Contaminations Impairing an Acetic Acid Biorefinery: Liquid-liquid Extraction of Lipophilic Wood Extractives with Fully Recyclable Extractants

Reactive extraction using organic solvents is regarded as an efficient method for carboxylic acid separation from dilute aqueous solutions. However, when biomass hydrolysates are used as feedstock, the contamination of extraction solvents with undesirable biomass constituents may cause problems at the industrial scale. To identify such impurities and quantify the extent of their negative influence on the extraction efficiency, a solvent of an existing acetic acid biorefinery was analyzed. High amounts of lipophilic wood extractives and minor amounts of solid humin particles were identified in the trioctylphosphine oxide-based solvent. Batch and single-drop extractions with model solutions revealed that wood extractives form complexes with trioctylphosphine oxide, thus reducing the distribution ratio and the mass transfer rate for acetic acid. Therefore, an additional purification process is required that removes the wood extractives from the solvent via liquid-liquid extraction. Five ethanolamine derivatives were screened as aqueous back-extraction solvents and the influence of the extraction parameters (amine concentration and temperature) and of multiple extraction stages was investigated. N,N-dimethylethanolamine was shown to remove 99% of fatty acids under optimized conditions. Residual amine was eliminated from the purified organic phase through washing with water or diluted acid. The aqueous back-extraction solvent could be recycled from the removed impurities via distillation. We showed that lipophilic wood extractives can contaminate organic solvents and severely impair their acetic acid extraction efficiency. We propose a novel method to remove these impurities using fully recyclable ethanolamines to restore the extraction capacity of the solvent. Adding this process to existing plants helps to increase process efficiency, maximize plant availability, and represents an important step towards valorization of previously untapped feedstocks for acetic acid production.