Influence of selected quench media used for direct contact condensation on yield and composition of fast pyrolysis bio-oils aided by thermodynamic phase equilibria modelling

Direct contact (quenching) condensation of fast pyrolysis vapours into fast pyrolysis bio-oils (FPBOs) is associated with the advantages of high efficiency, improved thermal performance and significant reduction in process cost. However, choosing a suitable quench media (QM) for the process remains a challenge and it has been a subject of ongoing investigations in the quest to optimising the recovery of FPBOs. In this study, the influence of four QM that include water, Isopar-V, ethanol and ethylene glycol on the product yield and composition of FPBOs were evaluated. Different ratios (0.5 and 2.0) of the respective QM to hot pyrolysis volatiles (mq/mv), which define the quenching temperature and extent of cooling were probed. Ethanol recovered the highest yield of the organic-rich condensate (ORC) fraction of the FPBOs, most particularly at the mq/mv ratio of 2.0, which produced 50 wt% more ORC than ethylene glycol and 75 wt% more than water and Isopar-V. Ethylene glycol proved to be the most efficient QM for the recovery of carboxylic acids, ketones and phenolic compounds in the ORC. Isopar-V largely demonstrated immiscibility with the ORC, however, some slight interactions with the ORC product were evident. For the water quench, owing to its significant interactions with the ORC, nearly all water-soluble compounds, particularly sugars ended up in this quench following condensation, making it valuable for the recovery of sugars directly during quenching. Phase equilibria model prediction of the quenching condensation process with the modified UNIFAC Dortmund (UNIFAC-DMD) thermodynamic model showed huge prospects specifically in predicting product yields and the chemical composition of the ORC. Nevertheless, the limitations of this model became apparent in scenarios that include predicting low concentrations of organics in water, association and hydrogen bonding interactions as well as uncertainties associated with pure component vapour pressure data of some compounds. Regardless, the study proves to be a significant addition towards optimising the selective recovery of valuable chemicals from FPBOs during direct contact condensation.