Supercritical Carbon Dioxide Separation of Carboxylic Acids and Phenolics from Bio-Oil of Lignocellulosic Origin: Understanding Bio-Oil Compositions, Compound Solubilities, and Their Fractionation

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