Hot water treatment of hardwood kraft pulp produces high-purity cellulose and polymeric xylan

Hot water treatments (HWTs) of unbleached hardwood kraft pulps under various process conditions were conducted to extract the xylan and thus produce a high-purity cellulosic pulp that could be used in dissolving applications. Increasing treatment temperature up to 240 °C increased the removal of xylan over the degradation of cellulose in birch pulp, but this effect was minor at higher temperatures. Addition of acetic acid lowered the treatment intensity needed to reach a certain degree of pulp purity, but did not improve the selectivity in xylan removal compared to water-only experiments. HWTs of eucalyptus pulp, with lower xylan content than birch pulp, produced cellulosic fibers with higher degree of polymerization at a given pulp purity. Under selected operational conditions (240 °C for 10 min) in a flow-through reactor, and provided that the HWTs were applied before bleaching, the chemical and macromolecular properties of water-treated pulps may be suitable for their conversion to viscose. Moreover, at high flow rates (200–400 mL/min), the extracted xylan was recovered from the aqueous hydrolysate in high yield and with relatively high molar mass (~10 kDa). Based on the results of this study, HWTs of hardwood kraft pulp are suggested as a simple and green method to produce high-purity cellulose and polymeric xylan for high value-added applications.