FAIR-CT96-5027 Simulation of oxygen gradients in a poorly mixed ethanol fermentation: modelling and experimental design of scale-up through a series of STRs

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Bulk Chemicals : FAIR Area 2.2 - Bioprocessing : FAIR Marie Curie Research Training Grants : Liquid Biofuels and Biogas

Objectives:

A thermotolerant yeast from soil samples obtained from a distillery in India has been isolated. The yeast strain designated Kluyveromyces marxianus var. marxianus IMB3 was capable of growth and ethanol production at temperatures between 25-48 degrees C. K. marxianus is a facultative anaerobe i.e. it can metabolise glucose in the absence of oxygen to produce ethanol and carbon dioxide. It has been classified as a respiratory yeast, since under aerobic conditions around 30% of the glucose metabolised is converted to ethanol. This led to a low biomass concentration in the fermenter during continuous fermentation and consequently a low ethanol productivity (13 ml/1.hour).This can be compared with fermentative yeast i.e. Saccharomyces spp. in which over 90% of glucose is converted to ethanol under anaerobic conditions, although growth cannot be sustained under anaerobic conditions.

Activities and Results:

This project aimed to:

• Confirm the results obtained elsewhere, in particular, to check carbon balances and by-product formation.
• Investigate the effect of scale-up on oxygen distribution in poorly mixed fermentation vessels.
• To study the effect of changes in oxygen transfer rates on ethanol productivity, growth and ethanol yield.
• Simulate plug flow through poorly mixed fermentation vessels using a series of small STRS.

It was found that high specific ethanol productivities can be achieved with this strain of Kluyveromyces during aerobic growth by intermittently switching the specific Oxygen uptake, qOpX levels to zero. This switch did not significantly affect the growth rate. Ethanol production was also seen at high specific growth rates under aerobic conditions. Using STRs in series to simulate plug flow through the reactor yielded some interesting results but the method in itself was complicated due to a number of factors. These included the need to incorporate cell recycle in order to simulate back-mixing and the increased complexity of measurement and analysis of data.

Conclusions:

It was also found that glycerol was the major by-product during aerobic and anaerobic conditions. The carbon balance also suggested that several other by-products were also formed.

Keywords: ethanol fermentation

Contacts

Participant

• Clemens POSTEN / Gesellschaft fuer Biotechnologische Forschung mbH Division of Biochemical Engineering / GERMANY