Liquid–Liquid Extraction for Recovering Low Margin Chemicals: Thinking beyond the Partition Ratio

Recovery of carboxylic acids from fermentation broths is an active area of research due to ongoing interest in utilizing renewable feedstock for chemical production. Several recent studies have focused on recovery via liquid–liquid extraction using reactive extraction solvents such as high molecular weight amines because they yield significantly higher partition ratios. However, these solvents tend to be more expensive than conventional physical extraction solvents. We have measured the liquid–liquid phase equilibrium behavior for extracting propionic acid from aqueous solutions at 26–91 °C using 1-butanol (physical extraction) and a blend of trioctylamine and 1-octanol (reactive extraction). As expected, the amine-based solvent system is more effective at extracting propionic acid. Additional analysis shows, however, that the 1-butanol process is still preferred in spite of its lower partitioning for propionic acid due to the high cost of the amine solvent relative to the product (propionic acid). Our study therefore shows that solvents must be evaluated based not only on the partition ratio but also on solvent cost, product cost, mutual solubilities, thermal stability, and ease of recovery.