Abstract
A trickle-bed reactor (TBR) when operated in a trickle flow regime reduces liquid resistance to mass transfer because a very thin liquid film is in contact with the gas phase and results in improved gas–liquid mass transfer compared to continuous stirred tank reactors (CSTRs). In the present study, continuous syngas fermentation was performed in a 1-L TBR for ethanol production by Clostridium ragsdalei. The effects of dilution and gas flow rates on product formation, productivity, gas uptakes and conversion efficiencies were examined. Results showed that CO and H2 conversion efficiencies reached over 90% when the gas flow rate was maintained between 1.5 and 2.8 standard cubic centimeters per minute (sccm) at a dilution rate of 0.009 h−1. A 4:1 molar ratio of ethanol to acetic acid was achieved in co-current continuous mode with both gas and liquid entered the TBR at the top and exited from the bottom at dilution rates of 0.009 and 0.012 h−1, and gas flow rates from 10.1 to 12.2 sccm and 15.9 to 18.9 sccm, respectively.
Highlights
Syngas fermentation is part of the hybrid conversion technology for the conversion of renewable feedstocks or gas waste streams containing CO, CO2 and H2 to biofuels and chemicals.Clostridium ljungdahlii, Clostridium carboxidivorans, Clostridium ragsdalei, and Alkalibaculum bacchi are among the microorganisms that metabolize CO, CO2 and H2 via the reductive acetyl-CoA pathway to produce ethanol, acetic acid and cell carbon [1,2,3,4]
The cell OD660 was 0.35 at 197 h when the trickle-bed reactor (TBR) was switched to continuous operation with a dilution rate of 0.012 h−1 and a gas flow rate of 1.9 sccm
19 g/L ethanol was reported in a two-stage continuous syngas fermentation in a continuous stirred tank reactors (CSTRs) followed by a bubble column with gas and cell recycling [23] and 48 g/L ethanol was reported in a CSTR with cell recycle using
Summary
Syngas fermentation is part of the hybrid conversion technology for the conversion of renewable feedstocks or gas waste streams containing CO, CO2 and H2 to biofuels and chemicals.Clostridium ljungdahlii, Clostridium carboxidivorans, Clostridium ragsdalei, and Alkalibaculum bacchi are among the microorganisms that metabolize CO, CO2 and H2 via the reductive acetyl-CoA pathway to produce ethanol, acetic acid and cell carbon [1,2,3,4]. Syngas fermentation is part of the hybrid conversion technology for the conversion of renewable feedstocks or gas waste streams containing CO, CO2 and H2 to biofuels and chemicals. One major advantage of the hybrid conversion process is the ability to utilize feedstocks such as municipal solid wastes, industrial fuel gases and biomass [5]. Challenges for this technology include mass transfer limitations, enzyme inhibition, low cell concentration and low ethanol productivity. Many researchers focused on improving ethanol productivity by optimizing media components, adding reducing agents, adjusting pH, adding nanoparticles and optimizing the bioreactor design to improve the mass transfer of CO and H2 in fermentation medium [7,8,9,10,11,12,13]
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