(507f) Reactive Separation of Formic Acid, Acetic Acid and Water From Biorefinery Feed

Painer, D., Graz University of Technology
Lux, S., Graz University of Technology
Siebenhofer, M., Graz University of Technology

In the biorefinery valuable bulk products are rarely provided in simple mixtures of appropriate composition. Formic acid/acetic acid/water mixtures from pulp industry provide an impressive demonstration for  non-separable systems from the distillation point of view. Isolation of highly pure acids is challenging since conventional rectification is more or less impossible, because of facing a binary high boiling azeotrope and a ternary saddle azeotrope.

Reactive separations give access to a variety of promising process concepts which allow effective product isolation from complex mixtures with thermodynamic separation limits. Separate esterification of formic acid and acetic acid are well investigated chemical reactions. However, the combination of esterification with distillation of either both carboxylic acids or selective esterification with distillation give access to a new approach for reactive separation of azeotropic mixtures.

Esterification with methanol will form low boiling esters ready for complete separation by distillation. Permanent removal of the esters shifts the equilibrium towards complete conversion of acids. Because of intramolecular catalysis no further catalyst is needed.

In a first step the vapor/liquid-equilibrium of the ternary system was verified experimentally at ambient pressure. For simulation of vapor/liquid equilibrium data the UNIQUAC model and the Hayden O’Connell equation were applied.

During process development the reaction kinetics of the individual esterification reactions were then investigated and kinetics was modeled. Simultaneous esterification of acetic acid and formic acid was then validated experimentally. Because of faster esterification of formic acid, the amount of methyl formate outweighs methyl acetate formation. Along with the difference in the boiling points of the formate and the acetate this gives access to selective removal of formic acid from the mixture.

Product isolation routines were validated with batch rectification, confirming highly selective product separation. Validation was completed with different feed compositions to demonstrate that the distillation limits of the ternary system may be avoided by chemically altering the substance properties, enabling a flexible technological toolbox for process design.


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