側流反應器架構於乙酸酯化之應用

Abstract

反應性蒸餾結合反應與分離於同ㄧ單元內，可以節省能源的消耗及設備成本。除了其潛在的好處外，反應性蒸餾本身仍存在操作與設計上的問題，特別是針對填充異相觸媒之反應蒸餾塔經常會有觸媒老化、置換及裝填方式硬體設計的問題。本研究以乙酸酯化生成乙酸乙酯及乙酸丁酯之系統為例，探討利用側流反應器架構來替代反應性蒸餾。根據Tang et al.(2005)對於乙酸乙酯系統反應性蒸餾之設計可以發現接近90%之反應量發生於反應蒸餾塔塔底部分，其餘10%則發生於10板反應板之內。此結果自然地使我們聯想到將所有觸媒填入塔底部的反應器/蒸餾塔偶合架構，稱為單一反應板反應蒸餾(Single Reactive Tray, SRT)。利用此架構可以解決傳統反應性蒸餾在操作上之問題。然而模擬的結果顯示，將所有觸媒填充到塔底並無法達到與反應性蒸餾相同之結果(轉化率約93%)，因此提出另ㄧ替代設計，保留原先單一反應板反應蒸餾架構，在塔的外部加入一反應器，稱為側流反應器架構(Side Reactor Configuration,SRC)。針對此架構其主要之設計變數為塔底填充觸媒量(Wcat,bot)、反應器達平衡轉化率之百分比(%Xeq)、抽出(Nss,w)與送回塔板位置(Nss,r)及抽出物流之流量(Fss)。運用系統化的方法進行側流反應器架構的設計，並以年總成本(TAC)為基準進行比較，進而獲得最適之設計結果。就年總成本而言，側流反應器架構相較於反應性蒸餾僅大約5%的差異。 接下來考慮乙酸丁酯系統，其反應物之沸點排序與反應的分佈都有別於乙酸乙酯系統，對於此系統，針對兩種架構進行探討，首先為一前置反應器加側流反應器架構(反應物進入前置反應器中進行反應後送入塔內)，另ㄧ架構為所有反應器皆為側流反應器之架構。結果顯示利用所有反應器皆為側流反應器之架構較一前置反應器加側流反應器架構效果要來的好。在使用四個側流反應器之架構下，其年總成本相較於反應蒸餾約高出25%。 針對本研究的兩個系統，雖然在年總成本上，側流反應器架構較反應蒸餾要來的高，但若考慮到觸媒置換的便利性而言，側流反應器架構將為替代傳統反應性蒸餾極佳之選擇。

Reactive distillation (RD) combines reaction and separation in a single unit to reduce energy consumption and capital investment. Despite of potential advantages, the reactive distillation may suffer from maintenance/design problems such as catalyst deactivation/replacement and hardware design (for catalyst packing), especially for heterogeneous catalyst such as ion exchange resin. In this work, an alternative design, side reactor configuration, is sought and the processes of interest are the production of the ethyl acetate (EtAc) and the butyl acetate (BuAc) via esterification. The reactive distillation study in (Tang, et al., 2005) reveals that almost 90% conversion takes place in the column base of the RD and the rest of the 10% conversion occurs in the 10 reactive trays. This naturally leads to a coupled reactor/distillation configuration where all of the catalyst is packed in the bottoms base, denoted as Single Reactive Tray reactive distillation (SRT) hereafter. This mitigates the maintenance problem associated with conventional RD. However, simulation results show that, with the same amount catalyst loading (Wcat,RD), the SRT configuration cannot achieve the same performance as the RD (~93% conversion). Another alternative is adding external reactors to the Single Reactive Tray distillation column and this is termed as the Side Reactor Configuration (SRC). Unlike conventional reactive distillation design, the SRC design is less clear and design variables include: catalyst loading in the column base (Wcat,bot), % of equilibrium conversion for side reactor (% Xeq), sidestream withdrawn and return trays (Nss,r), sidestream flow rate (Fss). A systematic design procedure is devised for the SRC design and the objective function to be minimized is the total annual cost (TAC). The results show that the TAC of the SRC only increases by a factor of 5% as compared to that of the RD. The butyl acetate (BuAc) system is considered next. It differs from the EtAc system in (1) relatively even reaction distribution across reactive zone and (2) boiling point distribution where both reactants are intermediate keys. Two possible SRC configurations are explored. One is the pre-reactor (only one inlet stream to the column) plus side reactors and the other is complete side reactor scheme where all reactors coupled with the column. The results show that the complete side reactor scheme performs much better as compared to the pre-reactor plus side reactor scheme. As the total number of reactor increases to four, the TAC only increases by a factor of 25% as compared to that of the RD. For the two cases studied, the TACs of the SRC is slightly larger than that of the RD, however, considering the ease of catalyst replacement, the Side Reactor Configuration offers attractive alternative to conventional reactive distillation.