(565d) Pervaporation, An Efficient Tool for Separation of Azeotropic Mixtures in Ester Synthesis

The bulk chemicals esters contribute to commodity chemicals in nearly unlimited variety. Though well investigated, the technology of ester formation may offer a variety of challenging characteristics regarding catalysis, operation conditions and downstream processing. Methyl acetate synthesis via esterification of acetic acid and methanol serves as a welcome model reaction, since methyl acetate forms low boiling binary azeotropes with methanol and the by-product water. Reverse hydrolysis reaction and transesterification are further examples facing the same issue.

The esterification reaction itself was catalyzed by commercial cation exchange resins. The operation temperature was kept well below boiling point. The reaction kinetics was modeled with an adsorption based approach accounting for the fact that the reactants show different adsorption behavior on the resin due to their different polarity. The reaction was performed under acetic acid limiting conditions and excess methanol and water were then separated from methyl acetate by pervaporation.

Pervaporation is a promising technology for separation of azeotropic mixtures. The main benefits are based on the moderate operation temperatures, partial evaporation and the nature of the driving force. This unit operation can handle higher water contents than reactive distillation technology. Moderate temperatures may suppress unwanted side reactions such as dehydration (ether formation) of methanol.

With membranes featuring hydrophilic surface properties methanol and water can pass the membrane, whereas methyl acetate is concentrated in the retentate phase. The binary mixture methyl acetate/methanol is the technological bottleneck of the isolation process. Appropriate membranes are needed for separation. Depending on the degree of crosslinking of the membranes and the composition of the feed, composite membranes show different swelling behavior which affects their selectivity and the permeate fluxes. Therefore membrane selection still needs experimental support and modeling of the permeate flux has to account for composition dependent swelling.

Based on the outcome of investigations pervaporation-assisted isolation of methyl acetate is an alternative route to the state of the art process.