High-Productivity and Eco-Friendly Bioethanol Production through Integration of Cell Immobilization Membrane Distillation-Coupled Fermentation and CO2 Capture & Fixation

  • 林 海亞

Student thesis: Master's Thesis


Bioethanol as one of renewable energy is considered an excellent alternative clean-burning fuel to replace gasoline Continuous bioethanol fermentation systems have offered important economic advantages in comparison with traditional systems Fermentation rates can be significantly improved when the continuous fermentation is integrated with cell immobilization techniques to enrich the cells concentration in the fermenter Growing cells of Zymomonas mobilis immobilized in polyvinyl alcohol (PVA) gel beads were employed in an immobilized-cell fermenter for continuous bioethanol fermentation from glucose The glucose loading dilution rate and cells loading were varied in order to determine the best condition employed in obtaining both high bioethanol production and low residual glucose at high dilution rates Higher glucose fermentation rate has been considered as the most important target as it leads to higher bioethanol titer and better bioethanol productivity To enable bioethanol fermentation at high glucose concentration with continuous-flow operations the Z mobilis cells were immobilized using polyvinyl alcohol (PVA) matrix modified with a certain amount of iron (III) oxide (Fe2O3) and the cell immobilization of Z mobilis was simply performed by using the enriched cells culture media harvested at the exponential growth phase The production of bioethanol was affected by the medium flow rates or hydraulic retention time (HRT) from 1 to 4 hour In addition the effects of both initial glucose loadings (100 g/L 125 g/L and 150 g/L) and cell loadings (20% (w/v) 40% (w/v) and 50% (w/v)) were also investigated On the other hand the bioethanol production performance of Z mobilis cells immobilized with cross-linked polymer of PVA with 1% (w/v) 2% (w/v) and 3% (w/v) of Fe2O3 and unmodified PVA-immobilized Z mobilis was compared Reusability of the immobilized Z mobilis or stability of beads was examined and found that the immobilized cells could be utilized for more than 20 days without losing their activity Biothanol productivity increased from 15 74 g/L/h to 31 09 g/L/h when the modified PVA-immobilized Z mobilis with 1% (w/w) of Fe2O3 was employed and the glucose loading was 125 g/L Moreover the HRT of 2 hour and cell loading of 40% (w/v) were considered as the optimum conditions to obtain the best bioethanol fermentation performance The modified PVA-immobilized Zymomonas mobilis cells were used to enhance the efficiency of bioethanol production under very high gravity (VHG) conditions Continuous bioethanol fermentation was integrated with in-situ bioethanol removal via vacuum membrane distillation (VMD) to overcome the problems associated with product inhibition and the resulting low bioethanol productivity during bioethanol fermentation The developed VMD-integrated VHG fermentation system can be successfully operated under a feeding glucose concentration of up to 300 g/L (or 30% (w/v)) obtaining a maximum bioethanol concentration of 127 39 g/L (or 16 14% (v/v)) an bioethanol productivity of 63 69 g/L/h and a glucose conversion of 84 93% In addition the production of bioethanol by fermentation must be accompanied by some by-products such as carbon dioxide (CO2) and organic acids These by-products can lower the quality and usability of bioethanol as a biofuel and also increase the amount of wastes to cause environmental pollution CO2 produced from fermentation is of high purity and is nearly a saturated gas (almost 100%) The majority of CO2 applications were dedicated to serving carbonated beverage and food processing/preservation Beyond these traditional applications one of the most potential ones is the production of algae-based biofuels through CO2 fixation by microalgae The advantages of using microalgae CO2 fixation include rapid growth rate and high CO2 fixation capability when compared to conventional plants and high oil/carbohydrate production The carbohydrate-rich microalgal biomass then can be used for bioethanol production in large scale applications Photosynthesis is often required for CO2 fixation The need of solar energy supply in photosynthesis reactions appears to limit the application of biological CO2 sequestration due to scale-up problems Furthermore many microorganisms considered for CO2 fixation have fastidious growth requirements Succinic acid is a common natural organic acid often found in humans animals plants and microorganisms As 1 mol CO2 is theoretically required for the synthesis of 1 mol succinic acid CO2 should play an important role in succinic acid production to promote the regulation of the PEP carboxykinase pathway Compared to carbon capture by microalgae this developed system encourages much higher CO2 fixation rate The highest carbon fixation rate achieved for microalgae cultivation was 2 06 g/L/d while it was 31 92 g/L/d (15 times higher) for succinic acid production Therefore production of succinic acid is also a feasible way of biological CO2 removal In the end bioethanol has to be produced from non-food sources Cellulosic biomass was used for bioethanol fermentation for the purpose of reducing greenhouse gas emissions and giving impacts to rural economy condition Cellulosic resources are in general very widespread and abundant Being abundant and outside the human food chain brings cellulosic materials such bagasse rice straw etc relatively inexpensive feedstocks for bioethanol production A 120 g/L of bagasse loading or corresponding to 62 79 g/L of glucose after hydrolysis was used to evaluate the feasibility of producing bioethanol with the modified-PVA immobilized Z mobilis cells The glucose conversion obtained was about 94 57% in average with a bioethanol titer and productivity of approximately 26 82 g/L and 13 41 g/L/h respectively during fermentation The results demonstrate that the modified PVA-immobilized Z mobilis cells are preferable cell immobilization system for cellulosic bioethanol production
Date of Award2014 Jan 24
Original languageEnglish
SupervisorJo-Shu Chang (Supervisor)

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