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汽爆秸秆高固酶解发酵过程强化的研究
刘志华
学位类型博士
导师陈洪章
2016-07
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业生物化工
关键词汽爆 高固酶解 木质纤维素乙醇 周期蠕动 过程强化
摘要

木质纤维素高固炼制过程具有可发酵糖浓度高,产物浓度高,分离成本低,废水排放少等诸多优势,是实现木质纤维素利用工业化的重要途径。围绕木质纤维素高固炼制过程存在的主要问题,即(1)预处理如何提高过程可发酵糖得率;(2)如何移除高固酶解发酵“固体效应”提高过程转化效率;(3)如何实现葡萄糖木糖的高效共发酵,以及如何构建各个单元的集成工艺体系来实现高效炼制等,本论文对木质纤维素炼制核心单元操作—预处理、酶解及发酵—进行了研究,研究结果如下:(1)针对常规汽爆预处理组分分离过程聚糖回收率低,降解物多及酶解效率偏低等问题,发明了一种低温维持高压爆破汽爆炼制新方法。该方法在最优条件下(160 °C,48 min),葡聚糖和木聚糖回收率分别为93.4%和71.6%;葡聚糖和木聚糖转化率分别为82.3%和79.6%,汽爆酶解过程葡萄糖、木糖和总糖得率分别达到77.3%、62.8%和72.3%,均优于常规汽爆(200 °C,6 min)。低温维持高压爆破汽爆新方法具有维持温度和处理强度低等优势,提高了聚糖回收率,降低了降解物得率,显著提高了酶解效率。(2)秸秆高固酶解体系存在“固体效应”,其导致酶解效率降低。基于仿生学原理,模拟瘤胃结构功能特点,发明了汽爆秸秆高固酶解周期蠕动强化新方法;开发了从秸秆中提取原料制备蠕动材料新工艺,即利用秸秆制备木糖和糠醛,通过糠醛合成聚四氢呋喃二醇制备聚四氢呋喃醇聚氨酯弹性体。(3)研究了周期蠕动对高固酶解聚糖转化率、体系粘度、酶失活的影响。相比于摇床振荡,15%-30%固体载荷条件,周期蠕动酶解葡聚糖和木聚糖转化率分别提高了4.1%-11.2%和2.8%-9.2%。相比于摇床振荡,15%-30%固体载荷条件,周期蠕动酶解表观粘度降低,固态到泥浆态转变点缩短6-14 h,纤维素酶失活降低3.2%-7.9%。随着固体载荷从18%增加到30%,相比一次加料酶解,补料周期蠕动酶解增加的葡聚糖和木聚糖转化率分别从5.3%增加到13.9%和从5.6%增加到11.1%,汽爆玉米秸秆转变点之前或者附近添加完成提高了酶解效率。结果表明周期蠕动是一种高效的强化方式,可以显著提高汽爆玉米秸秆高固酶解效率。(4)研究了高固酶解秸秆水分作用规律与酶解效率的关系,发现指数模型能够很好地表示未处理/汽爆玉米秸秆与水分的相互作用。研究发现汽爆增加了玉米秸秆葡聚糖含量,结晶度,比表面积,孔体积,平均孔直径,孔隙度,氧碳比,从而强化了玉米秸秆与水分的相互作用。汽爆通过增加玉米秸秆表面的结合水强度和含量,增加了聚糖对水和酶的可及性,其与酶解前36 h束缚水被释放共同作用提高了酶解聚糖转化率。此外,解析了影响秸秆水分相互作用的关键因子,为解决高固酶解水束缚和提高酶解效率提供了新的研究思路。(5)解析了周期蠕动对高固酶解水束缚的影响及其与酶解效率的关系。相比于摇床振动酶解,周期蠕动酶解水池峰高前24 h增加了7.7%-43%,增加的峰高变化趋势与增加的葡聚糖转化率变化趋势一致,说明周期蠕动在酶解初始阶段释放了束缚水,提高了酶解聚糖转化率。研究发现对高固酶解水束缚效应的影响按照吐温80,乙醇,木糖,葡萄糖和微晶纤维素顺序增加,亚微观颗粒和大颗粒残渣是影响水束缚效应的两个主要因素。(6)秸秆异质性与高固酶解效率密切相关,探究了选择性结构功能拆分及应力应变行为对汽爆及高固酶解的作用规律。与汽爆茎节和茎皮相比,汽爆髓芯,叶鞘,叶和整株玉米秸秆硬度较低,消耗总功少,有利于混合效率和酶解聚糖转化率提高。相比于摇床振荡,周期蠕动酶解12 h内体系硬度和总功消耗降低,聚糖转化率提高。选择性结构功能拆分可实现不同形态学部位在最佳汽爆和酶解条件转化,周期蠕动改变高固酶解体系的应力应变行为,从而提高聚糖转化率。高固酶解体系应力应变行为及其与酶解效率作用规律的解析,为高固酶解工艺开发和设计提供了新的理论支撑。(7)研究了S. cerevisiae IPE003高固酶解发酵,比较了几种不同酶解发酵策略,包括分步糖化发酵,分步糖化共发酵,同步糖化发酵和同步糖化共发酵。S. cerevisiae IPE003共发酵60 g/L葡萄糖和60 g/L木糖,葡萄糖和木糖分别在发酵12 h和36 h内消耗完,乙醇浓度,乙醇得率和乙醇产率分别达到53.8 g/L,92.4%和1.49 g L-1h-1。同步糖化共发酵汽爆玉米秸秆96 h后,葡聚糖和木聚糖转化率分别为82.0%和82.1%,乙醇浓度达到60.8 g/L,乙醇得率达到75.3%,乙醇产率为0.63 g L-1h-1。S. cerevisiae IPE003能高效地共发酵葡萄糖和木糖,提高了乙醇的浓度,得率和产率,解决了酶解高糖抑制、发酵乙醇浓度低、木糖难利用及废水排放多等问题。究结果表明低温维持高压爆破汽爆有效克服木质纤维素生物质抗降解性,提高了炼制效率;新型周期蠕动强化方式能够有效移除高固酶解体系“固体效应”,包括降低体系表观粘度、酶失活、水束缚及机械强度等,从而提高了高固酶解转化率和产率;S. cerevisiae IPE003能高效地共发酵葡萄糖和木糖,预处理酶解发酵的高效集成工艺能够有效提高秸秆高固转化过程效率。本论文研究为木质纤维素炼制及木质纤维素乙醇工业化奠定了理论基础。

其他摘要

High solids lignocellulose refinery has many advantages including higher fermentable sugar and product concentration, lower cost of separation, and less wastewater discharge. It is an important approach to realize lignocellulose refinery industrialization. The main challenges of high solids lignocellulose refinery for ethanol production includes (1) how to increase fermentable sugars by pretreatment; (2) how to remove the "solids effect" and improve the efficiency of high solids enzymatic hydrolysis and fermentation; (3) how to realize the high co-fermentation efficiency of glucose and xylose, and how to construct the integration process of each unit operation to achieve efficient refinery. In order to solve these problems, the core unit operation of lignocellulose refinery for ethanol production including pretreatment, hydrolysis and fermentation were studied in the present study, and the research results was given as follows:(1) The polysaccharide recovery was low, the degradation products yield was high, and the enzymatic hydrolysis efficiency was poor in conventional steam explosion (SE) pretreatment. A novel steam explosion pretreatment strategy, low holding