Design and Control of Reactive Distillation Processes for the Production of Butyl Acetate via Transesterification

In the production of Polyvinyl Alcohol (PVA), a large amount of methyl acetate (MeAc) is produced as by-product, i.e., 1.6 kg MeAc/1 kg PVA. Since methyl acetate has relatively low economical value, efficient reuse of MeAc is an important issue in the PVA plants. Generally, two process alternatives exist. One is hydrolysis where acetate is converted back to acetic acid and the other is transesterification where MeAc is converted to a higher value acetate. The second chemical route is explored here and, in this work, methyl acetate transesterification using butanol is studied. The transeasterification reaction is MeOH + BuAc ↔ MeAc + BuOH. Two plantwide designs using reactive distillation (RD) can be found in the literature. One is a four column configuration and the other is a three-column design. In both examples, attempts have been made to break the azeotrope between BuOH and BuAc to achieve high purity product. Carefully examining the process, a new flowsheet is proposed. It includes one conventional distillation column followed by a reactive column in which the azeotrope is reacted away in the RD column. Because of relatively low chemical equilibrium constant, “excess” reactant (MeAc) design is considered. A systematic design procedure is used to optimize the flowsheet. The results show that the two-column design is capable of producing high-purity butyl acetate and methanol with a TAC only 2/3 of the three-column design. Finally, the control structure design for this recycle plant is addressed. A systematic procedure is proposed for the plantwide control system design which starts from the RD column and then back to the distillation column. Three possible quality control structures are considered: two-temperature scheme, one-temperature-one-composition control and two-composition control. Dynamic simulation results show that good control can be achieved using two-temperature control and, moreover, steady-state composition offset can be mitigated using feedforward compensation for temperature set points.