Design and Control of Reactive Distillation System for Hydrolysis of Methyl Acetate

This work explores the hydrolysis of methyl acetate (MeAc) from the PVA plant, and produce high purity acetic acid and methanol by reactive distillation which improves the conversion of MeAc in conventional processes. The process consists of one reactive distillation column, two distillation columns and one recycle stream. MeAc is reacted with excess water, and we use Amberlyst 15 ion-exchange resin as heterogeneous catalyst. Due to low reaction rate and methyl acetate is a light component, total reflux operation in reactive column is required to reach high conversion of MeAc and avoid azeotropes in the separation process. The bottoms flow from the reactive column is fed to the second column, which produces acetic acid with 99mol% purity in the bottom and the distillate goes to the third column. The methanol product comes out from the top of the third column and a water-rich bottom stream is recycled back to the reactive column.
In order to simulate real situation, both pure feed and azeotropic feed (60mol% MeAc) systems are considered in optimum design based on total annual cost (TAC). The hydrolysis system is a recycle process, and relevance of each unit operation is strong. There should be an equilibrium between the cost of reaction and separation process to have the most economic structure. In optimal design procedure, two important design variable are identified: one is recycle flow rate (the amount of excess water) and the other is specification of acetic acid in the distillate of second column (pinch point exists between water-acetic acid system).
In process dynamics, three control strategies (CS1~CS3) are studied, including temperature and composition control. The system should keep stoichiometric balance for satisfying both product specifications. Therefore, in the control structure, fresh water feed is controlled by the liquid level of the third column. Temperature control results in offsets in product composition. Although composition control has slow response, it maintains product purity close to their set points. The result shows these schemes have good performance for disturbance rejection.