(371j) Synthesis and Global Optimization of Heat-Integrated Thermally Coupled Distillation Systems

The design paradigms of process industries have undergone a profound transformation due to the rapid advancement of computing prowess in modern industrial processes. This transformation is characterized by a shift towards digitalization and computerization, where intricate systems are crafted in the virtual realm before their physical instantiation. Through analysis, simulation, and optimization, digital designs uncover the complex behaviors and properties of industrial systems, leading to greater efficiency and innovation in the chemical industry ¹. In the pursuit of sustainability and economic viability, there is a need for functional designs that consume low energy, have minimal carbon footprint, and are cost-effective ^2,3. Thermally coupled distillation (TCD) ⁴ is a promising technology that offers a glimpse into the future of industrial efficiency. This highly energy-integrated distillation system achieves energy savings by facilitating direct mass and energy exchange between distillation columns. While dividing-wall columns are already present in industrial production, they are typically limited to single-partition configurations ⁵. To design dividing-wall columns with more complex configurations, a systematic program is required. This program is known as the TCD sequence.

Since the 1990s, there has been ongoing research into TCD sequences. However, certain technological puzzles remain unresolved. The industry has shown reluctance to adopt TCD because of the necessity for additional control over vapor transport between columns ⁶, unlike in dividing-wall columns. This requirement poses challenges. Conversely, dividing-wall columns require equal pressures on both sides, a constraint that is not applicable to TCD. This characteristic renders column pressure an additional optimization degree of freedom for TCD, presenting a distinct advantage.

This study presents a comprehensive design, simulation, and optimization of three configurations of a TCD system: the basic configuration, the thermodynamically equivalent configuration, and the more operable configuration. The thermodynamically equivalent configuration extends the basic configuration by allowing the rearrangement of column sections through thermal links, which can affect the capital cost of the system. The operable configuration focuses on solving the vapor transport problem, ensuring that the vapor always flows towards the lower pressure column. We present a modeling methodology for transforming process flow diagrams of TCD systems into computer-recognizable data structures. These data structures uniquely represent the process flow diagram while also participating in optimization as decision variables. We employed a meta-heuristic algorithm to globally optimize these configurations using a near-ideal system and a polar non-ideal system as examples. This study considers the simultaneous optimization of column pressure, thermodynamically equivalent configurations, nonlinear investment estimation models, and heat integration for the first time. The study results show that the TCD technique provides a maximum of 51% energy savings compared to the sharply separated conventional distillation sequence, considering thermodynamically equivalent configurations. However, employing the TCD technique for energy savings does not significantly reduce the total annualized cost. Since the heat supply for the TCD consistently requires high temperatures compared to the conventional distillation sequence, the utility cost contributes 43% to 55% of the total annualized cost. Additionally, the study reveals a limited variation in capital investment among the investigated configurations. Achieving further reductions in utility costs through the TCD principle poses challenges. Therefore, exploring dividing-wall columns to mitigate capital investment presents an appealing avenue for future research.

References

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