轉酯化產製碳酸二乙酯製程之設計與控制

Abstract

反應蒸餾具有突破共沸點限制、提高轉化率、減少設備成本等的特性，進一步的可以進行反應性隔牆蒸餾(RDWC)之節能設計，來減少傳統製程的設備成本與操作成本，由於RDWC同時具有反應蒸餾與隔牆塔設計的優點，且可透過熱耦合的方式(thermally coupled)來使整廠製程有效的提高能量使用效率，因此一般而言相較於傳統製程可以預期RDWC設計會有較低的年度總成本(total annual cost, TAC)。 在本研究中將探討兩個產製碳酸二乙酯的主題，第一個主題是重新回歸熱力學參數以修正先前文獻利用非均相觸媒碳酸鉀使反應物碳酸二甲酯與乙醇反應生成碳酸二乙酯的傳統製程設計，並進一步的將傳統製程兩塔合併為一塔進行反應性隔牆蒸餾的節能設計，除了可以減少設備成本之外，也因為消除了再混合效應使得提高能量使用效率並減少操作成本，因此在年度總成本上隔牆塔節能設計可以比傳統製程減少約13%。此外，進一步的研究結果發現當不使用過量反應物進料比時，在反應物進料比剛好等於反應速率表示式的係數比的條件下，如果增加精餾段板數，只需要一根反應蒸餾塔就可以完成產製碳酸二乙酯的目的，所以在設備成本與操作成本上相較於傳統製程都會大幅地降低，最後研究結果發現年度總成本可以比傳統製程減少約34%。動態模擬方面也針對單塔反應蒸餾塔探討了動態架構與控制策略，並進行閉環干擾測試以確認在進料流量±20%與進料組成-10%、-20%干擾進入程序時，還能維持住高純度的產品規格，最後結果顯示以雙點溫度控制加上利用前饋控制更改設定點的方式，可以有效的排除流量與進料組成干擾的影響。 第二個主題將探討如何從碳酸二甲酯與甲醇所形成的共沸物原料到產製碳酸二乙酯的路徑分析，共沸物進料組成源自於先前文獻，包含了17.86mol%的碳酸二甲酯與82.14mol%的甲醇，有四個不同製程設計可以利用此共沸物來產製碳酸二乙酯，其一是傳統使用四根蒸餾塔的設計，其二是將傳統四塔後面兩塔合併為一塔，做一隔牆塔節能設計，其三為不使用過量反應物乙醇，而是以劑量比為反應物進料比的方式來產製碳酸二乙酯，以上三種皆是先利用萃取蒸餾的技術將共沸物分離成純的碳酸二甲酯與甲醇後再與乙醇反應生成碳酸二乙酯，其四的設計為直接利用乙醇與共沸物中的碳酸二甲酯反應生成碳酸二乙酯，只需一根蒸餾塔即可完成反應和分離的目的，雖然此種設計可以大幅降低傳統製程複雜性，卻因為操作成本過高而使得此製程設計不具經濟效益，最後結果發現第三個製程設計擁有最低的年度總成本，可以比傳統製程節省約26%的年度總成本，每年的盈餘也是所有製程中最高的，說明了在利用共沸物作為進料產製碳酸二乙酯的路徑中，先將共沸物分離後再予以反應來產製碳酸二乙酯會較具有經濟優勢。

Break the azeotropes limits, increased conversion and reduced capital cost are well-known features of reactive distillation processes. An intensification design configuration of reactive dividing-wall column (RDWC) may be proposed to reduce equipment costs and energy consumption in conventional reactive distillation process. RDWC has both the advantages of reactive distillation column and dividing wall column and can avoid unnecessary energy consumption. In general, it is expected that the total annual cost will be lower by RDWC design compared with that of the conventional process. This research focuses on two topics about the production of diethyl carbonate (DEC). The first topic is that the thermodynamic parameters from previous paper are modified and redo the conventional process design for the production of DEC. Afterwards, RDWC design is proposed to not only reduce capital cost but also operating cost by eliminating remixing effect. As a result, the result shows that the TAC with RDWC design can be reduced by about 13% compared with that of the conventional process. Besides, further study finds that when the feed ratio of reactant is exactly at stoichiometric ratio instead of excess ratio, the reaction and separation task can be achieved by using only one RD column if the rectifying stages are increased. Hence, capital cost and operation cost can be dramatically decreased. The result shows that the TAC can be reduced by about 34% compared with that of the conventional process. Dynamic structure and control strategy are also discussed on one RD column design. Closed-loop disturbance tests are performed to confirm that when the ±20% flowrate disturbance or -10%、-20% feed composition disturbance enter the process, the distillation column can still hold the high-purity specifications of the two products. The results show that the feed flowrate and feed composition disturbance can be effectively eliminated with dual temperature control and feed forward control. The second topic concentrates on different design flowsheets for the production of DEC from the azeotrope composed of DMC and methanol. The azeotrope feed composition is from previous paper, and it contains 17.86mol% DMC and 82.14mol% methanol. There are four different design flowsheets in this topic. The first one is the conventional process which has four distillation columns. The second one is that RDWC design is applied to the conventional process. The third one is that the feed ratio of reactant for the production of DEC is exactly at stoichiometry ratio. All the above three design flowsheets separate the azeotrope firstly by extractive distillation, and then take excess ethanol to react with dimethyl carbonate. The fourth one is that ethanol is taken to react with this azeotrope directly to produce product DEC and methanol. Only one reactive distillation column is needed to accomplish the reaction and separation task. Although this proposed design method can decrease the complexity of the conventional process, it doesn’t have economic benefit because of its higher operating cost. The results show that the third design flowsheet has the lowest TAC and its TAC can be reduced by about 26% compared with that of conventional process. The third design flowsheet also has the highest earnings per year. This illustrates that the design flowsheet for the production of DEC from the azeotrope has more economic benefit when the azeotrope is separated firstly and then react with ethanol by stoichiometry ratio.