Production of caproic and caprylic acid through anaerobic fermentation of crops or residual and waste biomass has been regarded as an alternative to the conventional ways, where plant oils and animal fats are mostly used. The downstream processing of the fermentation broth is a particular challenge since the broth has a highly complex composition and low concentrations of the target products. In this study, the proof-of-principle for a separation cascade for caproic (C6) and caprylic acid (C8) produced in a maize silage-based fermentation process was demonstrated. For clarification of the fermentation broth, a filter press and a ceramic ultrafiltration membrane was used to remove coarse solids and to separate suspended particles and macromolecules from the fermentation broth, respectively. With both techniques, the dry matter content was reduced from 6.8 to 2.3% and a particle-free product solution was obtained. Subsequently, the carboxylic acids were extracted with oleyl alcohol by liquid-liquid extraction with an extraction efficiency of 85% for C6 and 97% for C8. Over the whole cascade, 58% of caproic acid and 66% of caprylic acid were recovered from the fermentation broth into the extract. Among all separation steps, solid-liquid separation with the filter press caused the major part of the product loss of 21% of each carboxylic acid. By using separation equipment with a better solid separation efficiency such as decanter centrifuges or belt filter presses this loss could be minimized.
In the published article, there was an error. The water solubility and the corresponding temperature of caproic acid quoted in the introduction was incorrect.A correction has been made to the Introduction, 2nd paragraph. This sentence previously stated:"They belong to weak acids with pKa values of 4.88 and 4.89 (Kim et al., 2021) and have water solubility values of around 1.1 and 0.8 g L −1 at 40 °C for C6 and C8, respectively, in their undissociated form (Yalkowsky et al., 2010)."The corrected sentence appears below: "They belong to weak acids with pKa values of 4.88 and 4.89 (Kim et al., 2021) and have water solubility values of around 10.1 and 0.8 g L −1 at 30 °C for C6 and C8, respectively, in their undissociated form (Yalkowsky et al., 2010)."The authors apologize for this error and state that this does not change the scientific conclusions of the article in any way. The original article has been updated.
Caproic acid (C6) and caprylic acid (C8) can be produced during anaerobic fermentation via chain elongation processes. This study focused on the selective recovery of C6 and C8 from a fermentation broth with liquid‐liquid extraction. Eight solvents, including alcohols, esters, an organophosphorus compound, and an amine, were evaluated based on their extraction efficiency and selectivity for C6 and C8. The highest extraction efficiency of C6 (46 %) and C8 (93 %) at pH 6.0 was achieved by 1‐octanol. Oleyl alcohol, 1‐decanol, and 1‐octanol had the highest selectivity for C6 and C8.
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