利用液-液分相與變壓蒸餾分離複雜三成分系統之節能設計與控制

Abstract

本論文探討具有三個二元最低共沸物、一個三元最低共沸物的複雜三成分系統分離，並以兩種不同進料組成之苯/異丙醇/水系統，進行新穎分離程序的說明與討論。由於新穎分離程序能在不添加額外物質進入分離程序的條件下，利用系統本身三成分間的液-液分相來跨越蒸餾邊界限制，並透過改變蒸餾塔操作壓力來擴增蒸餾區域與減少系統內回流。因此相較於萃取蒸餾與變壓蒸餾等常見的共沸分離程序，對多共沸物三成分系統的分離更具有經濟層面上的優勢。本論文也將為最具經濟效益的程序設計建立控制策略，並以動態模擬證明控制架構得以排除進料組成擾動與新鮮進料流量改變所帶來的干擾。 對於異丙醇蒸餾區域進料組成的苯/異丙醇/水系統分離而言，新穎分離程序相較於三塔雙側流萃取蒸餾分離程序，因為系統內回流流量能由273.10 kmol/hr大幅降低至131.16 kmol/hr，且熱源由原本的高壓蒸汽改用更便宜的中壓與低壓蒸汽，因此使用新穎分離程序設計能節省43.8%的操作成本與35.6%的年度總成本。透過部分熱整合以及加裝熱交換器預熱C2塔進料流等節能策略，可再節省27.7%的操作成本與18.6%的年度總成本。 對於水蒸餾區域進料組成的苯/異丙醇/水系統分離而言，新穎分離程序相較於三塔部分熱整合變壓蒸餾分離程序，因為系統內回流流量能由95.45 kmol/hr大幅降低至22.12 kmol/hr，且熱源由原本的中壓與低壓蒸汽改為全部使用較便宜的低壓蒸汽，因此使用新穎分離程序設計能節省64.6%的操作成本與56.4%的年度總成本。透過部分熱整合以及加裝熱交換器預熱C2塔進料流等節能策略，可再節省28.4%的操作成本與9.3%的年度總成本。 由於分離程序中的異丙醇蒸餾塔會發生多重穩態，使得塔板溫度與作動變數間存在輸出多重穩態以及輸入多重穩態，將導致動態控制更為困難。本論文根據開環路與閉環路敏感度分析，建立3種不同的控制架構，以排除進料組成擾動的干擾。結果顯示僅固定C2塔塔頂回流量對進料流流量比，使C1塔為雙點溫度控制，而其餘二塔進行單點溫度控制的控制架構三，在面對4種進料組成擾動時，皆維持各產物流純度在規範標準附近。最後再藉由對C1塔和C2塔的板溫控制器都進行進料流量補償溫度控制，使控制架構在面對10%新鮮進料流量改變時，亦仍能保持產物流的純度。 綜合上述，本論文提出一新穎分離程序，在分離會產生液相部分不互溶的多共沸物三成分系統時，相較於常見的共沸分離程序能大幅降低經濟成本，並透過探討不同進料組成相應的新穎分離程序設計，使本論文討論更全面且完善。此外本論文亦提出相應的控制策略，在不使用組成控制器的條件下，能有效排除進料組成擾動與新鮮進料流量改變帶來的影響。

In this thesis, a new energy-efficient design flowsheet is provided for the separation of complicated ternary system with three binary azeotropes, one ternary azeotrope, three distillation regions and liquid-liquid envelope. Without adding foreign components into the separation, a new energy-efficient design flowsheet is proposed in this work with two main ideas. The first idea is to use a decanter to overcome the limitation of the distillation boundary by taking advantage of the natural liquid-liquid separation in the system instead of only using distillation columns. The second idea is to change the operating pressure for the respective columns to move the pressure-sensitive distillation boundary in order to expand the operating distillation region and reduce the flow rate of overall recycle stream in the system. In this thesis, the separation of the benzene/isopropanol/water ternary system is taken as examples to illustrate the benefits of the proposed energy-efficient design. Two different design flowsheets for different feed compositions which are located in distinct distillation regions are provide to make the discussion more comprehensive. The control structure for the most economical design flowsheet is established to reject the disturbances in feed flowrate and feed composition. The first kind of feed composition is located in the isopropanol distillation region. The recycle flowrate of the proposed energy-efficient design flowsheet (131.16 kmol/hr) is much less than that of the triple-column double side-stream extractive distillation process design (273.10 kmol/hr) recently published in the open literature. Economical comparisons show that the annual operating cost and the total annual cost can be significantly reduced by 43.8% and 35.6%, respectively. Further energy-saving considerations of the proposed design are investigated. It turns out that by heat integration of the condenser in the C2 column with part of the reboiler in the C1 column together with the preheating feed stream of the C2 column by the bottom streams of C1 and C2 column. Further 27.7% of the annual operating cost and 18.6% of the total annual cost can be saved. The second kind of feed composition is located in the water distillation region. The recycle flowrate of the proposed energy-efficient design flowsheet (22.12 kmol/hr) is much less than that of the triple-column partial heat-integrated pressure-swing distillation process design (95.45 kmol/hr) recently published in the open literature. Economical comparisons show that the annual operating cost and the total annual cost can be significantly reduced by 64.6% and 56.4%, respectively. Further energy-saving considerations consist of the heat integration of the condenser in the C2 column with part of the reboiler in the C3 column and of the preheating feed stream of the C2 column by its bottom stream. Further 28.4% of the annual operating cost and 9.3% of the total annual cost can be saved. Besides, multiple steady states occur in the isopropanol column due to the nonideality of the azeotropic liquid phases. The phase splitting for the liquid composition drives the column into input multiplicity and output multiplicity conditions, which make the control system difficult. In this thesis, according to the open-loop and closed-loop sensitivity test, three different control structures are established to reject unmeasured feed composition disturbances without using online composition analyzer. The results show that the control structure which only fixes the C2 column reflux flowrate-to-C2 column feed stream ratio (RF2/C2F) and makes the C1 column become dual-point control can maintain all products at high-purity specifications. Furthermore, the usage of feedforward throughput-compensated temperature control for the C1 and C2 column can improve the closed-loop control performance when facing throughput changes. In summary, for separating the complicated ternary system, a new energy-efficient design flowsheet which makes use of natural liquid-liquid separation in a decanter together with varying operating pressure of respective columns is provided. These energy-efficient design thinking can be extended to other separation systems exhibiting similar characteristics in the ternary diagram with liquid phase splitting. In addition, the control structures are also established to reject the disturbances in feed flowrate and feed composition. Because no composition controller is required for the proposed control structure, it is expected that this control structure can also be applied in the industry.