SO2–ethanol–water (SEW) fractionation of spruce: kinetics and conditions for paper and viscose-grade dissolving pulps†
The study describes SO2–ethanol–water fractionation of spruce as a promising basis for future Biorefineries. The dissolution kinetics of lignin and hemicelluloses as well as the depolymerisation kinetics of cellulose during the fractionation process are expressed in terms of the H-factor, a parameter combining fractionation temperature and duration. The raw material moisture content is shown to have a very small effect on the fractionation kinetics. The liquor-to-wood ratio has little effect on the process during the (delignification) bulk phase, while in the residual phase the effect becomes pronounced. During the latter phase, lower liquor-to-wood ratios lead to higher residual lignin content, lower residual hemicelluloses and pulp viscosity, but higher hemicelluloses removal selectivity. Higher delignification rates and selectivities are obtained at ethanol-to-water ratios close to 1 : 1 (w/w) and high SO2 concentrations (≥12 w/w%). Based on the presented data, the optimum fractionation conditions for the production of paper and viscose-grade dissolving pulps are quantified.