Semicontinuous Aqueous Acetone Organosolv Fractionation of Lignocellulosic Biomass: Improved Biorefinery Processing and Output

Aqueous acetone organosolv fractionation of the lignocellulosic biomass using a batchwise operation is a robust technology option to produce cellulose, sugars, and lignin. Such fractionation is typically characterized by fast solubilization of most of the lignin and hemicellulose sugars in the early stages of the process, gradually followed by slower removal of the remaining, more recalcitrant part at later stages. As a result, most of the solubilized sugars and lignin experience a relatively long residence time in the hot liquor, leading to undesired sugar degradation and lignin depolymerization-condensation reactions. A lab-scale, semicontinuous countercurrent flow fractionation design is presented here as a solution to this issue and studied as an intermediate step toward an envisioned scaled-up design involving a series of percolation reactors coupled for liquid exchange. Counter-current semicontinuous processing (SCP) reduces the overall residence time of the solubilized sugars and lignin. While even slightly improving on fractionation performance, i.e., sugar and lignin solubilization, most importantly, SCP resulted in less sugar degradation and more hemicellulose oligomers compared to the simple batch process. SCP lignin proved to be of higher quality with increased β-O-4 content and, consequently, a higher abundance of lignin aliphatic hydroxyl groups, less formation of Hibbert ketones, and reduced condensation. The more native nature of the lignin is reflected in its improved reactivity in reductive partial depolymerization to lower molar mass and dispersity lignin building blocks. Overall, SCP thus provides a powerful and scalable strategy for improving the efficiency and effectiveness of aqueous acetone organosolv fractionation.