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生物质高固酶解发酵过程具有可发酵糖浓度高、产物浓度高、分离成本低、废水排放少等优势，是生物质能源工业发展的方向和绿色生物产业关注的热点。但由生物质固相基质复杂原料特性引起的理论认知不足及过程强化技术缺失，导致高固酶解发酵过程转化效率低、工业技术经济性难以突破。因此，从生物质固相多孔颗粒原料特性认知切入，可能是生物质高固酶解发酵取得突破的关键。本文首先对生物质原料多孔颗粒特性进行解析，研究高固酶解发酵过程传递反应规律，探索高固下高效的酶解与发酵过程强化技术与定向调控策略，指导反应器设计与工艺优化。主要取得以下研究结果：（1）木质纤维素类生物质的多尺度多孔结构影响其内水分分布及流动特性，进而影响其酶解效率。本文以汽爆秸秆为代表，探索木质纤维素多孔特性对高固酶解过程传递及酶解效率的影响。结果揭示毛管孔占据汽爆秸秆总孔体积的90%以上，液固比3~10内毛管水占据总水体积的90%~95%，又因毛管水可被外力驱动，因此毛管水成为高固酶解系统对流传递主要的传质载体。进一步，外力强化毛管水的对流传递能够提高酶解率58.3%~106.7%，而当酶解至12 h时，体系内毛管水消失，酶解效率高达96%不再增加。因此，多孔介质内毛管水传递是木质纤维素高固酶解限制性因素。以法向力为动力源的周期蠕动能强化颗粒内毛管水的流动，促进介质内流体和酶分子的高效传质，从而显著促进酶解率的提高。因此，在周期蠕动强化下，汽爆秸秆高固酶解的液固比可以降低至3，固形物含量提高至25%，酶解后糖浓可达160 g/L。同时，周期蠕动能有效降低酶添加量5~8倍，节约大量酶成本。（2）木质纤维素类生物质酶解是从固相基质到可溶性糖的固液相变过程，起始阶段固相多孔颗粒效应对酶解传递与可及性的影响不容忽视。从固液相变过程的认知切入，是深入认知其酶解机制并提高酶解效率的有效途径。本文首先建立木质纤维素酶解固液相变转折点的表征方法，并建立酶解固液相变动力学；第二，寻找出木质纤维素酶解相变的限速步骤、影响因素及突破限速步骤的强化措施。结果表明汽爆秸秆酶解过程固液相变判据为粘度0.493 Pa·s，压缩模量29.4 kPa。进一步，建立汽爆秸秆酶解固液相变分段动力学模型，得到汽爆秸秆相变时酶解率>0.85，揭示相变前阶段为木质纤维素酶解的关键阶段和限速阶段。且影响相变速率的因素有传质系数D2，酶加量E0、液固比LSR和颗粒度R0，相变速率随D2、E0、LSR的降低以及R0的增大而降低。进一步提出周期法向作用力强化固液相变新策略，与传统机械搅拌相比，周期法向力能显著提高传质系数D2，实现在低10倍酶加量E0、低7倍液固比LSR、大87倍颗粒度R0的系统下的快速相变和酶解，且降低10倍以上的过程强化能耗。（3） 固态发酵是生物质高固转化过程的特例和极致。由气、液、固三相组成的多孔介质固相基质特性引起固态发酵过程热质传递及过程监测困难。本文首先建立起红外热成像、可见光成像及低场核磁成像耦合数字图像处理技术的固态发酵过程监测系统，高效实时监测固态发酵过程菌体温度、菌体生长动态及基质内染菌情况，并突破基质床层内菌体生长动态监测。其次，建立起气相双动态固态发酵反应器内发酵过程菌体温度、品温及环境气体温度之间的关系，即 ，指导发酵过程温度调控及优化。最后，揭示气相双动态固态发酵反应器内的保水增湿效果，并通过理论计算揭示罐底水分蒸发面积，脉动压力，罐体体积以及泄压口面积是引起气相双动态固态发酵工艺增湿还是干燥的关键因素，通过调节这些因素指导压力脉动工艺优化。（4）从固态发酵基质力学特性角度，建立固态发酵基质物性与热质传递及发酵性能的关系，指导固态发酵培养基配置及厚层固态发酵填料方式设计。建立一个力学特性指标Imp（基质恢复力、凝聚力和弹性的乘积）来表征基质保水、透气及换热特性。结果表明Imp≤4.37×10-2有助于菌体生长，而Imp >4.37×10-2 意味着不利于发酵。Imp≤4.37×10-2可用来指导初始培养基的配制及发酵过程调控。此外，基于颗粒物质静力学特性的粮仓效应，即颗粒物质的库伦摩擦特性，导致其挡板或壁面分摊部分的基质自重应力，设计内置隔板的固态发酵厚层填料方式。在压力脉动反应器内、小试25 cm的填料高度下，发酵基质温度为34℃，生物量可达0.75 g/g干基。因此，在填料中增设隔板、降低装料单元面积，改善自重应力的固结沉降现象，且耦合周期气体压力脉动作用强化热质传递，能实现厚层固态发酵，增加装料系数和设备利用率。;High-solids enzymatic hydrolysis fermentation of biomass has a lot of advantages, such as high sugar concentration, high product concentration, low separation cost and less waste water discharge etc., which is the trend of biomass energy industry and the hot spot of green biological industry. However, the lack of theoretical knowledge and process strengthening technologies caused by the complex porous structure and particle characteristics of solid materials, leads to the low conversion efficiency and the difficulty of breaking through the industrial technical economy during the high-solids enzymatic fermentation. Thus, we believe that starting from the cognition of the characteristics of biomass porous solid particles, may be the key way to break through the biomass high-solids enzymatic hydrolysis fermentation. In this thesis, the characteristics of porous solid particle medium was analyzed, the transfer and reaction law during high-solids enzymatic hydrolysis fermentation was studied, effectively strategies of process intensification and targeted regulation were explored, and then to guide process optimization and reactor design for high-solids enzymatic hydrolysis fermentation. The main results are as follows:(1) The multi-scale porous structure of lignocellulosic biomass affects the water distribution and the water flow characteristics, and then affects the enzymatic hydrolysis efficiency. With steam-exploded straw (SES) as the representative of lignocellulose, the effects of the porous properties on the transfer and hydrolysis efficiency during high solids enzymatic hydrolysis of SES were explored. The results showed that the capillary pores accounted for more than 90% of the total pore volume of the SES, and the capillary water accounted for 90%~95% of the total volume of water at liquid-to-solid ratio (LSR) of 3~10. The capillary water can be driven by the external force, thus the capillary water becomes the main mass transfer carrier for high-solids enzymatic hydrolysis. Furthermore, after capillary water transfer driven by external force, enzymatic hydrolysis yield could be improved by 58.3%~106.7%, while when the capillary water disappeared at 12 h, the enzymatic hydrolysis yield was as high as 96% but no longer increased. Thus, capillary water transfer in porous media is the limiting factor for high solids enzymatic hydrolysis of lignocellulose. Moreover, the periodic peristalsis with the normal force as the power source was proposed to strengthen the capillary water transfer in the media. Periodic peristalsis could promote the efficient mass transfer of fluid and enzyme molecules in the media, and then significantly improve the enzymatic hydrolysis efficiency. Therefore, the LSR of SES can be reduced to 3, the solids content can be increased to 25%, and the sugar concentration can reach up to 160 g/L under periodic peristalsis. Meanwhile, periodic peristalsis can effectively reduce the enzyme dosage of 5~8 times, saving a lot of enzyme costs.