利用液-液分離和變壓蒸餾分離異丙醚/異丙醇/水三元混合物的節能程序設計與控制

Design and Control of An Energy-Efficient Process for the Separation of Diisopropylether/Isopropanol/Water Ternary Mixture by Liquid-Liquid Separation and Pressure Swing

羅雅欣(Ya-Hsin Lo)

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(2021年)

https://doi.org/10.6342/NTU202101911

摘要

本研究論文探討異丙醚(DIPE)/異丙醇(IPA)/水(water)的三成分共沸混合物之分離，此混合物在三成分圖中被蒸餾邊界分成異丙醚蒸餾區域、異丙醇蒸餾區域和水蒸餾區域，其具有三個二元共沸物、一個三元共沸物與液-液分相區域，而本研究探討進料組成在常壓下，位於接近在僅含少量異丙醇的異丙醚與水兩蒸餾區域之蒸餾邊界的分離系統。利用此三種成分本身在常溫常壓下具有液-液分相的現象，能跨越蒸餾邊界的限制；並在蒸餾塔操作時，改變不同蒸餾塔的操作壓力，使得蒸餾區域可因為壓力不同而變化，擴增蒸餾區域，使整體系統內部回流減少，並且都不需要添加額外的物質來達到分離成效。因此相較於變壓蒸餾、共沸蒸餾與萃取蒸餾等常見的共沸分離方法，本研究中的分離方法能夠使多共沸物三成分混合物的分離，具有更佳的經濟優勢。本論文探討兩種不同的穩態分離程序，分別為複合式的雙液液分相槽結合變壓蒸餾分離流程與複合式的單液液分相槽結合變壓蒸餾分離流程，並對其做更節能環保的熱整合策略，讓系統進行最適化分析，使年度總成本達最小化。最後再對兩種最經濟的設計流程，分別進行動態控制策略設計，證明進料組成擾動與新鮮進料流量改變所帶來的製程干擾，能夠有效地被排除。

利用複合式雙液液分相槽結合變壓蒸餾程序，相較文獻中較具經濟的傳統三塔進料預熱萃取蒸餾程序的回流流量，可由7293 kg/hr減少至2587.34 kg/hr。且三塔進料預熱萃取蒸餾之熱源由高壓、中壓、低壓蒸汽提供，但是在複合式雙液液分相槽結合變壓蒸餾設計中，皆由低壓蒸汽提供熱源，使得複合式雙液液分相槽結合變壓蒸餾程序能更加節省能源且環保。在最佳化熱整合設計當中，複合式雙液液分相槽結合變壓蒸餾程序可以透過部分熱整合及加裝熱交換器預熱異丙醚塔(C1塔)等節能策略，讓其操作成本減少50.51%，年度總成本減少34.33%；而利用複合式單液液分相槽結合變壓蒸餾程序，蒸餾系統的回流能減少至5354.08 kg/hr，熱源也都由低壓蒸汽提供，並且利用部分熱整合及加裝熱交換器預熱異丙醚塔的設計，讓其操作成本可以減少34.63%，年度總成本則減少24.85%。

本論文對複合式雙液液分相槽結合變壓蒸餾程序建立五種不同的控制架構，以排除進料組成及新鮮進料流量變化的干擾，結果顯示控制架構五：固定C1塔再沸器熱負荷對C1塔進料流量比(QR1/CF1)，C1、C2與C3塔皆為單點溫度控制，在面對4種10 mass%水進料組成擾動時，除了C進料組成干擾顯示出此控制架構對異丙醚產品純度控制稍差一點，其餘A、B、D進料組成干擾皆維持各產物純度在規範標準附近；而面對±10%新鮮進料流量改變時，控制架構五亦能控制住所有產物的純度。對複合式單液液分相槽結合變壓蒸餾程序建立兩種不同的控制架構，結果顯示控制架構二，根據開環路與閉環路敏感度測試找出適合的溫度控制板數，C1為雙點溫度控制，C2與C3塔皆為單點溫度控制，在面對4種20 mass%水進料組成擾動時，各組成干擾皆可維持各產物純度在規範標準附近且偏差極小；而面對±10%新鮮進料流量改變時，控制架構二也能保持產物的純度。

