Abstract
Product inhibition and the cost of downstream separations are two main barriers in using lignocellulosic biomass for bioethanol production. If bioethanol can be continuously removed from fermentation broth without affecting the fermentation process, significant gains can be achieved with bioethanol yields and process efficiency. Hot microbubble clouds generated by energy efficient means have been used to remove ethanol from dilute ethanol–water mixtures (∼4% [v/v]) maintained at 60 °C, and the effect of key operating parameters on the stripping rate has been studied. Numerical simulations of a hot microbubble rising in a dilute ethanol–water mixture were also performed to understand the instantaneous concentrations within the gas phase. Increasing the inlet gas temperature from 90 to 150 °C and decreasing the liquid height in the unit from 50 to 5 mm both increased the ethanol stripping rate. However, the benefit of increasing the gas temperature for maximum ethanol removal depended on the liquid height in the unit. Under all operating conditions, ethanol concentration was reduced below ∼2% [v/v] within ∼25 min of operation, demonstrating the potential of hot microbubble stripping for product removal from lignocellulosic fermenters. Implemented effectively in a fermenter, this technology could intensify the bioethanol production process and improve process economics.
Highlights
The world has heavily depended on fuels and chemicals derived from fossil resources that led to a significant increase in carbon dioxide emissions, estimated at ∼36 billion tones globally in 2018,1 and other forms of environmental pollution
This study demonstrated ethanol removal capability of hot microbubble clouds in dilute ethanol−water mixtures (∼4% [v/v]) maintained at 60 °C
Increasing the gas temperature at low liquid heights was found to be less effective in improving the stripping rate compared to that at high liquid heights
Summary
The world has heavily depended on fuels and chemicals derived from fossil resources that led to a significant increase in carbon dioxide emissions, estimated at ∼36 billion tones globally in 2018,1 and other forms of environmental pollution. Bioethanol, produced by fermentation of carbohydrates, is one such renewable fuel that can be blended with petroleum fuels[6] or, with some engine modifications, replace them completely.[7] Fermentation of biomass-derived carbohydrates, especially from “second-generation” feedstocks such as the non-edible parts of food crops, sugar-cane bagasse, forestry residues, switch grass, the organic fraction of municipal solid waste, etc., represents a sustainable way of producing fuels and platform chemicals that does not compete with resources that might otherwise be used for food.[3] Unlike first-generation (starch or sucrose-based) processes, utilization of biomass in second-generation processes presents additional economic constraints arising from the conversion of plant material to fermentable carbohydrates.[8] these can potentially be ameliorated by the use of organisms other than yeast, which are more process-suitable, such as catabolically versatile moderately thermophilic bacteria.[8,9] these are typically less tolerant to ethanol than yeast and suffer from product inhibition With this in mind, the current study is focused on the intensification of biofuel production by addressing two key issues: product inhibition and the cost of downstream separation. Numerical simulations of a hot microbubble rising in a dilute ethanol−water mixture were carried out using a lumped parameter model to understand the dynamic nature of the process, where the gas and the liquid are at different temperatures as opposed to traditional equilibrium calculations in which both phases are considered to be at the same temperature
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