(2) The enzymatic hydrolysis of lignocellulosic biomass is a solid-liquid phase transition (SLPT) process from solid particle to soluble sugar. The impact of porous solid particles effect on the mass transfer and accessibility at the initial stage of enzymatic hydrolysis is significant. Thus, from the perspective of SLPT, it is likely to be an effective way for in-depth understanding the enzymatic hydrolysis mechanism and improving enzymatic hydrolysis efficiency of lignocellulose. In this thesis, the method for characterization of SLPT turning point and the kinetics of SLPT during lignocellulose enzymatic hydrolysis were established firstly, then the rate-limiting step, influencing factors and targeted strengthening measures were proposed. Results showed that the criteria of SLPT for SES enzymatic hydrolysis were apparent viscosity of 0.493 Pa·s and compressive modulus of 29.4 kPa. The enzymatic hydrolysis conversions at SLPT turning point were >0.85, thus the stage before SLPT was the key stage for lignocellulose hydrolysis. The rate of SLPT was positively related with increasing of mass transfer coefficient D2, enzyme dosage E0 and liquid-to-solid ratio LSR but negatively related to increasing of particle size R0. The periodic normal stress can significantly improve D2, thus the rapid SLPT under the system with E0 10 times lower, LSR 7 times lower and R0 87 times higher is achieved, then enzymatic hydrolysis efficiency was improved significantly. (3) Solid-state fermentation (SSF) is an extreme case of high-solids conversion of biomass. The properties of porous solid substrate composed of gas, liquid and solid three phases resulted in the difficulties of heat and mass transfer and process monitoring. In this thesis, three SSF process monitoring systems based on infrared thermal imaging, visible light imaging and low-field nuclear magnetic imaging coupled digital image processing technology were established. The real-time mycelium temperature, mycelium growth dynamics and contamination can be monitored. Secondly, the relationships among the mycelium temperature, the average substrate temperature and the gas temperature during SSF process was established, that is, , to guide the temperature regulation and optimize the fermentation process. Finally, the effect of water retention and humidification in the gas-double dynamic SSF process was revealed. The theoretical analysis showed that the evaporation area A1, the pulsating pressure P, the volume of the tank V and discharge port area A2 are the key factors for the humidification or the desiccation of gas-double dynamic SSF process. Thus, those factors can be regulated to guide the pressure pulsation process optimization.(4) From the perspective of mechanical properties of SSF meidum, relationships among the medium’s physical properties and heat and mass transfer as well as fermentation performance were established, to guide culture medium preparation and thick-layer packing design of SSF. We established an integrated mechanical property index Imp to fully characterize medium physical properties of water retention, gas permeability, thermal conductivity and thermal diffusivity. When Imp ≤ 4.37×10-2, water retention and heat conduction was promoted in the medium, which was beneficial to cell growth. When Imp > 4.37×10-2, heat accumulation and poor permeability reduced fermentation performance. Thus, Imp could well comprehensively characterize physical properties of medium, which could be useful for guiding culture medium preparation and dynamic process control in SSF. In addition, based on the silo effect of the particulate matter, that is, the Coulomb's friction characteristics of the particulate matter result in the baffle or wall to share part of the self-weight stress of the medium, the SSF thick-layer packing pattern with built-in partition was designed. The center temperature of fermentation medium was 34°C and the biomass was up to 0.75 g/g dry basis under the bed height of 25 cm in the gas-double dynamic SSF bioreactor. Therefore, the addition of partitions in the packing, reducing the unit area of the packing, relieving the consolidation and settlement caused by self-weight stress, and coupled with pressure pulsating to enhance the heat and mass transfer, the thick-layer packing SSF and the increase of the loading coefficient can be achieved.
|张玉针. 生物质多孔颗粒特性及其高固酶解发酵过程的研究[D]. 中国科学院研究生院,2017.|
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