temperature and high explosion pressure (LHT/HEP), was developed. Under the optimal steam explosion conditions (160 °C, 48 min), glucan and xylan recovery was 93.4% and 71.6%, glucan and xylan conversion was 82.3% and 79.6%, glucose, xylose and total sugar yield reached 77.3% and 62.8% and 72.3%, respectively, which were better than these under conventional steam explosion pretreatment (200 °C, 6 min). LHT/HEP-SE had low holding temparture and pretreatment severity and it obviously increased the polysaccharide recovery, reduced the degradation products yield, and further improved the efficiency of enzymatic hydrolysis.(2) The "solids effect" of high solids enzymatic hydrolysis leads to the low efficiency of enzymatic hydrolysis. Based on the principle of bionics, a novel intensification method, periodic peristalsis, was developed and the corresponding equipment were consutructed by simulating the structural features of rumen. A novle preparation process of peristalsis materials was developed using agricultural straw as raw material. In this process, xylose and furfural was obtained from agricultural straw by SE to prepare polyurethanepolytetramethylene glycol (PTMG), which can be further used to prepare polyurethane.(3) The effects of periodic peristalsis on sugar conversion, apparent viscosity and enzyme activity loss in high solids enzymatic hydrolysis was studied. Compared with incubator shaker, periodic peristalsis increased glucan and xylan conversion by 4.1%-11.2% and 2.8%-9.2% at 15%-30% solid loadings, respectively. Compared with incubator shaker, periodic peristalsis reduced apparent viscosity, shortened transition point from the solid state to slurry state by 6-14 h, and reduced the cellulase loss by 3.2%-7.9% at 15%-30% solid loadings. With solids loading increasing from 18% to 30%, compared with that by batch enzymatic hydrolysis, glucan and xylan conversion increased from 5.3% to 13.9% and from 5.6% to 11.1% in fed-batch enzymatic hydrolysis by periodic peristalsis. Results suggested that steam exploded corn stover added completely before or near the transition point improved the efficiency of high solids enzymatic hydrolysis. Results showed that periodic peristalsis was an efficient intensification method to improve the efficiency of high solids enzymatic hydrolysis of steam exploded corn stover.(4) The interactions between biomass and water and its correlations with high solids enzymatic hydrolysis of steam exploded corn stover were studied. Results suggested that exponential model can express well the interactions between untreated/steam exploded corn stover and water. SE increased glucan content, biomass crystallinity, specific surface area, pore volume, average pore diameter, porosity, oxygen to carbon ratio, thereby enhanced the interactions between corn stover and water. SE increased the binding strength and water content on the surface of corn stover, hence increased the accessibility of polysaccharide to water and enzyme. Combined with these, the release of water constraint before 36 h improved the efficiency of high solids enzymatic hydrolysis. Additionally, the analysis of key factors affecting the corn stover-water interactions was helpful for solving the water constraint and improving the efficiency of high solids enzymatic hydrolysis.