Optimization Of Poplar Wood Residues Pretreatment And Kinetics Of Different Hydrolysis Measures

Selection and optimization of pretreatment operations on poplar wood processing residues,including chemical method (wet-oxygen, sodium hydrosulfite), mechanical method (platerefiner and miller) and biological method (fungi strain: Phanerochaete chrysosporium), prior tohydrolysis, were investigated in this thesis. Some modern analysis instruments such as X-RD,SEM, TG, FT-IR, HPLC and GC-MS were employed to characterize physical and chemicalchanges of poplar stocks and hydrolyzate with different pretreatment methods. To compareconversion effects of different hydrolysis conditions on production of reducing sugars, acidichydrolysis (concentrated and low-consistency sulfuric acid) and enzymolysis (cellulase) wereapplied, the hydrolyzed residual solids and hydrolyzate were analyzed, and kinetic modelswere explored. Finally, fermentation of enzymic hydrolyzate by yeast was carried out toproduce bio-ethanol.Wet oxidation, a lignin and hemicellulose-selective oxidation operation combining oxygenand alkali at higher pressure condition, was applied to pretreatment of poplar wood processingresidues. Results show that hemicellulose was removed effectively, and cellulose (cellulose63.26%, pentose2.77%) contents was enriched in pretreated stock and the performance ofenzymic hydrolysis was enhanced. The optimal conditions of wet-oxidation were proposed,such as: initial pH number of10, reaction time15min, elevated temperature195°C, and oxygenpressure1.2MPa. The optimal wet-oxidation conditions was verified and crystallinity ofpretreated stock decreased from85.42%to65.74%(by X-RD), fiber surface corrodedextensively (by SEM), de-lignification effectively (by FT-IR) and provided enzymic hydrolysisof pretreated poplar wood processing residues a reducing sugar yield of46.75%. It could beconcluded that this pretreatment operation can benefit to sequenced enzymolysis operation.Sodium hydrosulfite (NaHSO₃) pretreatment was based on the lignin dissolution withsulfonation reaction, resulting to enforced the stock enzymolysis. The proper conditions wereproposed as: dosage of NaHSO₃6%(w/w), solid to liquor ratio1:3, temperature200oC and time30min, lignin and hemicellulose was removed obviously (lignin content decreased from23.6%to16.7%, hemicellulose content referred as pentose from18.7%to1.66%). Afterenzymic hydrolysis, the reducing sugar yield of41.55%can be obtained. The crystallinity ofpretreated stock was decreased to31.57%, the specific surface area of cell-walls increasedwhich also can benefit to the followed enzymolysis.The proposed of mechanical method, breaking up and grinding, could increase the fiberspecific surface area to improve stock enzymolysis, however, the effect was limited. Theenzymolysis of pretreated stock was not increased corresponding with higher BET surface area(reducing sugar yield was8.11%and9.88%, respectively). The two kinds of stocks werefurther treated by sodium hydrosulfite, and the enzymolysis properties were increased greatly.The reducing sugar yield was45.17%and32.56%, respectively.Phanerochaete chrysosporium pretreatment could remove lignin in raw material andincrease reducing sugar yield. The proposed condition for this pretreatment as: inoculationamount25%, training28days at33oC environment quietly. After sterilization and alkalineextraction, the stock was enzymolysis by cellulase, and14.51%of reducing sugar yield couldbe obtained. Lignin in raw material was selectively removed (13.98%of raw material), GC-MSshowed the fracture of C_α-C_βbond and methylation of lignin during pretreatment.During these pretreatments, the effect of wet oxidation was the best, the effects ofmechanical and Phanerochaete chrysosporium pretreatment were limited. Therefore, wetoxidation pretreatment was chosed to discuss enzymolysis progress and fermentation.Cellulase was employed for hydrolysis of wet oxidation pretreated stock, the proposedcondition was: temperature49oC, time56h, enzyme loading38FPU/g. Under above condition,the cellulose conversion of96.39%could be obtained. The hydrolysate just contained glucose(97.54%) and xylose, and the crystallinity of residue dropped substantially. The kinetic ofenzymolysis can be expressed as: Y_C=17.84×T0.4244×(1-e^-0.2023t)×100%(R²=0.8658).Concentrated sulfite acid was employed for hydrolysis of Poplar wood processing residue,the proposed concentrated sulfite acid hydrolysis condition was: temperature50.97, time120min, H₂SO₄concentration60%. The optimal mesh number of poplar wood processing residue grain size was160-200. Under above condition, the cellulose conversion of90.81%can beobtained. The hydrolyzate contained glucose (60.49%) and xylose (19.84%), what's more,acetic acid and hydroxyl methyl furfural (HMF) was the main degradation products. Thecrystallinity of residue decreased greatly. The kinetic of concentrated sulfite acid hydrolysiscan be expressed as: YC=39.24×T^0.1808×(1-e^-0.0726t)×100%(R²=0.8105).Comparing with concentrated sulfite acid hydrolysis, low-consistency sulfuric acidhydrolysis was chosen. The proposed low-consistency sulfuric acid hydrolysis condition was:temperature of158oC, time5min, H2SO4concentration2.5%, and under these conditions,52.97%of cellulose conversion could be obtained. The hydrolyzate contained xylose (64.80%),glucose (18.93%) and small amount of degradation products. The crystallinity of residue wasdecreased.The fermentation process of Saccharomyces cerevisiae (with glucose as the substrate) wasoptimized as: fermentation temperature33oC, substrate concentration7.5g/L, initial pH5.25,inoculation amount5%, rotated speed160r/min and fermentation time48h. Under abovecondition,82.99%of ethanol conversion could be resulted. The ethanol conversion of70.83%can be obtained with enzymolysis hydrolyzate of wet oxygen pretreated stock as substract.Finally, material balances for the conversion of poplar wood processing residue to ethanol (wetoxidation pretreatment-enzymolysis-fermentation) was calculated. According to the effectiveexperimental results, the ethanol yield (equivalent to theory production) of50%can beobtained.