Bio-oil produced from the thermochemical treatment of lignocellulosic biomass is increasingly recognized as a potentially abundant source of renewable chemicals and fuels. Single ring phenolics and low molecular weight carboxylic acids are significant constituent compound groups found in bio-oil and are important end products or intermediate commodity chemicals. Fractionation of bio-oil using supercritical fluids (usually with CO2 as a solvent) is a relatively new process being investigated worldwide at both laboratory and pilot scales. Solubility data associated with supercritical carbon dioxide (scCO2) and the many chemical compounds in the complex bio-oil mixture are required to predict the extraction behavior of different bio-oil compounds. This article starts with a review of the composition of bio-oil in terms of the phenolic and low molecular weight carboxylic acid fractions which are potentially of commercial interest. Binary solubility data of major compounds in these bio-oil fractions with supercritical CO2 are summarized and discussed. Results from previously reported studies in which scCO2 is used as a solvent to recover bio-oil fractions are reviewed and collated. Density and temperature-based Chrastil type models are developed using available data for the solubility in scCO2 of some of the major bio-oil compounds. Finally, extraction of compounds from the complex bio-oil mixture is discussed in terms of the trends predicted by the respective individual binary solubility models.

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This study focused on understanding the relationship between dicarboxylic acid (DCA) yields derived from lignin and the structural attributes of their solid residues and corresponding acid-soluble lignin fractions. It is a continuation of the study by the authors on DCA production from bagasse lignin via hydrothermal liquefaction. It characterized the residues derived from the use of H2O2/chalcopyrite and sodium percarbonate for DCA formation, at temperatures between 60 and 300 °C with a reaction time of 3 h. FTIR, GPC, and 2D NMR were used to unravel the functional group changes, molecular weight distribution, and lignin substructures and linkages. The DCA yield correlated well, in a linear fashion, to the aromatic to aliphatic functional group ratio (AAFGR) and the degree of aromatic condensation (DAC). Supporting evidence of lignin depolymerization and repolymerization to explain the DCA yields was provided by GPC. Interestingly, the proportion of the α-oxidized substructure S′ was found to be an indicator of the extent of lignin depolymerization, and its ratio to the S substructure was found to correlate with DCA yield at reaction temperatures up to 200 °C.

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Supercritical fluid extraction (SFE) and fractionation of products from a complex mixture such as bio-oil, where many compounds are present in low concentrations, is a difficult process to model. This difficulty arises from the uncertainty associated with those interactions between mixture components for which fundamental vapor–liquid equilibrium (VLE) data are not available. In this work a novel extraction and purification concept is investigated using a predictive model developed from VLE data of binary solute–solvent systems; solute–solute interactions in the supercritical carbon dioxide (scCO2) phase are neglected. The predictive component of the work employs an equation of state (EOS) model to achieve the above task. The results of pilot plant trials utilizing a biocrude feedstock were shown to be in good agreement with the model predictions. Aspen Plus process simulations were developed for the extraction process which comprised supercritical extraction and subsequent purification steps utilizing distillation and multistage evaporation. A techno-economic analysis of different process designs were evaluated for comparison. In particular, distillation as the primary separation process with and without multistage evaporation were simulated to compare the economics of supercritical extraction to distillation. It was found from simulation results that distillation is a very energy intensive process, and total operating costs for it are always greater than supercritical extraction counterparts. Combining multistage evaporation with distillation will bring the total operating cost slightly lower than supercritical extraction processes. However, the internal rate of return (IRR) value was similar for both SFE and distillation combined with multistage evaporation processes. Solvent/bio-oil (S/B) ratio will have considerable impact on total profits of SFE process in relation to distillation combined with multistage evaporation.