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Energy Procedía 61 (2014) 2725 - 2728
The 6th International Conference on Applied Energy - ICAE2014
Process design for very-high-gravity ethanol fermentation
Yen-Han Lina*, Chen-Guang Liub
aDepartment of Chemical Engineering, University of Saskatchewan, Saskatoon, SK S7N 5A9, Canada _bSchool of Life Science and Biotechnology, Dalian University of Technology, Dalian, Liaoning 116023, China_
Abstract
Metabolic flux distribution may be altered by manipulating intracellular reducing equivalents. To favour ethanol synthesis by Saccharomyces cerevisiae, a reduced cytosolic environment is desired, otherwise biomass formation is favoured. Direct variation of intracellular NADH/NAD+ is difficult, however, indirect control through measurement of fermentation redox potential is applicable. To utilize fermentation redox potential into designing an ethanol fermentation process under very-high-gravity (VHG) conditions, correlations between yeast growth pattern and fermentation redox potential profile were established. Under VHG conditions, S. cerevisiae initially encounters osmotic stress resulting in a lengthy lag phase. As fermentation proceeds, the built-up of ethanol inhibits yeast propagation, resulting in sudden cell death and incomplete sugar conversion. Additionally, an operational scheduling for a continuous VHG ethanol fermentation, consisting of a chemostat device and an ageing vessel, was proposed and compared to the equivalent batch operation. Results show that the proposed operational scheduling is superior to the batch counterpart. Process design criteria for a chemostat device connecting to several equal-size ageing vessels were developed in an attempt to increase annual ethanol productivity.
© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.Org/licenses/by-nc-nd/3.0/).
Peer-reviewunderresponsibilityof theOrganizingCommitteeof ICAE2014
Keywords: redox potential; very-high-gravity fermentation; schemes;
1. Introduction
Fuel ethanol is made by fermentation worldwide and used in transportation as an alternative to fossil fuel. However, a relative high production cost limits its extensive applications [1]. Very-high-gravity (VHG) fermentation features low energy and water consumption, and high annual ethanol productivity, making it a promising technology for fuel ethanol industry [2].
'Corresponding author. Tel.: +1-306-966-4764; Fax: +1-306-966-4777; E-mail: yenhan.lin@usask.ca
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of the Organizing Committee of ICAE2014
doi: 10.1016/j.egypro.2014.12.289
Under VHG conditions, yeast encounters various stresses induced by high concentrations of dextrin and ethanol, resulting in sluggish ethanol fermentation and incomplete sugar utilization [3]. Oxygen is necessary during VHG fermentation in order to synthesize unsaturated fatty acids and sterols to maintain cellular membrane integrity and help cells adapt to negative effects [4]. Due to anaerobic characteristic of the fermentation, excess oxygen in broth will re-route metabolic pathway towards biomass formation and thus lower ethanol-to-sugar yield [5]. Hence, dissolved oxygen should be regulated precisely. For most fermentation processes, traditional dissolved oxygen probe is not sensitive enough to detect the presence of a trace amount of oxygen [6], so redox sensor becomes a better choice under such a circumstance. A design flowchart incorporating redox potential to VHG fermentation is illustrated in Figure 1.
Fig. 1 Design flowchart for redox potential-controlled VHG ethanol fermentation
2. Effect of redox potential control on batch fermentation
During ethanol fermentation, change of redox potential is mainly modulated by NADH (served as electron donor) and dissolved oxygen (served as electron acceptor). A typical redox potential profile resembles a bathtub curve, which can be subdivided into declining (equivalent to lag to initial exponential phase), basin (equivalent to mid exponential to stationary phase) and uprising (equivalent to late stationary to death phase) regions [5]. According to the design, fermentation redox potential will be maintained at a certain level by providing a proper amount of air into fermentation (called aeration-controlled scheme, ACS). The amount of air requirement is determined by the desired redox potential that is to be controlled.
Due to the substrate depletion and/or decline of cell viability, no or less substrate was available to generate enough reducing capacity to lower redox state. Meanwhile, the amount of liberated CO2 in the late stage of fermentation will be less than that liberated during rapid growth stage. As a result, more oxygen will dissolve into fermentation broth through agitation and diffuse through cellular membrane to elevate redox potential level. According to experimental observation, an uprising curve appearing during the fermentation reveals that the fermentation is about to finish. Without supplementing extra oxidants or reductants, redox potential measured during ethanol fermentation varies between -300 and 200 mV. Hence, an appropriate redox potential level could be obtained and correlate to the ethanol productivity [7, 8].
