Study On Process Intensification Of Pretreatment And Biodetoxification For Efficiency Elevation Of Lignocellulose Biorefining Processes

Pretreatment is the crucial step in lignocellulose biorefinery processes by breaking down the recalcitrance of lignocellulosic feedstock, followed by detoxification step to remove the inhibitory compunds generated from pretreatment step to ensure the efficiencies of consequent enzymatic hydrolysis and fermentation steps using pretreated materials as feedstock. In our previous study, dry dilute acid pretreatment with low water consumption and no waste water generation and biological detoxification process were developed. The combination of these two steps has been successfully applied to the production of ethanol, lipid and lactic acid from lignocellulose feedstock. However, these two processes were only realized in a bench scale thus mass transfer, heat transfer and low rate in large scale processes remained to be problems. In this thesis, the study is focused on the analysis of the mass and heat transfer and low rate involved in the biorefinery processes, and then the research on the chemical engineering, fermentation and metabolism mechanisms of microbe were carried out. Obvious improvements on the transfer efficiency and conversion rate of dry milling biorefinery process were obtained, which provides a powerful technical support for the industrial application of dry milling biorefinery in the future.The first part of this thesis is focused on the research of suitable pretreatment reactor in high solids loading process for solving the problems of mass and heat transfer at industrial scale. After the comparison with various agitators in the mixing performance at high solids loading, helical ribbon stirrer was chosen to apply in the dry pretreatment reactor. And then a helical ribbon stirrer reactor with the working volume of 20 L was designed. Compared to the pretreatment process in the traditional reactor without agitation, the efficiency of enzymatic hydrolysis and ethanol fermentation using dry pretreated corn stover as feedstock were both improved by the helical agitation. The cellulose conversion after hydrolysis and ethanol yield was increased by 2.4% and 17.4% in the pretreatment with helical agitation, respectively, while the generation of the inhibitors during pretreatment process was reduced. The helical agitation in the pretreatment step greatly improved the mass transfer between steam and solid materials. Also, the heat transfer from steam to solid materials during pretreatment was strengthened during pretreatment. The development of the helical ribbon stirrer reactor provides a suitable prototype of the pretreatment reactor for commercial cellulosic production.The second part of this thesis is an attempt to remove the routine impregnation step from dilute acid pretreatment to further improve the efficiency of pretreatment operation with reduced the investment cost. The results show that the bulk of solid material and minority dilute acid solution could be completely mixed in a short time under a low agitation rate when the sulfuric acid and solid materials were co-currently fed into the reactor for pretreatment. The results show that the cellulose conversion using the helical agitated corn stover without impregnation reached 84.77% and the cellulose conversion using the pretreated corn stover with 12 hours’impregnation reached 87.11%. The results were almost same. However, an obvious decline of 18.4% in cellulose conversion could be observed in the static pretreatment without impregnation, indicating that helical ribbon stirrer played an important role in the mixing between minority acid solution and bulk of solid materials. The removal of impregnation in industrial production simplified the problems of materials storage, transportation and reactor corrison, and greatly improved the economy and practicability of the pretreatment.The third part of this thesis is the detailed research about the impact of ash in the corn stover feedstock on the pretreatment efficiency. The results show that the increase of 63.9% and 42.1% in the cellulose conversion and ethanol yield was obtained using dilute acid pretreated corn stover as feedstock when the ash content was reduced from about 10% to less than 5%. The alkaline materials in the ash would neutralize the sulfuric acid added into the corn stover leading to a decrease of catalysis performance in the pretreatment process. Ash had a larger distribution in the feedstock with small size, thus some mechanic methods such as mechanic vibration screening or cyclone separator could be applied for de-ashing process in the industrial process. This study provides an important proof for the pre-pretreatment of lignocellulosic feedstock, and the ash content should be regarded as an important parameter for the evaluation of feedstock used in the biorefinery process.The fourth part of this thesis is focused on the low rate caused by the lack of mass transfer and fermentation control in the solid state biodetoxificaiton by Amorphotheca reslnae ZN1. Then the research on the rapid detoxification was carried out. Firstly, the metabolism of inhibitors in the solid state biodetoxificaiton and the consequent ethanol fermentation were analyzed, which showed that the termination of biodetoxification could be set at the time when furfural,5-hydroxymethyl furfural (5-HMF) and acetic acid were completely degraded. Then the inhibitor metabolism of the biodetoxification in the helical agitated bioreactor was researched. The results show that the increasing aeration rate to 1.00 VVM during biodetoxification greatly improved the degradation of furfural,5-HMF and acetic acid. Then the biodetoxification period could be shortened to 36 h equaling to 1/3 of the traditional one. Meanwhile, the distribution of the fungus was improved in the solid state biodetoxification leading to the retention of xylose (about 45 g/L in the hydrolysate). The results also show that the increase in oxygen supply by aeration didn’t improve the growth rate of the fungus, but improved the conversion rate from furfuryl alcohol to furoic acid. Thus the reducing power for inhibitors’ degradation was also overproduced leading to the further improve on the metabolism of 5-HMF and acetic acid. Therefore, the rapid biodetoxification realized in this study increases the potential of its application in the industrial process.Overall, the intensification of dry pretreatment and biodetoxification step raised in this thesis makes these two processes more suitable for the biorefinery process at industrial scale, which provides an important technical support for the application of dry milling biorefinery in the industrial production.