各類近沸或共沸混合物分離之節能設計與控制

Energy-saving Design and Control for the Separation of Close-boiling or Azeotropic Mixtures

陳怡均(Yi-Chun Chen)

指導教授 ： 錢義隆

國立臺灣大學/工學院/化學工程學研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.10510

摘要

本文探討各類近沸或共沸混合物分離之節能設計流程。首先討論近沸物質1,2-丙二醇和乙二醇之各種分離設計流程，包括單一蒸餾塔、多效熱整合、非勻相共沸蒸餾與萃取蒸餾系統，並透過熱整合或熱耦合 (thermally-couple) 的方式節能。若採用多效熱整合蒸餾系統，仍需要高蒸餾板數才能獲得高純度之產品，且為了避免低壓時存在之切線狹點，低壓塔設定為一大氣壓，反而會造成高壓塔再沸器不適合使用高壓蒸汽。若採用非勻相共沸蒸餾系統，由於進料組成中1,2-丙二醇成分較高，因此能預期非勻相共沸蒸餾塔內部流循環率將會很高。最後發現以三甘醇作為萃取劑之萃取蒸餾系統，並進行熱整合以將萃取劑回收流與進料流進行熱交換之設計流程最具經濟效益。
本文亦針對Pyridine與水的分離系統，討論萃取/蒸餾複合系統 (hybrid extraction/distillation system) 之程序設計與動態控制策略。探討n-propyl formate (NPF) 與diisopropyl ether (DIPE) 作為溶劑之萃取/蒸餾複合系統，並進行最適化分析。由於液-液相萃取塔不需要蒸汽，因此與非勻相共沸蒸餾系統或是隔牆塔設計相比，能大幅節省操作費用。若考量萃取塔各板與液-液分相槽中兩液相之密度差，將發現液-液相分離以DIPE作為溶劑實際上較可行。本文也討論萃取/蒸餾複合系統之動態模擬，並得知最適化流程與動態控制能力之間有經濟損益關係。最後進行閉環干擾排除測試，以確認儘管進料組成或是流率干擾，本文提出之控制策略仍能正確地維持高純度之產品規格。
本文也針對丙二醇甲醚與水的分離系統，討論改良之萃取蒸餾系統，以改善高沸點環丁碸作為萃取劑造成萃取劑回收塔塔底溫度過高的問題。結果發現以較低沸點但提升水對丙二醇甲醚之相對揮發度能力較差的N-Methyl-2-pyrrolidone (NMP) 作為萃取劑並無經濟效益。若繼續以環丁碸為萃取劑，但將萃取劑回收塔改為低壓塔，雖然設備成本增加，減少的蒸汽費用卻更加顯著，使得此低壓塔系統能夠大幅減少年度總成本。本文最後亦說明若欲以萃取/蒸餾複合系統進行丙二醇甲醚去水流程，必須找尋到合適的溶劑方能進行。
藉由討論上述之三個近沸或共沸物之節能設計流程，能發現儘管對於特定的混合物系統可以用不只一種方式來完成分離，但若是能夠選擇使用最適合之分離方式則可以大幅節省蒸汽費用與年度總成本。

關鍵字

近沸 ； 共沸 ； 多效熱整合 ； 非勻相共沸蒸餾 ； 萃取蒸餾 ； 萃取/蒸餾複合系統 ； 程序設計 ； 程序控制

並列摘要

In this thesis, energy-saving design and control for the separation of close-boiling or azeotropic mixtures will be discussed. For the separation of close-boiling mixtures, 1,2-propanediol and ethylene glycol, several alternative designs, including multi-effect distillation, heterogeneous azeotropic distillation, and extractive distillation, are investigated in order to save steam cost and total annual cost. Extractive distillation system using triethylene glycol as entrainer with a feed-effluent heat exchanger is considered to be the most economic design.
　　For the separation of pyridine and water, hybrid extraction/distillation system using n-propyl format (NPF) or diisopropyl ether (DIPE) as solvent is proposed. Since the main extraction column does not require steam, this design flowsheet can significantly save operating cost as compared to the previous designs based on heterogeneous azeotropic distillation. Considering the density differences in each stage of extraction column and in decanter, DIPE is a more practically favorable solvent for reaching liquid-liquid equilibrium in each stage. Dynamic control of this proposed system is also investigated. It is found that a trade-off between optimal steady-state design and dynamic controllability needs to be made in order to properly reject feed flow rate and feed composition disturbances.
　　For the separation of propylene glycol monomethyl ether (PM) and water, two modifications have been made in extractive distiilation system in order to overcome the drawback of high bottom temperature in entrainer recovery column due to using high-boiling sulfolane as entrainer. With relatively low-boiling n-methyl-2-pyrrolidone (NMP) as entrainer, steam cost reduces, but overall result does not show any encomic benefit due to NMP’s low enhancement of relative volatility of water over PM. Instead, decreasing the pressure of entrainer recovery column, significant reduction in steam cost and total annual cost can be obtained. For the same separation task, hybrid extraction/distillation system is also considered. However, a suitable solvent is important for effective extraction process but none of any has been found in open literature.
　　In summary, for a particular close-boiling or azeotropic mixture, there are more than one ways to achieve the separation task. However, significant savings on the steam cost and total annual cost can be achieved if the most suitable separation method can be selected.

並列關鍵字

Close-boiling ； azeotropes ； multi-effect distillation ； heterogeneous azeotropic distillation ； extractive distillation ； hybrid extraction/distillation system ； process design ； process control

參考文獻

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