(459c) Process Intensification in By-Product Recovery from Biobased Effluents

Authors: 
Siebenhofer, M., Graz University of Technology
Painer, D., Graz University of Technology
Lux, S., Graz University of Technology
Low-value by-product processing is an economic challenge common to many biobased processes. For recovery, the downstream processing must be very efficient, Value-adding conversion via chemical reaction may be beneficial. Both goals can be met with reactive separations. Chemical conversion of target constituents may simplify separation, and the market value of by-products may be increased by conversion.

Beside the main product fibres the pulping industry produces a black liquor containing lignin, hemicellulose, hydroxycarboxylic acids and carboxylic acids with a high water content. After thickening, this effluent is mainly used for thermal energy production via incineration. The volatile carboxylic acids are removed with the water during thickening. Separation of acids from the condensate is recommended for environmental reasons. State of the art recovery of carboxylic acids from aqueous effluents is reactive extraction followed by distillation for solvent regeneration. However, distillative solvent regeneration produces an azeotropic mixture of carboxylic acids and water causing energy intensive product isolation. Therefore, reactive distillation was investigated to intensify solvent regeneration and product isolation. Methanol was added as esterifying agent to the laden solvent containing formic acid and acetic acid, converting the carboxylic acids into their corresponding low boiling methyl esters. Esterification improves the separation of the acids and the reactive extractant and simplifies separation of the distillate products due to lower boiling points of the methyl esters compared to the acids. A homogeneous (4‑dodecylbenzenesulfonic acid) and a heterogeneous catalyst (Amberlite IR120) were tested to speed up the reaction rates in reactive distillation. The admixture of 5 w% of 4‑dodecylbenzenesulfonic acid and a proton equivalent amount of Amberlite IR120 in batch reactive distillation experiments provided complete conversion of formic acid and 88 % yield of methyl acetate in the distillate. High solvent regeneration efficiency at low temperature and ambient distillation pressure is achieved. The heterogeneous catalyst enables simple separation from the solvent in the reboiler. In homogeneous catalysis catalyst separation step may be challenging. To circumvent an additional separation step the influence of the homogeneous catalyst in the solvent phase on extraction efficiency of acid extraction was investigated. Loss of solvent load capacity is of minor impact. Due to the preference of the reactive extractant to strong acids, the catalyst 4‑dodecylbenzenesulfonic acid is kept in the solvent phase during extraction without losses. During distillation the catalyst-extractant aggregate is cleaved at operation temperature in the reboiler and therefore available for catalysis of esterification, enabling combination of extraction and reactive distillation without catalyst separation.

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