(5) The effects of periodic peristalsis on the water constraint and its relationship with the efficiency of high solids enzymatic hydrolysis were analyzed. Compared with that by incubator shaker, the pool peak height of water pool in high solids enzymatic hydrolysis increased by 7.7%-43% before 24 h by periodic peristalsis, which was consistent with the increase trend of glucan conversion. During the initial stage of hydrolysis, the periodic peristalsis released the water constraint, and thus improved sugar conversion. Results suggested that the effects of chemicals on water constraint in high solids enzymatic hydrolysis followed the order: Tween 80, ethanol, xylose, glucose and microcrystalline cellulose. Submicroscopic particles and large particles residue are two main factors affecting water constraint.(6) Biomass heterogeneity is closely related with the efficiency of high solids enzymatic hydrolysis. The selectively structural and functional fractionation and the stress-strain behavior of agricultural straw were studied in SE and high solids enzymatic hydrolysis. Compared with steam exploded steam node and stem rind, steam exploded pith, leaf sheath, leaf and whole corn stover had lower hardness and less total work done, which facilitated the efficiency of mixing and high solids enzymatic hydrolysis. Compared with incubator shaker, periodic peristalsis reduced the hardness and total work done of high solids enzymatic hydrolysis within 12 h, and thus increased sugar conversion. Selectively structural and functional fractionation obtained the higher conversion efficiency of different morphological fractions under their optimum steam explosion and enzymatic hydrolysis conditions, respectively. Periodic peristalsis changed the stress-strain behavior of high solids enzymatic hydrolysis, and thus increased the sugar conversion. The analysis of the stress-strain behavior and its correlations with the efficiency of enzymatic hydrolysis provided a new theoretical basis for the development and design of high solids enzymatic hydrolysis.(7) The high solids enzymatic hydrolysis and fermentation process was studied by S. cerevisiae IPE003, and several fermentation strategies including separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), separate hydrolysis and co-fermentation (SHCF), and simultaneous saccharification and co-fermentation (SSCF) were compared. 60 g/L glucose and 60 g/L xylose were co-fermented by S. cerevisiae IPE003 within 12 h and 36 h, respectively, and ethanol concentration, yield and productivity reached 53.8 g/L, 92.4% and 149 g L-1h-1. Glucan and xylan conversion were 82% and 82.1%, respectively, and ethanol concentration, yield, and productivity was 60.8 g/L, 75.3%, and 0.63 g L-1h-1 in SSCF of steam exploded corn stover after 96 h. S. cerevisiae IPE003 can efficiently co-ferment glucose and xylose, and increase the concentration, yield and productivity of ethanol.In summary, LHT/HEP-SE overcome biomass recalcitrance and improved the refinery efficiency of agricultural straw. The novel intensification method, periodic peristalsis, can effectively remove “solid effect” and hence enhance the efficiency of high solids enzymatic hydrolysis. S. cerevisiae IPE003 can co-ferment glucose and xylose effectively; the integration process of pretreatment, enzymatic hydrolysis and fermentation can effectively improve the refinery efficiency of agricultural straw with high solids. These results provided a theoretical foundation for lignocellulose refinery and lignocellulosic ethanol industrialization. 

语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/22888
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刘志华. 汽爆秸秆高固酶解发酵过程强化的研究[D]. 北京. 中国科学院研究生院,2016.
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