Solution to Lestak’s Heat Transfer Problem for Dividing Wall Columns and Implications for Practical Operation
Ehlers, Christoph
Schroder, Moritz
Fieg, Georg
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Ehlers C., Schroder M., Fieg G., 2018, Solution to Lestak’s Heat Transfer Problem for Dividing Wall Columns and Implications for Practical Operation, Chemical Engineering Transactions, 69, 463-468.
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Dividing wall columns (DWCs) are distillation columns containing a vertical inner wall in the area of the feed and the side product stage. Thereby, three high-purity fractions can be obtained in one single column. For the same task, two conventional distillation columns are needed. In many cases, DWCs thus have the potential to reduce investment costs. At the same time, it is also possible to save energy compared to a sequence of conventional columns. These benefits, however, can only be realized if the column is designed and operated in a sensible manner. If not, small internal changes of heat and material streams often cause a significant increase of energy demand. A famous paper about this topic was published by Lestak et al. (1994). The authors show simulation results indicating that small heat streams across the inner wall might strongly affect the energy demand of the DWC. A detailed explanation for Lestak’s observation, however, was missing for many years. Ehlers et al. (2015) showed that the high sensitivity observed by Lestak et al. (1994) was due to resulting changes of the internal component splits in the prefractionator of the column. The authors also showed that such sensitivity can effectively be prevented with an additional temperature control (Ehlers et al., 2015). The first part of this paper shows how these findings help to gain a precise understanding of the internal processes inside dividing wall columns. Based on that, the second part focuses on the practical operation of DWCs. It is shown how a suitable control concept will allow to run the column in an energy-efficient manner even if different input parameters are not known exactly. Such a control concept is very important for industrial processes, especially when high-purity products are required. Combining simulation results with practical issues in industry, this paper aims to give relevant insights into this special type of distillation column.
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