Continuous removal of ethanol from dilute ethanol-water mixtures using hot microbubbles

Joseph Calverley,William B Zimmerman,David J Leak,H.C Hemaka Bandulasena

doi:10.1016/j.cej.2021.130511

Abstract

Product inhibition is a barrier to many fermentation processes, including bioethanol production, and is responsible for dilute product streams which are energy intensive to purify. The main purpose of this study was to investigate whether hot microbubble stripping could be used to remove ethanol continuously from dilute ethanol–water mixtures expected in a bioreactor and maintain ethanol concentrations below the inhibitory levels for the thermophile Parageobacillus thermoglucosidasius (TM242), that can utilize a range of sugars derived from lignocellulosic biomass. A custom-made microbubble stripping unit that produces clouds of hot microbubbles (~120 °C) by fluidic oscillation was used to remove ethanol from ~2% (v/v) ethanol–water mixtures maintained at 60 °C. Ethanol was continuously added to the unit to simulate microbial metabolism. The initial liquid height and the ethanol addition rate were varied from 10 to 50 mm and 2.1–21.2 g h−1 respectively. In all the experiments, ethanol concentration was maintained well below the inhibition threshold of the target organism (~2% [v/v]). This microbubble stripping unit has the potential to operate in conjunction with a 0.5–1.0 L fermenter to allow an ethanol productivity of 14.9–7.8 g L−1h−1 continuously.

Highlights

Bioethanol is a prime example of an alternative to petroleum fuels which has the potential to make the road transport sector more sus tainable and environmentally friendly
The initial amount of ethanol in the microbubble stripping unit (MSU) and the ethanol addition rate is proportional to H0; reduction in the concentration is relatively slower for the higher liquid heights
Even though the results found in this study looks promising, experiments and mathematical modelling were related to pure ethanol–water mixtures mimicking a fermentation system

Summary

Introduction

Bioethanol is a prime example of an alternative to petroleum fuels which has the potential to make the road transport sector more sus tainable and environmentally friendly. In batch systems concentrated sugar solutions are inoculated with a fermenting organism, typically the yeast Saccharomyces cerevisiae, which over time metabolises the sugars into a mixture of products, mainly carbon dioxide and ethanol under fermentative conditions. Fed-batch processes benefit from improved speed of fermentation as it removes substrate inhibition, where the high concentration of sugar reduces the metabolic rate over a certain threshold, typically ~150 g/L for S. cerevisiae [4]. In both cases, over the course of the fermentation the ethanol concentra tion reaches a point at which it begins to inhibit the growth of the or ganism until the fermentation process ceases completely [5]. The fermentation medium undergoes various unit operations such as distillation to recover and purify ethanol up to the azeotropic concentration and is further processed up to fuelgrade ethanol using different separation techniques [6]

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Conclusion