Hot microbubble gas stripping for bioethanol recovery

Growing concern of the impact of carbon dioxide gas emissions on increasing global temperatures via the greenhouse effect is increasing interest in low-carbon technologies for energy generation. Bioethanol production by the thermophilic fermentation of pretreated lignocellulose is an attractive low-carbon fuel option that can reduce the carbon intensity of the transport sector, but the fermenting organisms often suffer from severe product inhibition effects.

This thesis presents work undertaken using dense clouds of hot microbubbles to reduce the impact of product inhibition on this fermentation process in a manner that is not harmful to the biological culture. While ethanol stripping often demonstrates lower mass transfer rates than other mass transfer operations, its high thermal efficiency and its high affinity to microbial systems (a combination of low shear mixing and a relatively low potential for fouling) make stripping the obvious choice for this application.

Complemented by numerical simulations, dilute ethanol and water mixtures (~4% v/v) at 60°C (the optimum temperature of Geobacillus thermoglucosidasius) are contacted with dense clouds of hot microbubbles (90 – 150°C) to assess the impact of the inlet gas temperature and initial liquid height on the stripping rate and quality.

Subsequently, a simulated ethanol generation rate was included in the experimental setup to imitate the ethanol generation of a microbial process. The competing ethanol generation and stripping rates would therefore provide key information regarding the location of a steady state concentration, which is important for assessing the rate of ethanol generation because of the severe product inhibition demonstrated by this microorganism. Therefore, a simulated ethanol generation was included in the second stage of the experiment with a rate appropriate to G. thermoglucosidasius strain TM333 to demonstrate the ability of the microbubble stripping process to maintain sub- or low-inhibitory conditions inside the microbubble stripping unit.

Finally, microbubble stripping was incorporated into a fermentation process as a sidearm extraction unit to assess the impact of the microbubble system on the fermentation, as well as the effects of the fermentation, which contained salts and antifoaming agents which could affect the rate of mass transfer. This process demonstrated a fermentation reaction that consumed five times the amount of substrate (hydrolysed waste bread) than would have been consumed in a batch process, demonstrating that this could allow fermentation to occur for extended periods of time, with a peak product ethanol concentration of 13.5% (v/v), higher than the product of the traditional process using baker’s yeast which is ended between 11 – 12% (v/v). This result represents a positive step towards the production of bioethanol from lignocellulosic material which would have a more significant environmental benefit and reduce negative social and political impacts of bioethanol production.