Solvent selection and optimisation of the liquid-liquid extraction of volatile fatty acids from the aqueous stream of the HTL-WO process for the production of aviation biofuels

Hydrothermal liquefaction (HTL) is a thermochemical process used to produce advanced biofuels. Besides bio-crude oil, HTL generates an aqueous stream containing organic compounds that can be decreased in concentration or transformed by wet oxidation (WO), producing water rich in volatile fatty acids (VFAs) and ammonium. This study explores liquid-liquid extraction to recover and valorise VFAs from the HTL-WO aqueous stream. Using molecular simulation with the COSMO-RS method, six terpenes and four conventional organic solvents were selected for experimental investigation. First, synthetic aqueous mixtures of VFAs were tested, followed by a real aqueous sample obtained from the HTL-WO of a straw and manure mixture. Optimisation of operating variables for acetic and propionic acid extraction was carried out. Using geraniol, citronellol, verbenone, and methyl isobutyl ketone (MIBK) as solvents, with a solvent-to-feed ratio (S/F) of 2, extraction temperature of 303.2 K, and water pH of 2, the recovery of VFAs was maximised in the real stream. Extraction yields exceeded 50% for acetic acid and 80% for propionic acid in both batch and continuous-parallel extraction using a packed column. Under the same conditions, cross-current multistage extraction was studied. Finally, counter-current extraction in a column was simulated with the Kremser method, showing that over 99% of VFAs in the aqueous stream could be extracted in five stages. These results demonstrate the effectiveness of terpenes as green solvents for VFA recovery in HTL-WO process water. • HTL-WO aqueous stream is rich in VFAs, recoverable via liquid-liquid extraction. • Terpenes compete with conventional solvents to extract VFAs from aqueous samples. • Optimal VFAs extraction from HTL-WO water uses S/F 2, 303.2 K, and pH near 2. • Acetic and propionic acids yield ~99% using 5-stage countercurrent extraction.