利用半批次反應蒸餾生產乙酸異丙酯與乙酸乙酯製程之最適化

Abstract

批次反應蒸餾(Batch reactive distillation)結合了批次程序的彈性及反應蒸餾的優點。半批次反應蒸餾(Semi-batch reactive distillation)是從批次反應蒸餾的一種衍生製程，在批次反應蒸餾中使用一反應物或萃取劑作連續性進料，除了原有批次反應蒸餾的優點外更可以突破共沸點的限制。 在乙酸異丙酯的系統中，最低共沸點是在兩相區之中，為一三成份共沸物，分離之兩相在油相中還殘留3%的反應物需要被回收，而水相純度只有95%，根據Qi, Wei; Malone, M. F. Semi-batch Reactive Distillation for Isopropyl Acetate Synthesis. Ind. Eng. Chem. Res. 2011, 50, 1272-1277，使用批次反應萃取蒸餾需要很大的回流比及板數，他們同時也提出利用半批次反應蒸餾可以增進生產效率及產品純度，使用乙酸為連續性進料來降低塔頂異丙醇的濃度，接著再使用逆轉式批次蒸餾塔(Inverted batch distillation column)進一步分離，把水從油相產物中分離而得到高純度之酯產物。 在乙酸乙酯的系統中，最低共沸點雖然和前者一樣為三成份共沸物，但落在兩相區之外，因此無法以批次反應蒸餾來得到高純度的產物，只能適用半批次反應蒸餾。在此系統中使用乙酸為連續性進料來降低塔頂乙醇的濃度使蒸餾液回到兩相區之中，其餘設計和乙酸異丙酯相似。 在這篇論文中，研究兩系統之最適化，找到最低能耗之參數。在半批次反應蒸餾塔之產物中，乙酸和醇同為不純物，而逆轉式批次蒸餾塔只能去除醇，因此固定在半批次反應蒸餾塔產物中乙酸之不純度上限，調整醇之不純度，當醇之不純度上升時半批次反應蒸餾塔能耗減少，逆轉式批次蒸餾塔能耗增加，因此醇之不純度可以視為連接兩單元間最重要的變數，可以找到一個最佳的值來達到最低能耗。

Batch reactive distillation (BRD) is an integrated process which combines reaction and distillation in a batch processes. Semi-batch reactive distillation (SBRD) is an alternative design from BRD in which reactant or entrainer is added as a side feed continuously. Besides the advantages of BRD, SBRD can circumvent limitations due to azeotropes. In the synthesis of isopropyl acetate by BRD, the lowest-boiling point is a ternary heterogeneous azeotrope located in a two-liquid region. After separation the organic phase still contains 3% reactant and the aqueous phase contains only 95% H2O. According to Qi, Wei; Malone, M. F. (Semi-batch Reactive Distillation for Isopropyl Acetate Synthesis. Ind. Eng. Chem. Res. 2011, 50, 1272-1277), batch reactive distillation requires an additional entrainer, a large reflux ratio and a large number of stages. They also show that by using acetic acid as a side feed into the column continuously the concentration of isopropyl alcohol in the distillate can be reduced efficiently. A second non-reactive inverted batch distillation (IBD) can be employed to separate the product IPAC from water. Hence, SBRD has the potential to improve the production efficiency. For the ethyl acetate synthesis, the lowest-boiling point is a ternary azeotrope, but it is not in two-liquid region. Therefore, a BRD is not feasible in this case. However, for the SBRD, using HAC as a side feed can drag the distillate composition into two-liquid region, closer to the ETAC-H2O edge. It also needs a second non-reactive IBD to separate the product ETAC from water. A similar design to the IPAC system is used. For both systems, we study the optimization of this process to find the values of parameters that minimize the energy consumption. Since the acetic acid and the alcohol are impurities in first column (SBRD) product and the second column (IBD) can only reduce alcohol impurity, in the SBRD we set a constraint on acetic acid impurity and adjust the constraint on the alcohol impurity which is the most important variable affecting both columns. When increasing the constraint on the isopropyl alcohol impurity, the energy required for the SBRD decreases and the energy required for the IBD increases. Therefore, we can find the optimal value of the isopropyl alcohol constraint that minimizes total energy consumption.