Time-optimal control of dividing-wall distillation columns

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Ind. Eng. Chem. Res. 2010, 49, 9195–9208


Time-Optimal Control of Dividing-Wall Distillation Columns
Alexandru Woinaroschy* and Raluca Isopescu
Department of Chemical Engineering, “Politehnica” UniVersity of Bucharest, 1-5 Polizu Street, Bucharest, Romania

Time-optimal control of startup traditional distillation columns by iterative programming proposed by Woinaroschy for ideal[Ind. Eng. Chem. Res. 2008, 47, 4158] and nonideal mixtures [Ind. Eng. Chem. Res. 2009, 48, 3873] is extended to the case of dividing-wall distillation columns. The minimization of distillation startup time is performed by iterative dynamic programming employing randomly chosen candidates for admissible control. The control variables are the reflux ratio, the reboiler heat duty, and the side-draw flowrate. The dynamic distillation model proposed by the author in the previous papers is applied. Two illustrative case studies for the separation in a dividing-wall column with sieve trays and lateral downcomers are presented as follows: the separation of an ideal benzene-toluene-ethylbenzene ternary mixture and the separation of a nonideal methanol-ethanol-1-propanol mixture. In another case study,a conventional two-column system is presented in comparison to the dividing-wall column. As in the cases of traditional distillation columns, the startup time decrease and the corresponding reboiler energy savings are significant for each of the control variables.
Introduction Distillation is the most commonly used separation process in chemical and petrochemical plants. Its main disadvantage ishigh energy consumption, and this is the reason why thermally coupled distillation systems are studied to provide challenging solutions for industrial implementations. The dividing-wall distillation column (DWC), which represents the Petlyuk column built in a single shell, is a very promising alternative for both energy and cost savings. Several theoretical studies have reported important energysavings in thermally coupled columns, including the DWC, as compared to classical separation schemes.1-5 Some practical applications of this technology are also reported, underlying the thermal efficiency and cost reduction.6-8 The most important parameters that are mentioned to influence the energy consumption in a DWC are the feed composition and pressure.8 The DWC led to about 40% energy savings,and the efficiency of DWC increases when the middle component is in a large amount in the feed.9 A case study for a common hydrocarbon mixture separation in oil refiners demonstrates that the heat transfer units reduction and the use of a single column shell provide a decrease up to 23% of the capital cost as compared to a classical two-column separation sequence.10 Despite the advantages of theDWC, the industry is still reluctant to introduce it on a large scale due to not yet wellestablished design procedures and fear of control issues.5 The design of a full thermally coupled column can not follow the conventional multicomponent design procedures when information about interlinking streams is unknown.11 A design procedure based on a three-column model was developed by Triantafyllou andSmith.12 In their model, the final structure has to be iteratively adjusted, as the number of trays on both sides of the dividing wall must be the same. This problem also arises when a commercial simulator is used to establish the final column topology. A structural design methodology for full thermally coupled distillation columns is developed by Kim,11 based on tray to tray calculations, assumingvery large reflux ratios in the main column and ideal trays. From an operational
* To whom correspondence should be addressed. Tel: +40-214023902. Fax: +40-21-3185900. E-mail: A_WOINAROSCHY@ chim.upb.ro.

point of view, the thermally coupled columns seem to raise more problems than simple distillation columns. The operational complexity of a DWC derives from the increased number of freedom...