Study On Hydrothermal Pretreatment With PH-Controlled Assisted Kraft Pulping Of Eucalyptus

With the increasing improvement of people's living standard,the demand for paper increases gradually.Lignocellulosic biomass is the main raw material for pulp and paper.The shortage of wood raw materials in China leads to great challenges for the pulp and paper industry.It is extremely urgent to develop cleaner and more efficient pulp and paper making technology combining with biomass refining theory and realizes high value utilization of raw materials.Hydrothermal pretreatment is an economical and environmentally friendly biomass pretreatment technology.The application of pH-controlled hydrothermal pretreatment(pH-HP)in pulp and paper is of great significance to the sustainable development of wood raw materials.In this study,eucalyptus is used as raw material.The effect of the final pHvalue of hydrolyzed liquids on the monose,oligose and total sugars was studied by using the pH-HP,and the residual solids(RS)under the optimal pretreatment conditions were characterized to analyze the influence mechanism of pretreatment on the main components and structure of eucalyptus.The residual solids are cooked by kraft pulping(KP)to analyze the effect of pH-HP on pulp properties.The kinetics of delignification during pulping of residual solids was studied.It provides technical and theoretical basis for eucalyptus pH-HP assisted kraft pulping.The effects of pHof hydrolyzed liquids on the residual solids,total dissolved solids(TDS),carbohydrate dissolution and lignin degradation of eucalyptus during hydrothermal pretreatment were studied.The results show that the equilibrium between the residual solid and the total dissolved solid is obtained at the pH4.0.The dissolution rate of hemicellulose was the highest,the degree of cellulose and lignin was the lowest,and the sugar components in hydrolysate were mainly in oligomer form.The residual solids were characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD)and Fourier infrared spectrometer(FTIR).The lignin from raw material and residual solids was extracted and its structure was determined by 2-dimension nuclear magnetic resonance(2D-NMR).The results showed that the surface damage degree of the residual solid was less,the fiber crystallinity increased by2.1%,hemicellulose related functional groups were significantly weakened,and lignin structural degradation was inhibited in the pHof hydrolyzed liquids at 4.0,compared with the traditional hydrothermal pretreatment.It is proved that pH-HP can not only realize the efficient extraction of hemicellulose,but also protect the physical and chemical structure of the residual solid,which is beneficial to the subsequent high-value utilization.The effect of the pH-HP on the subsequent kraft pulping process and the pulp properties was studied.By controlling the maximum cooking temperature,alkaline charge and sulphidity,the pulp yield number,kappa value and viscosity are taken as index of comparison.The results show that the pretreatment is beneficial to reduce the cooking energy consumption and chemical dosage when the pulp properties are similar.The content of cellulose in pulp was significantly increased,hemicellulose and acid lignin were reduced,the crystallinity of fiber was slightly increased,the lignin-related groups and distribution on the surface of fiber were reduced,and the removal effect of S-type lignin structure in pulp was significant,compared with the traditional kraft pulping without hydrothermal pretreatment.Kraft pulping after pH-HP can reduce energy consumption and chemical dosage,reduce lignin content on fiber surface,and improve slurry performance.The kinetic model of delignification was established for hydrothermal pretreatment of residual solids followed by kraft pulping.The kinetic constants of each influencing factor were calculated and the complete kinetic equation was obtained:d L/dt=e^16.53e^-5744.7/T×[Na OH]^1.11×[Na₂S]^0.27×L^-2.70.Through fitting calculation,the reaction series n=-2.70 of lignin removal rate L was obtained,and the fitting coefficient reached 0.9737.The reaction series of sodium hydroxide a=1.11,and the fitting coefficient is about 0.9679.The reaction series B of sodium sulfide was 0.27,and the fitting coefficient was about 0.9435.The reaction activation energy E=47.76 k J.mol^-1 calculated by Arrhenius law.The activation energy of the pulping after pretreatment is lower compared with other delignification kinetics studies.