Development of a Self-Cycling Fermentation Approach to Improve Productivities for Ethanol Production

• Author / Creator

  Jie Wang

• Biofuels have great potential to help secure the global energy supply and reduce greenhouse gas emissions. Taking cellulosic ethanol as an example, it is a biofuel produced from lignocellulosic material (e.g. wood and corn stover), which has the most abundant polymer on the planet. The production process starts with pretreatment and enzymatic hydrolysis, which generates sugars for fermentation into ethanol. The ethanol is then distilled into a high purity product. Despite the extensive technological developments achieved over the years for cellulosic ethanol production, the current industry is still faced with economic challenges, hindering its rapid expansion.
  This thesis identifies the current industrial fermentation approach—performed in batch—as one of the primary limiting factors. This process requires extensive labor and long fermentation times, resulting in low productivity. In contrast, a self-cycling fermentation (SCF) approach with significant improvements in productivities were observed compared to conventional processing techniques. SCF is a semi-continuous, cycling fermentation technique that can be operated for a number of cycles; when cells arrive at stationary phase, half of the culture volume is automatically harvested and replaced by fresh medium to start the next cycle. Despite the integration of an SCF operation strategy into many microbial cultivation systems under aerobic conditions, there has been no successful report on its application to stable ethanol production.
  This work aims to integrate the SCF approach into ethanol fermentation by automating the process and improving overall productivity. The first study mimicked the SCF approach using synthetic medium in shake flasks, where half of the culture volume was manually removed and replaced with sterile medium for a total number of five cycles. As a result, stable patterns for glucose consumption and ethanol production were observed for SCF after cycle 1, but using only about 1/3 of the fermentation time of batch fermentation. This proved a proof-of-concept that SCF can help significantly increase ethanol volumetric productivity (the amount of ethanol produced by a cycle per working volume per cycle time) compared to batch fermentations performed under similar conditions.
  To apply a real SCF strategy into ethanol production, the process needs to be automatically monitored and driven by a feedback control parameter. The second study successfully identified a real-time sensing parameter—gas flow rate—that revealed the flow rate of gas evolved from the fermenter under anaerobic conditions. With the incorporation of the gas flow meter, an SCF system using synthetic medium was successfully operated for ethanol production in a 5-L fermenter. A stable and robust behavior was observed, and the system was automatically operated for 21 cycles. More importantly, the ethanol volumetric productivity of SCF was substantially improved by over 35%, compared to batch.
  Finally, to explore the use of feedstocks that would be more representative of the cellulosic ethanol industry, the enzymatic hydrolysate of wood pulp was fed into the newly established SCF system for ethanol production. The same feedback control parameter, gas flow rate, successfully drove the fermentation for 10 cycles, with remarkable improvement in ethanol volumetric productivity (54-82%) compared to batch. Interestingly, during the operation of SCF, cell flocculation was consistently observed as cycle number increased; this improvement will facilitate the downstream separation process. Taken together, this work demonstrated that an advanced cycling fermentation strategy can significantly improve ethanol productivity, which will contribute to a reduction of production cost for the cellulosic ethanol industry, thus helping to overcome energy and environmental challenges.

• Subjects / Keywords

  • biorefinery
  • diauxic growth
  • Tween 80
  • anaerobic fermentation
  • yeast nitrogen base
  • annual ethanol productivity
  • self-cycling fermentation
  • Manual cycling fermentation
  • cellulosic ethanol
  • ergosterol
  • wood pulp
  • online monitoring parameter
  • ethanol volumetric productivity
  • flocculation
  • batch fermentation
  • co-products
  • gas flow rate

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-nz0a-p784

• License

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