Extraction Of Hemicellulose By Hot Water To Reduce Adsorbable Organic Halogen Formation In Chlorine Dioxide Bleaching Of Bagasse Pulp

The study has drawn a conclusion from the research background in pre-extracting hemicelluloses by hot water and existing situations in this field over domestic and international scales.This technology provides significant merits for traditional paper and pulp industry where low energy consumption and environmental benefits would increase added values and improve competitiveness.Moreover,the study also provides an outline of utilizing solid phase and liquid phase after pre-extraction technology.Liquid phase:1)hydrolyze extracts from hot water;2)separate and purify hydrolysates;3)ferment sugar ethanol.There are three alternatives for utilizing solid phase:1)extraction by hot water for following pulping,bleaching and papermaking process;2)advanced separation and hydrolyzation for woody biomass remains in order to make aromatic hydrocarbons and glucose.In addition,based on the mechanism of absorbable organic halogen(AOX)formation during the chlorine dioxide bleaching,the reduction technology of AOX had been put forward.It mainly can be divided into four aspects.For example the process optimization,chemical pretreatment,biological pretreatment and the latest technology is the composite processing technology which is composed of the physical or chemical pretreatment and biological pretreatment.The by-product of sugarcane bagasse as the main raw material in this topic,it systematically studied extraction of hemicellulose by hot water to reduce adsorbable organic halogen formation in chlorine dioxide bleaching of bagasse pulp,the concrete research content is as follows:1)Bagasse hemicellulose efficient extraction and characterization remaining solids;2)Extraction of hemicellulose by hot water to reduce adsorbable organic halides formation in chlorine dioxide bleaching of bagasse pulp;3)Kinetics study of oxidation of the hemicellulose model compounds by chlorine dioxide;4)Molecular simulation of reaction mechanism for hemicellulose model compound during chlorine dioxide bleaching.To reduce the degradation of cellulose and obtain high molecular weight of hemicellulose from theextracts,pH pre-corrected hot water pretreatment was developed by employing 3.9 M sodium hydroxide.The response surface model was established to optimize the extraction process.The speciescomposition and purity of hemicellulose extract was analyzed by High Performance Liquid Chromatography(HPLC).The obtained solid remainder was analyzed by Fourier Transform infrared spectroscopy(FTIR)and Scanning Electron Microscope(SEM).The results showed that the component of xylose in hemicellulose extract was similar with commercial xylan.FTIR and SEM were shown to be able to evaluate solid remainder composition and surface characterization of the bagasse.The biggest balance between solid remainder and dissolved solid was obtained.Not only the yield of dissolved solid was improved,but the structure of solid remainder was also proved,which was beneficial to pulping and papermaking.To study the influence of the content of hemicellulose on the AOX formation in chlorine dioxide bleaching,a pH pre-corrected hot water pretreatment was developed.The influence of lignin on AOX formation was eliminated by the control of hot water extraction time,AOX formation was decreased by 33.27%under optimal hot water extraction conditions.The chemical composition of bleaching effluent was analyzed by Gas Chromatography-Mass Spectrometer(GC-MS),and the formation of chlorobenzene and chlorophenol was inhibited by removal of hemicellulose,the ring structures of which were degraded into chain macromolecules and small molecules.The hemicellulose structure of raw material and bleached pulp was characterized by ATR-FTIR and Nuclear Magnetic Resonance(NMR).This finding revealed differences in the structures of the bagasse hemicelluloses and the solid residual hemicelluloses.AOX formation was affected by the structure of hemicellulose.A new process combining biorefinery and clean pulp and paper was developed through these experiments.A study of the kinetics of the oxidation of D-xylose,a hemicellulose model compound,by chlorine dioxide under simulated bleaching conditions was performed.The final reaction product,chloroacetic acid,was detected by GC-MS.The kinetics were expressed as dW/dt =2.72e'592.5/T[H+]0.05[ClO2]O.11X0.92.The kinetics was close to first-order,with a good agreement between the experimental and modeling data.The reaction activation energy was 4.93 kJ-mol-1,the low reaction activation energy indicated that hemicellulose was highly reactive with chlorine dioxide.The results will provide a theoretical basis for reducing AOX formation based on the perspective of hemicellulose.In order to study the generating mechanism of chloroacetic acid by D-xylose during chlorine dioxide bleaching,the molecular orbital energy distribution of D-xylose,xylitol and 3-hydroxyglutaric acid was calculated by the quantum chemical method.Three reaction paths were designed.The thermodynamics and kinetics of the reaction process of D-xylose constitutional unit were studied by searching the transition state of D-xylose during the reaction process.The results showed that the biggest heat of reaction was-234.33 kJ·mol-1 and the minimum reaction activation energy 44.44 kJ·mol-1,they appear in the reaction path 2.The best reaction path 2 was obtained.Xylitol was generated by D-xylose degradation,and then chloroacetic acid was generated by a series of oxidation,fracture and substitution reaction on xylitol.