Acetic acid extraction from aqueous solutions using fatty acids

• Acetic acid extraction from aqueous solutions was studied using fatty acids as solvents.
• Fatty acids performed comparable to other oxygen bearing solvents.
• The extraction of hexanoic acid was modeled with a homogeneous extraction model.
• Heptanoic acid showed higher selectivity than hexanoic acid.

A major challenge for production of acetic acid via bio-based routes is cost-effective concentration and purification of the acetic acid from the aqueous solutions, for which liquid–liquid extraction is a possible method. A main challenge in extraction of acetic acid from dilute aqueous solutions is to develop a solvent that allows for high distribution and selectivity in the extraction, and an effective and preferably simple recovery method to regenerate the solvent and recover the product. In this study fatty acids were proposed as extracting agents for acetic acid because of their ability to form double hydrogen bonded complexes that, compared to e.g. acid–base complexes were expected to be easier to regenerate. Experimental extraction studies with hexanoic acid and heptanoic acid showed that although the distributions were lower than were expected, these fatty acids are applicable for the extraction of acetic acid from dilute aqueous solutions, and an extraction model with homogeneous organic phase complexation was developed and found to describe the experimental equilibria for hexanoic acid accurately. The partition and complexation coefficients were determined for the extraction of acetic acid with hexanoic acid: Kp,HAc = 0.145 ± 0.019 (–) and Kcomp = 0.338 ± 0.039 (L/mol). With a selectivity of the fatty acids for acetic acid over water of up to 12 for hexanoic acid, and up to 24 for heptanoic acid, significant concentration through extraction is possible, and further purification through fractionation during thermal recovery is considered.