hsu, P. H. C.; Chien, I. L. Critical Assessment of the Energy-Saving Potential of an Extractive Dividing-Wall Column. Ind. Eng. Chem. Res. 2013, 52, 5384-5399.