(303k) Process Alternative for Methyl Acetate Conversion Using Reactive Distillation: Transesterification Versus Hydrolysis
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. This paper provides a quantitative comparison of two process alternatives for the methyl acetate conversion. First, methyl acetate is converted back to acetic acid and methanol via the hydrolysis route. Second, methyl acetate is transformed to butyl acetate via transesterification using n-butanol. Because both transesterification and hydrolysis reactions are reversible, the reactive distillation is chosen as the major reaction unit in the design of entire process units. Due to low chemical equilibrium constants in both reactions, excess reactants design is economically favorable as compared to the "neat" design of the reactive distillation. At the conceptual design stage, the relative positions of reactive zone and separation sections are determined followed by a systematic design procedure is used to generate detailed process flowsheets. The total annual cost (TAC) is used to evaluate the competitiveness of process alternatives. Detailed process development is described as follows.
The methyl acetate hydrolysis has received considerable attention in the last decade [1-3]. Generally, the design objective is to achieve high conversion for the limiting reactant in a single reactive distillation column [1-3] and, typically, water is the excess reactant. Thus, the product stream contains relatively low purity acetic acid and methanol along with large amount of water, and further separation in necessary to recover high purity acetic acid. In this work, the design of the entire hydrolysis plant is studied. The process consists of one reactive distillation column, two distillation columns and one recycle stream with the product specification of 99 mol% acetic acid. The design procedure of Tang et al.  is used for the design of reactive distillation column and TAC is used to design evaluation. The resultant flowsheet has the following structure. In the reactive column, fresh and recycle water are fed to the top, fresh methyl acetate/methanol (MeAc/MeOH, 60 mol% MeAc) is introduced to the stripping section. Total reflux operation in reactive column is required to reach 99% conversion of MeAc . The bottoms flow from the reactive column is fed to the second column, which produces acetic acid 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 this work, two important design variables are identified: one is the % excess of water and the other is the acetic acid in the distillate of second column (Because of a pinch point exists at the water end between water-acetic acid system). This leads to a TAC of 2.2 million for a production rate of 50 kmol/hr acetic acid.
Because of relative high TAC for the MeAc hydrolysis, an alternative is sought, which is to convert methyl acetate into butyl acetate. The transeasterification reaction is MeOH + BuAc = MeAc + BuOH. Two plantwide designs using reactive distillation can be found in the literature [6-7]. One is a four column configuration  and the other is a three-column design . Carefully examing the process, a new flowsheet is proposed, which includes one conventional distillation column followed by a reactive column. Fresh methyl acetate/methanol azeotropic feed and the recycled methanol are fed to the first column, which produces methanol in the bottom and MeAc/MeOH mixture (close to azeotropic composition) at the top. The distillate of the first column and fresh butanol are fed to the reactive column. Butyl acetate product is withdrawn from the column base, while the methanol-rich distillate is recycled back to the first column. It is found that the configuration with two columns is capable of producing high-purity butyl acetate and methanol with a TAC of 1.2 millions for the acetate production rate of 50 kmol/hr.
The results presented here provides useful information for the production planning for the PVA plant, and, more importantly, correct reactive distillation configuration as well as the entire plant structure is identified.
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