分離三成分共沸混合物之三塔變壓蒸餾的程序設計

Abstract

一般的蒸餾技術是很難分離三成分共沸混合物，因共沸物與蒸餾邊界的存在，不利於蒸餾技術的應用，然而，可以藉由觀察一三成分混合物之蒸餘曲線圖，若其蒸餾邊界隨著壓力的移動有明顯的改變，三塔變壓蒸餾會是一個合適的選擇來分離此混合物。本論文將討論利用三塔變壓蒸餾來分離具有不同數量共沸物之三成分混合物。本論文利用Aspen Plus v8.4此商用模擬軟體來模擬所有的穩態程序。 第一個系統為丙醇/丙酮/甲苯，在常壓下形成一個共沸物，在其蒸餘曲線圖中形成一條蒸餾邊界且其隨著壓力的移動十分明顯，變壓蒸餾程序為一合適之分離程序。在傳統的變壓蒸餾中，面對到此混合物會先將最輕之組成丙酮從系統中分離，接著再進入後續的雙塔變壓蒸餾程序分離剩下的兩個成分。三塔變壓蒸餾程序是利用蒸餘曲線圖中邊界的移動來分離三成分混合物，且採用丙醇-甲苯-丙酮此種分離次序。 甲醇/乙醇/甲乙酮此種混合物在蒸餘曲線圖上常壓時會形成兩個共沸物與一條蒸餾邊界，對於一般的具有兩個共沸物之三成分混合物，三塔變壓蒸餾是很難應用在此混合物上的，然而，甲醇-甲乙酮之共沸物在高壓時會消失，因此傳統變壓蒸餾與三塔變壓蒸餾皆可應用在此混合物上。乙醇/乙酸乙酯/四氯化碳此種混合物在常壓下會形成三個共沸物，其蒸餘曲線圖中之蒸餾邊界隨著壓力移動明顯，此研究將討論三塔變壓蒸餾不同的分離次序對年度總成本的影響。己烷/甲醇/乙酸甲酯之混合物在常壓時形成四個共沸物，其邊界也會隨著壓力改變而移動，此三塔變壓蒸餾程序最適化後得到最低的年度總成本。 本研究採用連續疊代法來做四個系統之最適化，其最適化程序之中存在多個變數，因此本研究採用較為簡化的最適化程序，並以年度總成本為目標函數，完成最適化程序。

Triple column pressure swing distillation (TCPSD) can be a promising method to separate azeotropic ternary mixtures. In order to adopt this configuration, a pre-requisite is that the distillation boundaries move apparently under different operating pressures, and this can be checked through the residue curve map (RCM). In this thesis, a rigorous and extensive study on TCPSD is performed, with several separation system containing different amount of azeotropes as demonstration. All of the simulation work is implemented in Aspen Plus v8.4. The first separation system is the Propanol/acetone/toluene system, which forms an azeotrope. In conventional pressure swing distillation process (CPSD), the lightest component acetone is separated at first column, with propanol and toluene obtained from the flow-up binary pressure swing distillation. For TCPSD, the separation sequence is propanol-toluene-acetone by using the moving distillation boundary based on different operating pressure. The second system studied is the Methanol/ethanol/methyl ethyl ketone (MEK) mixture, which forms two azeotropes, one between methanol-MEK and another between ethanol-MEK. It RCM presents one distillation boundary at atmosphere. For a typical mixture with two azeotropes, it is hard to obtain three product with high purity by TCPSD. However, for this system, methanol-MEK azeotrope pair disappears at high pressure, so TCPSD can be used in this mixture. The third system, Ethanol/ethyl acetate (EtAc)/carbon chloride (CCl4) mixture, forms three azeotropes. By investigating the pressure-sensitive moving boundaries, the complex azeotropic mixture can be separated by TCPSD with a different separation configuration discussed in this research. The fourth system, Methanol/methyl acetate (MeAc)/ hexane system, forms four azeotropes and its triangle diagram shows several distillation boundaries which moves apparently as pressure changes. Throughout the four systems studied in this thesis, sequential iterative method is used to optimize all the flowsheets with minimized TAC as the objective function. Because there are quite a few design and operating variables need to be optimized, some simplification is need to reasonably relieve the tasks. After that, the optimal cases of CPSD and TCPSD are directly compared, in order to get a deeper insight into the feasibilities and advantages when applying TCPSD configuration to different separation systems.