(223e) Stacked Side-Stream Distillation Sequences for Energy Intensification of Multi-Component Separations

Distillation is ubiquitous in the chemical and related industries, and it consumes a great deal of energy worldwide [1]. Numerous methods have been proposed to reduce the energy consumption of distillation processes through intensification, including cyclic distillation [2], thermal coupling [3], heat pumps [4], side-streams [5] and column stacking [6]. (Column stacking refers to the strategy of adjusting the column pressures so that the condenser of one column can power the reboiler of another column.) Each of these methods separately has been shown to significantly reduce energy consumption compared to an ordinary column sequence where all heat duties are provided by process utilities.

Thermal coupling, including its implementation as a dividing wall column (DWC), has received a great deal of attention in the literature due to its potential to improve thermodynamic efficiency and reduce cost and energy consumption [7]. One significant source of inefficiency in multi-component separation sequences is the remixing effect, where the concentration of the intermediate component reaches a maximum part-way down the first column, but this material is then remixed with heavy component near the bottom of the column. Dividing-wall columns can mitigate the remixing effect [3, 7].

In this work, a new configuration, a stacked side-stream sequence (SSS), is proposed for multicomponent distillation [8-11]. As the name suggests, the idea is to combine two known technologies: column stacking and side-streams. The side-stream can mitigate the remixing effect, and significant additional energy savings is achieved by stacking the two columns.

To assess the potential of this new strategy, 11 flowsheet alternatives, including three types of dividing wall columns and the proposed stacked side-stream configuration, were optimized for seven chemical mixtures at ten feed compositions (770 total flowsheets) [10]. The proposed process is compared not only to a conventional sequence, but also to other intensified alternatives including thermally coupled sequences and dividing-wall columns. The results show that if column stacking is not permitted, a DWC is preferred in 39 out of 70 cases and DWCs reduce cost by 19.1% on average. However, if stacking is permitted, a stacked side-stream sequence is preferred in 58 out of 70 cases and an SSS saves 21.0% on average compared to the best conventional sequence.

To further assess the potential of this method, detailed energy and exergy analyses were conducted for a mixture of benzene, toluene and xylene for five process alternatives, including the conventional sequence, the best DWC process, and a stacked side-stream sequence [9]. The results were used to construct a series of Pareto fronts with cost and thermodynamic efficiency as the two optimization variables. The results showed that the SSS outperformed the DWC and all other alternatives at every point on the Pareto front.

To demonstrate the industrial relevance of the proposed method it was applied to two mixtures of industrial significance reported in the literature [8]. The SSS was found to outperform the best DWC and all other alternatives considered for both mixtures. Finally, for completeness, the method was applied to a large canonical set of side-stream configurations [11]. The results showed that without stacking, the best side-stream sequence costs 80.3% of the conventional sequence, but when stacking is permitted the best SSS costs only 69.3% of the conventional sequence, a further savings of 11 percentage points or 56% additional savings. Possible directions for future research will also be discussed briefly.

References

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