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The Effects of Pretreatment Methods on Ethanol Production from Sweet Sorghum Stalk Juice and Bagasse

Author: CaoWeiXing
Tutor: LiuRongHou
School: Shanghai Jiaotong University
Course: Environmental Engineering
Keywords: Sweet sorghum Bio-ethanol Juice storage Juicefermentation Pretreatment Enzymatic hydrolysis
CLC: TQ223.122
Type: PhD thesis
Year: 2012
Downloads: 97
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Abstract

As a kind of non-food crop, sweet sorghum is becoming a worldwideresearch hotspot recently. There are realistic significances to resolve theenergy shortage problems by developing bioethanol production from sweetsorghum. As far as bioethanol production from sweet sorghum concernednowadays, the seasonally harvested raw material can’t meet the requirementof continuous bioethanol production of the whole year. Also, the lowfermentation efficiency severely restricts its industrialization progress.The objective in this thesis is to utilize the whole plant of sweetsorghum for bioethanol production. Firstly, storage study applying formicacid and chitosan addition to juice from the stalk was carried out to prolongthe storage period of sweet sorghum juice. And the verification tests to determine the fermentation effect have been done. Then different traceelements were added into the juice which has been clarified. Results showedthat some trace elements could significantly improve the ethanolfermentation efficiency. After that, optimization studies of saccharificationconditions using the grain have been performed. And co-fermentation testsusing the saccharification mash and sweet sorghum juice were conducted inorder to realize the high efficiency utilization of the whole plant forbioenergy production. Finally for the residue of sweet sorghum stalk, fivedifferent pretreatment methods were applied to make this kind oflignocellulosic raw material much easier to be saccharified. The enzymatichydrolysis and ethanol fermentation results showed that the applied range ofbioethanol production from sweet sorghum was greatly broadened.Results from the storage tests applying formic acid addition andchitosan to preserve sweet sorghum juice showed that both of themcontributes to sugar conservation in juice as well as bioethanol fermentationafter storage. Among the treatments mentioned in the thesis, adding0.1%（v/v） formic acid into the juice was the best method. Under this conditionthe soluble sugar loss rate in sweet sorghum juice after storage for40d is15.90%. And the ethanol content and ethanol yield after fermentation for30h using immobilized yeast was39.94g/L and75.49%, respectively. Clarification treatment was applied to sweet sorghum juice usingchitosan solution. The optimal treatment condition was chitosanconcentration0.42g/L, pH value5.4and temperature29.6℃, respectively,which were determined by Respond Surface Method. The verificationexperiments showed that the optimization results bore quite a high degree ofconfidence level. The clarity of juice with optimized conditions was90.62±0.12%and the total soluble sugar content was130.32±0.22g/L,which showed that the optimization results were genuine and believable.Furthermore, fermentation kinetics studies were carried out both ofclarification juice and the control. The results showed that experimental datafitting degree is reasonable. And the kinetic related parameters indicated thatclarification not only increased the maximum specific growth rate andpopulation of yeast cell, but also accelerated the consumption rate of sugarcontent thereby improved the bioethanol yield.Plackett-Burman design was carried out to screen3kinds of traceelements （Biotin, CoCl₂·6H₂O and MnCl₂·4H₂O） which would exertpositively impact to bioethanol production from sweet sorghum juice from8trace elements. The addition range was determined by single factorexperiments. After that Box-Behken method was conducted to get theoptimized addition amount of the3trace elements, which were7.70mg/L, 5.74mg/L and11.97mg/L for MnCl₂·4H₂O, CoCl₂·6H₂O and Biotin,respectively. Verification tests were performed under the optimizedconditions. Results showed that the ethanol yield efficiency was5.63%higher than the control, which would offer a reference to the future pilotscale ethanol production with trace elements addition.Single factor experiments were brought to determine the extent ofliquefaction and saccharification effects for sweet sorghum grain. CCDmethod was applied to get the applicable technological conditions forliquefaction and saccharification. The optimal conditions for liquefactionwere as follows:20%of substrate concentration,5.0u/g substrate ofenzymatic activity of liquifying enzyme,5.50of pH value,65℃oftemperature, and30min of liquefaction time. And the optimalsaccharification conditions for saccharification were enzymatic activity ofsaccharifying enzyme:594u/g substrate, pH value:4.1, temperature:51.4℃;time:80h. Then the grain saccharification mash after clarification wasco-fermented with sweet sorghum juice to produce ethanol. And the resultsdemonstrated that co-fermentation could increase the reducing sugar contentand free amino nitrogen （FAN） content in the fermentation broth as well asthe fermentation rate. Moreover, the higher the volumetric percent ofclarified saccharification mash was added, the higher the fermentation rate was obtained. The highest ethanol concentration, productivity and yieldwere obtained at the80%volumetric percentage of clarified saccharificationmash in the mixed fermentation liquor, which were65.64±0.57gL¹、5.47±0.04gL¹h¹and89.29±0.77%, respectively. These results were6.8%、33.5%and13.7%higher than the control which using pure sweet sorghumjuice.The residue of sweet sorghum stalk was analyzed to determine its maincharacteristic and then was pretreated. The results manifested that the fivedifferent methods mentioned in this thesis all benefited the cellulosehydrolysis efficiency of stalk residue. Preatment with2%NaOH plus hightemperature and pressure and then immersed by5%H₂O2was the mostappropriate pretreatment method, after which the highest glucose and xylosecontent in hydrolyzate were obtained. Besides, the highest cellulosehydrolysis rate （74.29%）, enzyme saccharification rate （90.94g sugar/100gdry material） and ethanol concentrate （6.12g/L） was obtained with thismethod, which were5.88,9.54and19.13times higher than the control,respectively. Furthermore, it could be observed by the SEM analysis thatthere were significant structural changes of stalk residue after thepretreatments. Also some certain breakage and changes of the chemicalbonds caused by pretreatment were deduced though FITR analysis. Characteristic analysis after pretreatments dedicated that cellulose contentand total sugar yield were related to each other, so were glucoseconcentration, cellulose hydrolysis yield and ethanol concentration. Thiskind of pretreatment method would offer a reference for the bioethanolproduction from lignocellulosic stalk residue in the future.In sum, the storage period of sweet sorghum juice will be extended atleast by40days by adding certain mount of formic acid. And clarification ofthe juice using chitosan as well as adding trace elements including Biotin,MnCl₂·4H₂O and CoCl₂·6H₂O will improve the bioethanol yield effectively.Co-fermentation by applying sweet sorghum juice and saccharification mashfrom grain will increase the ethanol concentration in the fermentation broth.And pretreatment at high temperature and pressure with diluted alkaline plushydrogen peroxide soaking pretreatment will contribute to the hydrolysisrate of residue of sweet sorghum stalk. All of these methods will broadenthe range of raw material source as well as prolonged the industrializedproduction period of bioethanol.

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