Conference Paper

261925 Comparison of Syngas Fermentation Reactors for Ethanol Production

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Abstract

Ethanol can be produced from biomass feedstocks or municipal solid wastes (MSW) using a hybrid thermochemical-biochemical conversion process called gasification-fermentation. In the gasification-fermentation process, biomass is gasified and converted to synthesis gas (syngas; primarily CO, CO2 and H2). The advantage of gasification-fermentation over the saccharification-fermentation process is that gasification utilizes cellulose, lignin and hemicellulose components of the biomass, leading to a greater potential for high carbon conversion efficiency and high ethanol yield from the same amount of biomass. Syngas fermentation rates are subject to gas-liquid mass transfer limitations since CO and H2 have low solubilities in the fermentation broth. Successful syngas fermentation balances the kinetic capacity of the cell inventory with CO and H2 supply that is set by the mass transfer characteristics of the reactor used. Continuously stirred tank reactor (CSTR), trickle-bed reactor (TBR) and hollow fiber membrane reactor (HFR) were compared for syngas fermentation with regards to gas utilization (including mass transfer assessment), cell growth, and product formation. The volumetric mass transfer coefficient (KtotA/VL) for O2 in the three reactors was measured. The HFR provided the largest KtotA/VL followed by the TBR. The mass transfer characteristics in each reactor were affected by agitation speed, and gas and liquid flow rates. Correlations for KtotA/VL for each reactor were developed to predict mass transfer during syngas fermentation by Clostridium ragsdalei to produce ethanol and acetic acid. Although scale-up issues are different for each reactor, the comparison of the three reactors provides guidance as to which reactor(s) should be evaluated for scale-up, especially if one reactor significantly outperforms the others regarding ethanol yield and productivity and syngas utilization efficiency. Further, insight gained from the analysis of the three reactors can be extended to other fermentation processes.

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... Higher selectivity of biological catalysts reduces undesired byproducts and improves ethanol yield and downstream processes (Griffin and Schultz 2012;Abubackar et al. 2011). However, lower gas-liquid mass transfer and production of inhibitory compounds during fermentation were noted to be the main constraints to syngas fermentation into ethanol (Munasinghe and Khanal 2010;Huhnke 2013;Lee P 2010). ...
... High gas flow rate adversely affects the gas conversion (Bredwell et al. 1999). Devarapalli et al. (Huhnke 2013) have also reported that gas-liquid mass transfer characteristic was affected by gas, media, and effluent flow rates. The culture may have also become inhibited by the increased supply of CO. ...
... Ethanol concentration was noted to be dependent on the mass transfer efficiency (Munasinghe and Khanal 2010;Huhnke 2013;Lee et al. 2012), and the characteristics of the biofilm (Shen et al. 2014). The mass transfer was also dependent on the reactor configuration, agitation speed, syngas, and media flow rates (Huhnke 2013;van Kasteren JMN et al. 2005). ...
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Background In a conventional syngas fermentation process, gas was released into the fermentation broth through a single orifice or multiple orifices, except the hollow fiber membrane reactor. Consequently, a simplified bioreactor has been developed employing an innovative gas supply and effluent extraction systems. ResultsA continuous stirred tank bioreactor (CSTBR) has been developed by incorporating an innovative gas supply and effluent extraction system to ferment syngas into ethanol. The working volume of the bioreactor was controlled to 2 L. The CO gas was fermented in the developed bioreactor by using a microorganism (Clostridium ljungdahlii) with different gas (5–15 mL/min), media, and effluent flow rates (0.25–0.75 mL/min) and stirrer speed (300–500 rpm). Gas was diffused into the fermenting broth through an aqueous aeration tube commonly used in the small household aquarium, placed at the bottom layer throughout the periphery. The effluent was extracted from the top layer of the broth by using a membrane separator. Ethanol and acetic acid concentrations were varied from 0.17–1.17 and 8.50–23.68 g/L-effluent, respectively. Conclusions It seems that the performance of CSTBR can be enhanced with an innovative gas supply system, which may reduce the gas bubble size and result in higher lateral velocity at the releasing point, especially, throughout the periphery instead of the center of the reactor through a single or multiple orifice.
... Moreover, reactor design is important; for demonstration, often bubble column reactor technology is applied. Also, continuously stirred tank reactor, trickle bed reactor, and hollow fiber reactor are under investigation (Devarapalli et al. 2012;Verma et al. 2016). ...
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