(408a) Isolation of Low Grade Biobased Bulk Chemicals
Focus of this project was the intensification of isolation of acetic acid and formic acid from black liquor evaporation condensates. This ternary mixture exhibits a binary high boiling azeotrope and a ternary saddle point azeotrope with the low boiling component water in high excess. Liquid-liquid extraction is well investigated and perfectly suited for separation of the carboxylic acids from the aqueous feed. However, solvent regeneration and byproduct isolation are still too energy intensive.
Combining distillation with an appropriate chemical reaction may intensify the solvent regeneration. Esterification of the carboxylic acids with a low molecular weight alcohol produces low boiling esters. These esters can be continuously removed from the reaction broth by distillation, acting positively on the equilibrium composition of the aqueous broth towards complete conversion of the target constituents. Expectedly catalytic acceleration of esterification would contribute to intensification. By using a reactive extractant with preference to acids the liquid-liquid equilibrium is improved compared to physical extraction and the acidic catalyst is kept in the solvent phase during extraction due to adduct formation with the reactive extractant. Admixture of solvent-soluble acidic coextractants may for sure limit the extraction capacity of the solvent, but provide the catalytic support. In solvent regeneration via reactive distillation the elevated temperature in the reboiler releases the catalyst from the adduct.
For technology assessment and process validation experiments were performed with Cyanex®923 (a Lewis basic phosphonate extractant) for extraction plus 4-dodecylbenzenesulfonic acid for catalysis. With this solvent mixture complete conversion and isolation of formic acid was achieved and methyl acetate yield of 88 % was obtained. For comparison distillative isolation of acids would cause an economic disaster and isolation by extraction/distillation would end up in the ternary azeotropic mixture.