(400ag) Control of Stacked Side-Stream Distillation Sequences

Global climate change is perhaps the greatest environmental challenge of the 21^st century. Distillation has been estimated to consume three percent of all energy produced by humans each year [1]. While alternatives to distillation such as membranes have garnered increased attention, distillation remains by far the most widely-used method for separating and purifying mixtures of volatile substances. Accordingly, strategies for reducing energy consumption in distillation processes by energy intensification have also received a great deal of attention [2].

Probably the most commonly studied and recommended strategy for reducing energy consumption in multi-component distillation sequences is thermal coupling, including the embodiment of thermal coupling as a dividing-wall column [3]. Dividing-wall columns can mitigate the so-called remixing effect, where (for a three-component direct sequence) the concentration of the intermediate species reaches a maximum part-way down the column below the feed and is then re-mixed toward the bottom of the column.

An alternative energy intensification strategy for multicomponent distillation is the stacked side-stream sequence suggested by Ward and coworkers [4-7]. In this strategy, a liquid side-stream is employed to lessen the remixing effect while reboiler-condenser heat exchange further significantly reduces energy consumption. Ward and coworkers found that the stacked side-stream sequence worked better than a dividing-wall column and all other alternatives for xx out of xx mixtures and for two other industrially-relevant ternary separations.

In addition to energy savings, operability and control are important considerations when contemplating an energy intensification strategy. Control of dividing-wall columns has received a great deal of attention in the literature [8]. By contrast, we are not aware of any control studies of a stacked side-stream sequence. A challenge for operation and control of a thermally coupled sequence is the control of the vapor split when there is only a single reboiler. Stacked side-stream sequences do not require a vapor split, but they have another challenge for operating and control, namely that two control degrees of freedom are lost when the reboiler and condenser are integrated.

In this work, control strategies for stacked side-stream sequences are developed and the dynamic performance of stacked side-stream sequences is compared with that of dividing-wall columns and conventional two-column sequences. Strategies for addressing the lost control degrees of freedom when the reboiler and condenser coupled are proposed, compared and discussed. The results show that the stacked side-stream sequence can be satisfactorily controlled and that the control performance of the stacked side-stream sequences is somewhat better than that of the dividing-wall column. Thus in addition to greater energy savings, stacked side-stream sequences have the additional advantage of somewhat better control performance.

References

1. Hewitt, G.; Quarini, J.; Morrell, M. More efficient distillation. Chem Eng-London 1999, (690), 16-18
2. Tang, W. T.; Chien, C. K.; Ward, J. D. A review of energy intensification strategies for distillation processes: Cyclic operation, stacking, heat pumps, side-streams, dividing walls and beyond. Sep Purif Technol 2025, 357,
3. Kooijman, H. A.; Sorensen, E. Recent advances and future perspectives on more sustainable and energy efficient distillation processes. Chemical Engineering Research & Design 2022, 188, 473-482
4. Tang, W. T.; Chien, C. K.; Ward, J. D. Stacked Side-Stream distillation sequences. Chem. Eng. Sci. 2023, 280,
5. Tang, W.-T.; Ward, J. D. Energy and exergy analysis of a stacked complex sequence and alternatives for ternary distillation. Sep Purif Technol 2023, 304, 122384
6. Tang, W. T.; Ward, J. D. Stacked complex sequences for ternary zeotropic distillation. Comput Chem Eng 2022, 161,
7. Tang, W. T.; Ward, J. D. Comparison of Separation Alternatives for Two Industrial C6-C7 Aliphatic Hydrocarbon Mixtures Including Stacked Complex Sequences. Ind Eng Chem Res 2022, 61 (36), 13488-13504
8. Donahue, M. M.; Roach, B. J.; Downs, J. J.; Blevins, T.; Baldea, M.; Eldridge, R. B. Dividing wall column control: Common practices and key findings. Chemical Engineering and Processing-Process Intensification 2016, 107, 106-115