(184e) A Synthesis Framework for Structure Constrained Thermally Coupled Distillation Sequences Including Divided Wall Columns

The optimality of distillation sequence decides the energy consumption of multicomponent zeotropic mixture separation directly[1]. Thermally coupled structure can effectively eliminate the backmixing of intermediate components, which is proved to be one of the most promising energy saving configurations. It is reported that in some cases thermally coupled sequences can decrease energy consumption for more than 30 percents and reduce total annualized cost for nearly 20 percents compared with that of conventional ones[2-4]. However, most of the highly thermally coupled structures can't be utilized in chemical industry due to the difficulties caused by equipment manufacture, automatic control and practical operation[5]. Besides, distillation sequence with feasible thermally coupled structure, like divided wall columns (DWC), is usually optimized through enumeration method which can't be adopted to zeotropic mixture separation with more than five components[6-7].

Therefore, a novel synthesis framework for thermally coupled distillation sequences with structure constraints is proposed. To obtain structure constraint equations, a systematic methodology for qualitatively analyzing the complexity of thermally coupled distillation sequences and a strategy for developing structure constraints are given. The concepts of couple degree and maximum section number of mass loop to qualitatively describe the complexity of system as well as the mechanism to classify the reason of thermally coupled structure generation are proposed. Based on these, a series of structure constraints are generated considering technologies applied in industry (DWC). With these constraints, a mathematic model for thermally coupled distillation synthesis based on state-task network superstructure is developed. A five components zeotropic mixture case study is carried out demonstrating that the existence, position and number of thermally coupled structure is related to the feed condition and investment cost. Besides, a six components zeotropic mixture case study is also carried out, the resulting sequence can decrease energy consumption for 30.8 and 21.1 percents and reduce total annualized cost for 21.5 and 16.2 percents respectively compared with that of conventional ones and thermally coupled sharp split ones, demonstrating the feasibility of the proposed methodology in large scale zeotropic mixture separation.

Reference

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