乳酸純化製程之反應蒸餾系統設計與最適流程控制

Abstract

近年來由於對環境保護意識的逐漸重視，在自然環境中遇熱可分解的環保素材如環保乳膠手套等環保物質在現今越來越受到重視。而環保乳膠手套正是乳酸的聚合產物，因此乳酸的需求也因此逐年增加，乳酸有兩種形式，L型和R型，其中L型的乳酸可以製造出較高品質的乳膠產品。 由於乳酸的傳統製程是必須透過發酵反應，而製程中其中會含有許多的雜質和未反應的原料有待分離，因此乳酸純化的議題近年來也越來越受到重視，純化方式有很多種，例如：分離方式加入熟石灰再經過過濾或是加入溶劑做萃取分離或是利用電極吸引分離，但是都會有乳酸回收率不高或是總處理量不大的缺點。因此利用乳酸與醇類的酯化以及乳酸酯與水的水解反應來純化乳酸具有高的處理量以及較高的乳酸回收率等優點。 本研究針對粗乳酸的純化製程做出設計，利用五種不同的醇類來設計此純化製程，並透過年總成本來選擇最經濟的純化製程，結果顯示甲醇以及丁醇有最少的年總成本，兩者會依據不同的回償年限而改變兩者的排名。 接下來針對甲醇以及丁醇系統的最適化穩態流程做控制架構的設計以及控制性能的測試。其中，控制架構以溫度控制以及組成控制為主，最後也嘗試串級控制，並且將原本的最適化的穩態架構做推廣，將水回流接上以期能夠有更好的能源運用。結果顯示甲醇系統的控制效果優於丁醇系統，可在較短的時間內達到新的穩態結果:甲醇的控制架構能通過流量正負10%以及乳酸5%以及重成分雜質3%的進料組成的擾動;丁醇的控制架構能通過流量正負10%以及乳酸5%以及重成分雜質3%的進料組成的擾動。

Process designs for the continuous recovery of lactic acid from fermentation broth by reactive distillation (esterification and hydrolysis with C1 to C4 alcohols) are developed and optimized to minimize cost. The best designs are qualitatively different for different alcohols because of differing volatility ranking of reactants and products and the formation of a two liquid phase zone in some cases. The results suggest that the methanol and butanol processes are the most attractive solution. The costs of these two processes are similar, and the ranking depends on the payback period (when payback period is 3 butanol is more economical, vise versa). The ethanol and isopropanol processes are more expensive because an entrainer is required to break the alcohol/water azeotrope. Control structures for the methanol and butanol processes are designed and tested for the performance through feed flowrate, lactic acid composition and impurity composition disturbances. Designed control structures can be separated into 2 scenarios. One is 1-recycle stream of recovered alcohol. Another one is 2-recycle streams with recovered alcohol and water. Methanol system can implement 2 recycle streams and butanol system can implement 1 recycle stream. Results show that 1-recycle scenario is more robust than 2-recycle scenario because it has a more straight forward flowsheet. Combined composition and temperature control structures can handle more disturbances and maintain the constraints. Both methanol and butanol control structures can pass flow rate and composition disturbances. Methanol control structure is more robust than butanol control structure because it can handle the disturbances in less time.