(365f) Reactive Distillation Combined with Pervaporation for Biobased By-Product Recovery

Painer, D., Graz University of Technology
Lux, S., Graz University of Technology
Siebenhofer, M., Graz University of Technology
In downstream processing the biobased industry often struggles with complex multicomponent mixtures. By-products in effluent with low concentration may complicate the wastewater treatment and limit complete usage of biomass. In pulping, an effluent containing mainly acetic acid and formic acid is produced from black liquor evaporation. State of the art process for effluent treatment is reactive extraction followed by several distillation steps for solvent recovery and product isolation. Reactive extraction combined with alternative separation methods were investigated to reduce the high energy consumption and apparatus demand for solvent regeneration and isolation of acetic acid and formic acid from black liquor evaporation condensate. The aqueous mixture of acetic acid and formic acid forms inseparable azeotropes.

Reactive distillation and pervaporation are perfectly suited for separation of azeotropic mixtures. Reactive distillation overcomes azeotropes by changing the substance properties via chemical conversion. With carefully chosen esterification the market value of the by-products may be increased, and the significant change of the physico-chemical properties and product separation decrease the cost of product isolation. For separation of acetic acid, formic acid and water reactive distillation with methanol as esterifying agent was successfully performed, but with low reaction rates without catalyst. For successful solvent regeneration high reaction rates and high separation efficiency of the carboxylic acids are needed. The reaction rates were raised by admixture of 5 wt.% 4-DBSA the catalyst 4-dodecylbenzenesulfonic acid (4-DBSA) to the solvent. Batch reactive distillation experiments of laden solvent showed complete conversion of formic acid and a methyl acetate yield of 88 % in the distillate. The feasibility of catalyst admixture to the solvent phase was successfully tested. The catalyst has a minor impact on the liquid-liquid equilibrium. The distillate contains the low boiling methyl esters and excess methanol. Water is not transferred to the distillate. This product mixture exhibits a low boiling azeotrope of methyl acetate and methanol.

Pervaporation is able to isolate azeotropic mixtures because the separation does not depend on the vapor-liquid equilibrium; it is the result from solution-diffusion properties of the components in the membrane. Therefore, pervaporation with hydrophilic membranes was investigated for the separation of methanol from the methyl esters. Pervaporation experiments with methanol/methyl acetate mixtures, the bottleneck in the system, were performed with the commercial membrane PervapTM 4155-70. With the ternary mixture, the viability of separating methanol from both methyl esters was confirmed.

The implementation of alternative separation tools shows a high potential for raising the product yield and for reducing energy demand. Simplification of process concepts satisfies the specific needs of the biobased industry.