Kraft Cooking Kinetics of Poplar Wood and the Impact of a Mechano-Enzymatic Pre-Treatment

Kraft cooking kinetic of poplar wood (Populus deltoides and Populus nigra) was investigated using chips of controlled size, and various enzymatic/mechanical pretreatments of the chips were attempted to observe the effects on pulp quality. Batch Kraft cooks of small amounts of untreated wood chips was carried out in mini-autoclaves at 140 - 165°C. Delignification rate constants and apparent activation energy during the bulk phase were determined. Various mechanical pretreatments, using a modular screw device (MSD) with or without xylanase impregnation prior to the cooks, were also investigated. Cooks were monitored through pulp yield, kappa number, fiber morphology, viscosity-average degree of polymerization (DPv) and residual effective alkali (REA). The results showed a variable, temperature-dependent activation energy of about 100 kJ/mol for the bulk delignification with a non-linear Arrhenius behavior suggesting a shift from reaction-controlled to diffusion-controlled kinetics as temperature increased . The effectiveness of xylanase impregnation was limited, due to chips thickness that limits enzymes penetration. MSD and MSD-Xylanase pretreatments significantly reduced fiber length (from 871 µm to 845 µm and 784 µm, respectively) and increased fines content (from 20.3% to 26.3% and 29.1%, respectively), indicating degraded fiber quality after the cooks of pretreated chips. Variability in pulp yield and REA was too high to reach statistical significance. Finally, it was shown that neither of these pretreatments could improve pulp quality in terms of fiber morphology and pulp strength properties, but this study opens the way towards utilization of pretreatments to improve pulp fibrillation for nanocellulose production.