Extractive Isolation of Acetic Acid and Formic Acid from Green Liquor Condensates

Acetic acid and formic acid are representative byproduct constituents of pulping processes. During evaporation of spent cooking liquor they are transfered into the condensate phase.

Condensate treatment is expensive and a waste of valuable bulk products.

Separation of acids from the condensate phase by liquid-liquid extraction with phosphonate based solvents such as Cyanex 923 is a state oft he art recovery step.

Isolation of highly pure acids from the extract phase is challenging since conventional rectification is more or less impossible because of the formation of a binary high boiling azeotropic mixture of formic acid and water  and formation of a saddle azeotrope of the ternary mixture of acetic acid, formic acid and water. Complete isolation of constituents by distillation is not possible.

By combining the process steps esterification, rectification and pervaporation the separation problem can be solved successfully.   

Due to the difference in acid strength formic acid may be esterified with methanol selectively in a first step, preparing the major amount of aqueous acetic acid for further downstream processing.

Alternatively both acids may be esterified with excess methanol in heterogeneously catalysed reaction with the catalyst Amberlyst 15 and separated from the aqueous carrier by combination of rectification and pervaporation