本研究針對聚乙烯醇工廠所產生的乙酸甲酯(MeAc),以反應蒸餾塔將其水解為更具價值的乙酸及甲醇,改善傳統製程中低轉化率的缺點。系統由一個反應蒸餾塔、兩個分離塔及一迴流股組成。乙酸甲酯與過量的水進行反應,以Amberlyst 15離子交換樹脂作為異相觸煤。由於水解反應速度慢且乙酸甲酯沸點低,反應蒸餾塔操作上須採塔頂全回流方式以達到高轉化率,避免後續分離程序中醇酯共沸物形成。塔底產物送入第二根塔進行分離,於塔底得到乙酸,產品規格為99mol%。塔頂產物送入第三根,塔頂得到甲醇,塔底大量的水迴流至反應蒸餾塔中。 為了模擬真實工廠操作,將乙酸甲酯進料條件分為純進料與共沸進料(60mol% MeAc)系統,並藉由計算年總成本(TAC)進行最適化設計。水解乙酸甲酯系統為迴流程序,單元之間操作上有其關聯性。在反應與分離程序成本間,應取得一平衡點,以符合最經濟的系統架構。在最適設計步驟中,我們發現兩個最重要的變數:迴流股流量(水過量幅度)及第二根塔頂乙酸規格(乙酸與水之間有狹點存在)。 在系統動態方面,我們提出三種控制架構(CS1~CS3),包含溫度控制與濃度控制。為了確保兩產品組成均達到規格,在控制架構中,以第三根塔底液位控制新鮮水入料量,於系統中維持化學計量平衡。在溫度控制下將造成產品組成偏差;而濃度控制雖然響應較慢,但最終產品濃度符合規格,由模擬結果可知這些架構對於進料條件擾動有良好的排除效果。
This work explores the hydrolysis of methyl acetate (MeAc) from the PVA plant, and produce high purity acetic acid and methanol by reactive distillation which improves the conversion of MeAc in conventional processes. The process consists of one reactive distillation column, two distillation columns and one recycle stream. MeAc is reacted with excess water, and we use Amberlyst 15 ion-exchange resin as heterogeneous catalyst. Due to low reaction rate and methyl acetate is a light component, total reflux operation in reactive column is required to reach high conversion of MeAc and avoid azeotropes in the separation process. The bottoms flow from the reactive column is fed to the second column, which produces acetic acid with 99mol% purity in the bottom and the distillate goes to the third column. The methanol product comes out from the top of the third column and a water-rich bottom stream is recycled back to the reactive column. In order to simulate real situation, both pure feed and azeotropic feed (60mol% MeAc) systems are considered in optimum design based on total annual cost (TAC). The hydrolysis system is a recycle process, and relevance of each unit operation is strong. There should be an equilibrium between the cost of reaction and separation process to have the most economic structure. In optimal design procedure, two important design variable are identified: one is recycle flow rate (the amount of excess water) and the other is specification of acetic acid in the distillate of second column (pinch point exists between water-acetic acid system). In process dynamics, three control strategies (CS1~CS3) are studied, including temperature and composition control. The system should keep stoichiometric balance for satisfying both product specifications. Therefore, in the control structure, fresh water feed is controlled by the liquid level of the third column. Temperature control results in offsets in product composition. Although composition control has slow response, it maintains product purity close to their set points. The result shows these schemes have good performance for disturbance rejection.
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