| Summary: | 碩士 === 國立成功大學 === 化學工程學系碩博士班 === 100 === Bioethanol produced from lignocellulosic materials has been considered as one of the most promising fuels to replace the fossil fuel. Immobilized yeasts or bacteria have been frequently used in continuous system due to its feasibility for repeated uses with high biomass retention during the continuous process. In this work, glucose and xylose liberated from lignocellulosic materials were used to produce ethanol by PVA-immobilized Zymomonas mobilis and suspended Pichia stipitis. The optimal condition for batch glucose fermentation by immobilized Zymomonas mobilis was a particle loading of 20%-w/v and an initial sugar concentration of 50 g/L. The batch co-fermentation system between PVA-immobilized Z. mobilis and P. stipitis reached its best condition when glucose was first consumed by Z. mobilis, followed by xylose consumption by P. stipitis.
The fed-batch strategy was employed for glucose fermentation by immobilized Z. mobilis. It was found that product inhibition occurred when the ethanol concentration was higher than 70 g/L. Continuous fermentation could prevent the ethanol accumulation, so in this study, continuous fermentation using continuous stirred tank reactor (CSTR) was employed for bioethanol production. Optimization of the CSTR system for PVA-immobilized Z. mobilis shows that the optimum ethanol productivity (4.78 g/L/h) can be achieved at a high particle loading (35%-w/v) and a high glucose loading (10.64 g/L/h), while optimum glucose conversion (96.87%) was achieved at high particle loading (35%-w/v) but a lower glucose loading (8.64 g/L/h).
For pentose fermentation, continuous fermentation using CSTR by PVA-immobilized P. stipitis was implemented. It is found that even with the full nutrient support, the ethanol production rate was very slow (less than 0.2 g/L/h), indicating that immobilized cells might be not suitable for xylose fermentation. The reason for this may be related to mass transfer (especially oxygen transfer) limitations. Alternatively, fed-batch study using suspended P. stipitis exhibited better bioethanol production performance than the continuous fermentation using immobilized cells. In addition, the combination of continuous pentose fermentation by P. stipitis with membrane separation can maintain high cell concentration and ethanol productivity, whereas the decrease in performance still occurred during long-term operations due the cell activity decay.
The cellulosic ethanol fermentation was also investigated. The experiments were divided into two groups: batch SHF ＆ SSF ethanol production from acid pretreated sugarcane bagasse and continuous SHcF ＆ SScF ethanol production from alkaline pretreated sugarcane bagasse. The batch SHF process gave maximum ethanol concentration of 6.24 g/L (79.09% of theoretical yield) for PVA-immobilized Z.mobilis cells and 5.52 g/L (69.96% of theoretical yield) for CA-immobilized Z. mobilis cells, with ethanol productivity of 1.52 g/L/h and 0.92 g/L/h for PCA and CA cells, respectively. The SSF study gave a maximum ethanol concentration of 5.53 g/L (70.09% of theoretical yield) for PVA cells and 5.44 g/L (68.95% of theoretical yield) for CA cells, with ethanol productivity of 0.691 g/L/h and 0.679 g/L/h for PVA and CA cells, respectively. For continuous cellulosic ethanol production system, the SHcF system process displayed an overall process efficiency of 70.65% of theoretical yield with an average ethanol productivity of 1.868 g/L/h, while the SScF process gave an overall efficiency of 81.18% of the theoretical yield with an average ethanol productivity of 0.705 g/L/h. Comparing with the performance obtained from the related studies, the result from the proposed methods appears to be highly competitive and are thus suitable for cellulosic bioethanol production and have a good potential for industrial applications.
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