含有對二甲苯之醋酸去水製程設計與控制

Abstract

醋酸去水及回收是製造純對苯二甲酸中重要的程序之一。進料中含有水、醋酸與些許不純物，如醋酸甲酯與對位二甲苯。醋酸去水程序中常使用醋酸酯作為挾帶劑，但醋酸酯是製程中額外加入的成分，會在程序中產生預期外的反應。 純對苯二甲酸製程中的反應物對位二甲苯亦可作為醋酸去水的挾帶劑。此研究中將以非均勻相共沸蒸餾的設計分離高純度的醋酸與水並根據年度總成本找出最佳操作條件，程序設計上將討論側分流設計對程序的影響。模擬發現使用對位二甲苯作為挾帶劑可以節省挾帶劑補充所需的花費及避免醋酸酯殘留於回收的醋酸中對純對苯二甲酸製程的影響。側分流的設計可避免挾帶劑從塔底流失，但年度總成本較高且無明顯效益。 動態方面將對無側流的設計提出適當的控制策略。利用開環靈敏度測試與閉環靈敏度測試提出單點、雙點溫度控制策略，使用的操作變數為再沸器熱負載與水相回流比。此控制策略可有效的排除進料中組成的擾動與產能變化。

Acetic acid dehydration and recycle is an important process in purified terephthalic acid (PTA) production. The feed contains water (H2O), acetic acid (HAC) and some impurities, such as methyl acetate (MA) and p-xylene (PX). Acetic acetate is commonly used as entrainer in acetic acid dehydration, but it is an additional component which will cause unexpected reactions in the PTA process. The reactant (PX) in PTA process also can be used as the entrainer. In this work, the heterogeneous azeotropic distillation will be used to separate high purity acetic acid and water and the optimum design with minimal total annual cost (TAC) will be investigated. In the process design, effect of side-draw to the process will be discussed. Using PX as entrainer not only can save the cost of entrainer makeup but also prevent the effect of acetic acetate remaining in the recycled acetic acid in the PTA process. It is found that the side-draw design can avoid entrainer loss from recycled acetic acid, but need higher TAC and cannot provide obvious advantage. Some feasible control strategies for no side-draw design are then investigated subsequently. The open-loop and closed-loop sensitivity analyses techniques are adopted to develop the single-point as well as the dual-point temperature control strategies, where the manipulated variables are reboiler duty and aqueous reflux ratio. These strategies can reject feed composition disturbance and throughput change effectively.