藉由萃取蒸餾以分離含最高共沸溫度之混合物的製程設計與控制

Design and Control of Separation of Maximum Boiling Azeotrope by Extractive Distillation

王彥翔(Yen-Hsiang Wang)

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(2018年)

https://doi.org/10.6342/NTU201800824

摘要

萃取蒸餾為一常見的方法用來分離近沸物或共沸物，公開的文獻大部分都是探討含最低共沸溫度之混合物的分離，因此長期下萃取蒸餾的架構及想法是以分離含最低共沸溫度之混合物的立場發展出來。用萃取蒸餾來分離含最高共沸溫度之混合物有兩點較特別之處，其一重夾帶劑會與共沸組成產生蒸餾邊界，當萃取蒸餾塔的塔底組成座落在蒸餾邊界上，夾帶劑回收塔的塔頂就無法得到另一個高純度產品，因此在選擇夾帶劑方面，不只要考慮提升相對揮發度的能力，也要考慮最低的夾帶劑流量與新鮮進料流量之比例。另外，含最高共沸溫度之混合物的共沸組成靠近沸點較高的化合物，因為新鮮進料中的重物質分離較困難，可以視夾帶劑進料位置至塔底都為萃取段，如果輕物質容易分離的話，在不需要夾帶劑作用之下也能在塔頂得到高純度的輕物質。
本文探討三個含最高共沸溫度之混合物的分離，其一是丙酮與氯仿分離系統之程序設計及程序控制，以N-甲基吡咯酮(NMP)為夾帶劑時，最低的夾帶劑流量與新鮮進料流量之比例較低，且提升相對揮發度的能力較二甲基亞碸(DMSO)與乙二醇(EG)還強，年度總成本(TAC)與兩者比較分別節省21 %與26.2 %。以N-甲基吡咯酮(NMP)為夾帶劑之萃取蒸餾系統中加上一進料/出流熱交換器(feed-effluent heat exchanger, FEHE) 進行熱整合之設計流程最具經濟效益。本文也討論此熱整合設計流程之動態模擬與控制，比較單點溫度控制策略及雙點溫度控制策略，最後進行閉環干擾測試，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。結果以雙點溫度控制為較佳的控制策略。
其二是苯酚與環己酮分離系統，萃取蒸餾系統中採用TEG為夾帶劑，結果發現在年度總成本方面，傳統上夾帶劑回流有加裝冷卻器的系統比未加裝冷卻器的系統還要高，由於輕成分在沒有夾帶劑的作用之下本來就很好分離，因此高溫的夾帶劑進料可以往塔底靠近，能夠進一步地降低再沸器熱負載。本文也討論最具經濟效益的熱整合之設計流程的動態模擬與控制，以雙點溫度控制策略的基礎上，探討夾帶劑流量方面的控制選擇，最後夾帶劑流量與夾帶劑回收塔的塔頂流量成固定比例可以有效地排除進料組成及流率干擾。
其三是二甲基乙醯胺與醋酸分離系統，選擇出的兩種夾帶劑三甘醇(TEG)及四甘醇(TTG)分別在提升相對揮發度及最低的夾帶劑流量與新鮮進料流量之比例各有優勢，結果不加裝冷卻器的萃取蒸餾系統中以兩者為夾帶劑的年度總成本並沒有太大的差別，而且夾帶劑進料位置在新鮮進料位置的下方。不過在熱整合之設計流程方面，由於TTG系統中所使用的夾帶劑流量較多，因此在熱交換器換熱的限制之下，新鮮進料可以換到較多的熱。結果為以TTG為夾帶劑的熱整合之設計流程最為節能。
藉由上述之三個含最高共沸溫度之混合物的萃取蒸餾設計流程，能發現夾帶劑選擇對於萃取蒸餾的重要性。另外由於重成分較難分離的特性，當塔頂輕成分非常好分離出之下，以高溫夾帶劑進料可以更有效地節能。

關鍵字

萃取蒸餾 ； 含最高共沸溫度之混合物 ； 重夾帶劑 ； 程序設計 ； 程序控制

並列摘要

Extractive distillation is a common tool for separating close boiling and azeotrope mixtures. There are many papers in opening literature studying the separation of minimum boiling azeotrope. Thus, the long-term development of the configuration and concept about extractive distillation is based on the separation of minimum boiling azeotrope. There are two special points concerning the separation of maximum boiling azeotrope by extractive distillation. First, a distillation boundary will be formed between heavy entrainer and azeotropic composition. When the bottom composition of the extractive distillation column is located on the distillation boundary, the other high purity product cannot be obtained at the top of the entrainer recovery column. Therefore, for selecting the entrainers, it considers not only the ability to enhance the relative volatility, but also the feasible minimum entranier-to-feed ratio. Second, the azeotropic composition is close to the high boiling point component. Because the separation of heavy component from fresh feed is difficult, the stage from the entrainer feed location to the bottom of the column can be considered as the extractive section. If the light component is separated easily, high purity light component can be obtained at the top of the column without the need for entrainers.
For the process design and control of the separation of acetone and chloroform, when using N-methyl-2-pyrrolidone (NMP) as entrainer, the feasible minimum entranier-to-feed ratio is the lowest and the ability to enhance the relative volatility is greater than using dimethyl sulfoxide (DMSO) and ethylene glycol (EG) as entrainers. The total annual cost (TAC) can be separately reduced by 21 % and 26.2 %. Extractive distillation using NMP as entrainer with a feed-effluent heat exchanger (FEHE) is considered to be the most economic design. For the dynamic simulation of heat integrated flowsheet, comparison of single temperature control and dual temperature control, and closed-loop disturbance test was performed to confirm that despite the feed composition or flow rate disturbances, the control strategy proposed in this paper can get high purity product. As a result, a dual temperature control strategy is a preferred control option.
For the separation of phenol and cyclohexanone (CYC), triethylene glycol (TEG) is used as the entrainer in the extractive distillation. As a result, it is found that in TAC, the system with a cooler on the entrainer recycle flow is higher than that without a cooler. Because the light component is easily separated without the action of the entrainer, the high temperature entrainer feed will be closer to the bottom of column and further reduce the reboiler duty. This study also discusses the dynamic simulation of heat integrated system and finds control strategies for entrainer flow based on a dual temperature control strategy. Finally, fixing the ratio of entrainer flow to distillate of the entrainer recovery column can effectively eliminate feed composition and feed flow disturbance.
For the separation of N,N-dimethylacetamide(DMAC) and acetic acid(HAC), TEG and Tetraethylene glycol (TTG) serve as the candidate entrainers and each has advantages in enhancing relative volatility and the feasible minimum entranier-to-feed ratio. As a result, there is no difference in TAC of both entrainers in an extractive distillation without a cooler and the entrainer feed position is below the fresh feed . However, in the design process of heat integration, due to the large amount of entrainer in the TTG system, the fresh feed can obtain more heat under the heat exchange limit of heat exchanger. Finally, heat integrated process with TTG as entrainer is the most economic design process.
In summary, for maximum boiling azeotrope, the importance of entrainer selection for extractive distillation can be observed. In addition, because the heavy component is difficult to separate, when the light component can easily be separated out at column top, the high temperature entrainer can save energy more efficiently.

並列關鍵字

extractive distillation ； maximum boiling azeotrope ； heavy entrainer ； process design ； process control

參考文獻

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