Separation and purification of caproic acid from chain elongation fermentation broth: A comparative study between adsorption and reactive extraction

Sequential arrested anaerobic digestion followed by chain elongation (AAD-CE) is a promising biological process for producing caproic acid from wet organic waste, such as garden and food waste (commonly referred to as green waste). However, the downstream process for caproic acid recovery represents a significant portion of the overall production cost, necessitating the development of more economical alternatives. The aim of this study was to evaluate two different caproic acid separation and purification methods: adsorption using activated carbon followed by desorption with NaOH and ethanol, and reactive extraction using the biocompatible solvent oleyl alcohol as a diluent and tri-n-butyl phosphate (TBP) as an extractant. The results of batch experiment showed that 95.2% of the caproic acid in the fermentation broth was adsorbed onto activated carbon at a concentration of 80 g/L, while 82.8% was recovered after two cycles of desorption with ethanol supplemented with 0.1 M NaOH. In contrast, the reactive extraction method demonstrated that oleyl alcohol alone could recover approximately 90.2% of caproic acid at 30 °C and pH 4.0, with mixing for 60 min. The addition of TBP further enhanced recovery, achieving a rate of 96.3%. Continuous downstream process simulation and a cost assessment revealed that reactive extraction had lower direct fixed capital and annual operating costs compared to adsorption/desorption, particularly for the production of protonated caproic acid. • Activated carbon adsorbed up to 95.2% of caproic acid from fermentation broth. • The supplementation of TBP (4:1) into oleyl alcohol extracted 96.8% caproic acid. • The caproic acid downstream process design and cost was modeled. • Caproic acid recovery by reactive extraction was a more cost-effective method.