Research On Impact Of Pre-extracted Hemicellulose On Poplar Wood Chemimechanical Pulping Performance And Antifouling Mechanism In Janus Composite Membranes

Currently,the primary challenge facing the pulp and paper industry is the transformation of pulp production towards low-carbon,economical,and biomass-refining directions to achieve green innovation in high-yield pulp production.This research project is based on the concept of biomass refining and has developed a high-yield pulp production process involving the pre-extraction of hemicellulose.The primary goal was to address key technical challenges in pulping and facilitate the transformation of extracted hemicellulose into high-value applications.The study investigated the impact of hemicellulose on wood storage modulus（E’）,cell wall microstructure,and the underlying mechanisms.This was accomplished by extracting hemicellulose from hardwood and applying it to the pretreatment of chemi-mechanical pulp（CMP）.Furthermore,the effect of hemicellulose pre-extract on reducing the energy consumption of refining while improving the fiber quality and fiber surface fibrillation of CMP was discussed.The pre-extracted hemicellulose was utilized for the hydrophilic modification of hydrophobic membranes,creating a novel avenue for high-value hemicellulose utilization.Based on the concept of integrated forest bio-refinery（IFBR）,the pulping process was designed with hemicellulose extraction and recovery,and pilot-scale production was achieved in the pulp production.The main research contents and results are as follows:（1）The effect of hemicellulose on storage modulus（E’）and microstructure of poplar wood and its mechanism were investigated.The hemicellulose was extracted from poplar using Na OH solution under different conditions,and the E’of wood samples with different degrees of hemicellulose removal was investigated.At 15.9 mg/g of hemicellulose removal,the wood E’decreased from 1074.3 MPa to 415.9 MPa due to hydrolysis of the lignin-carbohydrate（LC）bonds between hemicellulose and lignin.At 74.7 mg/g of hemicellulose removal from wood after treatment with 1:5 liquor ratio at the same Na OH concentration,the hemicellulose dissolved into the impregnating solution and E’decreased to 200.5 MPa,which was 81.4%lower than that of the wood without hemicellulose removal.This indicated that the LC bonds as well as the free hemicellulose that was not detached from the wood affects the viscoelasticity of the wood.The characterization of wood cell wall by Confocal Raman Microscopy（CRM）showed that the hemicellulose extraction was mainly in the middle lamella（ML）and secondary（S）layers of the fiber cell wall,indicating that the hemicellulose removal by Na OH pretreatment loosened the structure of the wood cell wall and affected the viscoelastic behavior of the wood.（2）Building upon the theoretical foundation of reduced wood viscoelasticity resulting from hemicellulose pre-extraction,we applied this concept to the pre-treatment process within chemical mechanical pulping（CMP）.The effect of hemicellulose on refining energy consumption,pulping and refining performance was investigated.The results indicate that,compared to the control samples,employing a 1:5 liquor ratio alkali pre-treatment under conditions of 40.8%hemicellulose extraction from wood led to a remarkable 68.3%reduction in refining total energy consumption（SEC）.This resulted in a paper breaking length exceeding 4 km and an 84.7%decrease in fine content in pulp.Analysis of fiber morphology and observation of microfibrils on the fiber surface through SEM suggest that the significant hemicellulose extraction may lead to potential sliding between the cell wall cellulose and lignin when subjected to the shear forces in the refining process.Therefore,the possible mechanism for the reduction in refining energy consumption could be attributed to the promotion of fiber separation from the lignin matrix due to this sliding,concentrating the shear forces on the lignin that envelops the cellulose,reducing energy consumption while minimizing fiber damage.Research on the high-yield pulp（AMP）indicates that it enhances fiber surface fibrillation during the PFI beating process.Furthermore,optimization of hemicellulose dissolution through single-screw pre-extrusion was also investigated.The optimized effect of Model-screw pre-extrusion on hemicellulose extraction was also investigated to reduce the pretreatment time and temperature.（3）A hemicellulose-modified hydrophobic Polyvinylidene fluoride（PVDF）membrane surface was prepared for membrane distillation（MD）.The aim was purifying oil-contaminated seawater and realizing a valuable application for hemicellulose extracted from the CMP process.The quaternized hemicellulose（QHC）was trapped in a three-dimensional network by Polyvinyl acetate（PVA）and cross-linked by Glutaraldehyde（GA）,to fabricate Janus membranes.The results show that the QHC modified PVDF membranes have higher hydrophilicity and underwater oleophobicity.The electrostatic interaction between the quaternary ammonium substituents and water molecules produced a hydration layer on the membrane surface,and the underwater oleophobic angle was increased from 0°to 146.8°,preventing the adsorption of surfactant and oil droplets.The zeta potential of the membrane surface changed from-32.2 e V to 21.5 e V,and the positive charge repelled mineral ions in seawater by electrostatic repulsion,improving the antifouling performance of the membrane.The Janus membrane exhibited a stable water vapor flux with J/J₀ consistently greater than0.96 with oily saline wastewater in lab direct contact membrane distillation（DCMD）experiments.（4）Based on the IFBR concept,a mechanical pulping process for hemicellulose extraction and recovery was designed,and a pilot production was conducted on the pulping section.The key feature of this process is the pre-extraction of hemicellulose before refining,followed by the utilization of the wood or bamboo chips,which have undergone pre-extraction of hemicellulose,for the production of high-yield pulp.Additionally,we have explored its application in unbleached kraft pulp.Furthermore,the pre-extracted hemicellulose is either collected as coagulated precipitate or directly modified in wastewater to produce crude hemicellulose by-products.The alkali present in the wastewater is chemically recovered,contributing to cost savings and reduced production costs.This study not only complements our understanding of the mechanism by which hemicellulose extraction facilitates the separation of hardwood or bamboo fibers in chemical mechanical pulping but also employs this mechanism as a theoretical foundation to develop a chemical mechanical pulping process with hemicellulose pre-extraction.This achievement realizes the low-energy preparation of hardwood or bamboo high-yield pulp and promotes the high-value application of hemicellulose.Our research expands the prospects of comprehensive production and high-value utilization of hardwood or bamboo high-yield pulp as well as crude hemicellulose by-products.