3. Kinetic model for VHG fermentation
That experimentally determine an optimal redox potential control towards high ethanol productivity is not straightforward. However, mathematical modeling approach can narrow the gap to the optimal control region. Modeling redox potential could not only explain the performance of yeast cells under different redox potential and VHG conditions, but also could predict final ethanol concentration under various combination of fermentation conditions by process simulation.
Kinetic models relating redox potential control to ethanol fermentation have been proposed [9, 10]. Liu et al. developed a kinetic model by incorporating ethanol toxicity concentration and VHG conditions into consideration [10]. Various operating diagrams were then designed, making it easy to choose different combinations of fermentation operating conditions when needed. According to their simulated results, the best operation condition under redox potential-controlled VHG fermentation was to control redox potential level at -140 mV along with 275 g glucose/l feed, where 133.2 g ethanol/l could be obtained.
4. Development of different redox potential controlled schemes
The performance of VHG ethanol fermentation can be further improved by applying redox potential control. This can be achieved by two approaches: 1) to search for the optimal redox potential setting under different glucose feed; and 2) to extend the exponential growth phase by prolonging the redox potential control period.
The previously mentioned aeration-controlled scheme (Section 2) has a short redox potential-controlled period. In order to extend controlled duration, two schemes were suggested: 1) glucose-controlled feeding scheme (GCFS) where glucose was supplemented along with dissolved oxygen presented in the feed stream; and 2) a combined chemostat and aeration-controlled scheme (CCACS) where a constant glucose was fed along with air supply determined by redox potential-controlled device.
The GCFS extends the redox potential-controlled period by offering enough glucose for yeast to propagate and to maintain the residual glucose at a low level. As a result, the ethanol yield is increased noticeably. The operation of GCFS likes a fed batch, as such the buildup of ethanol causes yeast cessation, resulting in incomplete fermentation.
The CCACS is a set of continuous equipment that feeds the fresh medium into a chemostat fermenter at a constant dilution rate and discharge spent broth into ageing vessels as the same rate in order to keep the chemostat working volume invariable. Sterile air was used to adjust the fermentation redox potential as a planned level. In the chemostat fermenter, both intracellular and extracellular factors should reach their respective states as a steady state is attained. Thus a constant growth rate and yeast viability are sustained under the pre-determined redox potential levels, which is helpful to prolong the redox potential-controlled duration and to maximize the benefits from redox potential control. The CCACS achieved the longest controlled period and the highest ethanol yield among all three schemes.
However, a chemostat device along could not result in zero glucose discharge; the higher the glucose feed, the higher the unconverted glucose in the effluent stream. To overcome unspent glucose problem found in the CCACS, the incorporation of ageing vessel design into fermentation operation was developed [11, 12].
5. Economic evaluation for redox potential-controlled VHG ethanol fermentation
Process modeling and economic evaluation of a newly developed process is a logic approach to analyze the feasibility for possible large-scale production. A lot of time and money could be saved through such an approach. Yu et al., based on Kwiatkowski's work, developed a redox potential-controlled VHG ethanol fermentation process model to carry out a feasibility study of such a process. The
concluded results were positive. They also suggested that the injection of CO2 into underground geological formations is a promising strategy to reduce atmospheric CO2 emissions [13].
6. Conclusions
Fermentation redox potential control plays a unique role in regulating oxygen supply during the course of anaerobic fermentation [14]. It is not only altering the redistribution of metabolite flux to enhance ethanol productivity, but also increasing yeast viability under VHG conditions, resulting in short fermentation time. To further improve the fermentation efficiency under redox potential control, an understanding of embedded biological mechanism is essential. One feasible approach is to carry out microarray analysis to identify those genes relevant to the changes of fermentation redox potential. Once identified, proper molecular biology approaches can be developed to augment gene function under redox potential conditions.
Acknowledgements
The authors acknowledge contributions from Wan-Shan Chien, Sijing Feng, and Fei Yu. Reference
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