A new generation of organosolv biorefinery: Birch fractionation in gamma-valerolactone, full valorization of pulping streams, and solvent recovery

Abstract

This thesis presents a comprehensive biorefinery concept based on wood fractionation in gamma-valerolactone. Firstly, the stability of gamma-valerolactone (GVL) was evaluated in a wide range of pH (pH = 0-9) with a focus on pulping conditions (150-180°C). GVL hydrolyzes in a highly acidic environment and forms equilibrium with 4-hydroxyvaleric acid (4-HVA) (4 mol%) while under highly alkaline conditions GVL ring irreversibly opens and forms 4-hydroxyvalerate (>10 mol%). Nevertheless, GVL/ 4-HVA equilibrium remains stable under typical pulping conditions. The second part of this thesis investigated the alkaline-catalyzed fractionation with prehydrolysis, acid-catalyzed and alkaline-catalyzed fractionation of silver birch in 50 wt% GVL (180°C, 120 min). Although prehydrolysis could remove a substantial amount of hemicelluloses, the alkaline catalysis did not improve the overall quality of pulp in comparison to uncatalyzed or acid-catalyzed fractionation. Acid-catalyzed fractionation produced highly pure but significantly destroyed cellulose. After the optimization of fractionation conditions, the temperature of 150°C, time of 120 min, 55 wt % GVL, and the liquor-to-wood ratio of 4 were selected as the optimal conditions for dissolving pulp production.The produced dissolving pulp resembled the properties of commercial acid sulfite pulp, and it could be readily converted to fibers by Ioncell® technology. The mechanical properties of spun GVL-fibers and GVL-yarn exceeded the ones of commercially regenerated cellulose fibers and yarns. 70 % of dissolved lignin was isolated from the spent liquor by the addition of water (1:1), which was further processed as a polyol via an isocyanate-free route into polyhydroxyurethane. Most of the hemicelluloses were dissolved into spent liquor mostly in the form of monomeric xylose. It was demonstrated that xylose can be easily isolated by cooling crystallization from spent liquor in the presence of residual GVL.The last part of the thesis focused on GVL recovery from spent liquor by liquid CO2 extraction. In the laboratory setup, the recovery of GVL was 67 %, while 9 % of GVL remained in the raffinate. The low recovery rate was limited by the number of separation stages and short column height. Nevertheless, the advanced modeling validated based on laboratory experiments demonstrated that almost 99% of GVL could be recovered and recycled back to pulping