綜合上述，對於異丙醚/異丙醇/水的三成分共沸混合物之分離，利用複合式雙液液分相槽結合變壓蒸餾程序能得到最節省能源，並且為最經濟的設計流程；而利用複合式單液液分相槽結合變壓蒸餾程序，本研究設計出其他文獻所缺少的控制策略，並且控制架構能在不使用組成控制器的條件下，有效排除大量進料組成干擾與新鮮進料流量改變所帶來的影響。

關鍵字

液-液分相 ； 變壓蒸餾 ； 熱整合 ； 多共沸物三成分系統 ； 程序設計與控制

並列摘要

In this thesis, the separation of the diisopropyl ether (DIPE)/isopropanol (IPA)/water ternary system was studied. The ternary diagram of this mixture is divided into diisopropyl ether distillation region, isopropanol distillation region and water distillation region by the distillation boundaries. Besides, this system has three binary azeotropes, one ternary azeotrope and a liquid-liquid envelope. Without adding foreign components into the separation process, the limit of the distillation boundary can be crossed by using natural liquid-liquid phase separation; furthermore, by changing the pressure to expand the distillation regions, the flow rate of overall recycle stream in the system can be reduced. Therefore, compared with pressure-swing distillation and extractive distillation methods, the systems proposed in this study have better economic advantages. This thesis discusses two different steady-state separation procedures, which are a separation process including two decanters combined with pressure swing distillation and a separation process including one decanter combined with pressure swing distillation, implementing energy-saving strategies and using optimal analysis to minimize total annual cost (TAC). Finally, the control strategies of the two most economical design flowsheets are established to eliminate the disturbances in feed flowrate and feed composition.

The recycle flowrate (2587.34 kg/hr) of the design flowsheet including two decanters combined with pressure swing distillation is much less than that (7293 kg/hr) of the extractive distillation process including triple columns with preheated feed recently proposed in the open literature. Heat sources of the extractive distillation process are high-pressure, medium-pressure, and low-pressure steam, while in the novel process including two decanters, only low-pressure steam serves as the heat source, so the case including two decanters can save energy and is environmentally friendly. Further energy-saving techniques of the proposed design are investigated. It turns out that the installation of two sets of heat integration units can save 50.51% of operating cost and 34.33% of TAC: integration of partial condenser of C1 column and reboiler of C3 column; feed-effluent heat exchanger for C1 column. In the the case including one decanter, low-pressure steam also serves as the only heat source. Moreover, the energy-saving design is investigated and the result shows that operating costs can be reduced by 34.63% and TAC can be reduced by 24.85%.

In order to deal with disturbances for the system, five different control structures without using composition analyzer are established for the design flowsheet including two decanters combined with pressure swing distillation. The results show that control structure five, which fixes the ratio of C1 column reboiler duty to C1 column feed flowrate and has single-point temperature controllers in C1, C2 and C3 column, is able to maintain all products at high-purity specifications, except for the interference of feed composition C, leading to a slight deviation in diisopropyl ether product purity. When facing throughput changes, control structure five can also keep all products at high purity specifications. In addition, two different control structures are established for the design flowsheet including one decanter combined with pressure swing distillation to eliminate four feed composition disturbances without using composition analyzer. The results show that the best control structure to maintain all products at high purity specification is control structure two, which is designed according to open-loop and closed-loop sensitivity tests, and has two-point temperature controllers in C1 column, while C2 and C3 columns both have single-point temperature controllers. When facing throughput changes, control structure two can also keep all products at high purity specifications.

In summary, for separating the complicated ternary mixture of diisopropyl ether/isopropanol/water, the most energy-saving and economical design process is the design flowsheet including two decanters combined with a pressure swing distillation. Design of control strategies have always been lacking in other papers. Our design flowsheet including one decanter combined with pressure swing distillation can effectively eliminate a large number of feed composition disturbances and fresh feed throughput changes without using any composition controller.

並列關鍵字

liquid-liquid separation ； pressure-swing distillation ； heat integration ； multiple azeotropes ； process design and control.

參考文獻

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