CO2-enhanced extraction of acetic acid from fermented wastewater

Abstract

The industrial process of recovering fermentation-based volatile fatty acids (VFAs) utilizes H₂SO₄ to acidify the fermentation broth containing VFA-salts [e.g. Ca(CH₃COO)₂] to enable formation of molecular VFAs. Molecular VFAs are then recovered by liquid–liquid extraction. However, acidification with H₂SO₄ results in production of large quantities of salts (e.g. CaSO₄). Using CO₂ rather than mineral acids for acidification of fermentation broth is an environmentally benign alternative which eliminates salt formation. In this study, CO₂ was applied in pressures up to 40 bar to enhance the efficiency of extraction of acetic acid (HAc) from fermented wastewater. HAc extraction under atmospheric conditions was also investigated to obtain benchmarks. The ionic liquid [P_666,14][Phos] and trioctylamine (TOA) dissolved in n-octanol were applied as solvents to extract HAc from fermented wastewater model solutions containing HAc (1 wt%) and various salts resulting in pH ranging from 2.8 to 6. A more pronounced increase in extractability of HAc, expressed as HAc distribution (D = [HAc]_solvent/[HAc]_aqueous), was observed for [P_666,14][Phos] with increasing CO₂ pressure. A mathematical model taking into account carbonic acid equilibria and dissociation of HAc and salts showed that the measured influence of CO₂ cannot be explained by the effect of CO₂ on aqueous phase pH. Thus, it may be concluded that the pressurized CO₂ has altered the fluid properties of the solvents and made them more accessible for HAc. This suggests that applying pressurized CO₂ may enhance extraction efficiency of processes other than those involving extraction of volatile fatty acids.