Downscaling Industrial-Scale Syngas Fermentation to Simulate Frequent and Irregular Dissolved Gas Concentration Shocks

Metabolic Versatility of acetogens in syngas Fermentation: Responding to varying CO availability

Syngas fermentation using acetogenic bacteria offers a promising route for sustainable chemical production. However, gas-liquid mass transfer limitations and efficient co-utilization of CO and H2 pose significant challenges. This study investigated the kinetics of syngas conversion to acetate by Acetobacterium wieringae and Clostridium species in batch conditions under varying initial CO partial pressures (19 - 110 kPa) in batch cultures. A. wieringae strains, exhibited superior growth in all gas compositions, with a maximum growth rate of 0.104 h-1. The distinct CO, H2, and CO2 consumption patterns revealed metabolic flexibility and adaptation to varying syngas compositions. Notably, A. wieringae strains and C. autoethanogenum achieved complete CO and H2 conversion, with C. autoethanogenum also exhibiting net CO2 uptake. These findings provide valuable insights into the distinct metabolic capabilities of these acetogens and contribute to the development of efficient and sustainable syngas fermentation processes.

Real Case Study of 600 m3 Bubble Column Fermentations: Spatially Resolved Simulations Unveil Optimization Potentials for l-Phenylalanine Production With Escherichia coli

Large-scale fermentations (»100 m³) often encounter concentration gradients which may significantly affect microbial activities and production performance. Reliably investigating such scenarios in silico would allow to optimize bioproduction. But related simulations are very rare in particular for large bubble columns. Here, we pioneer the spatially resolved investigation of a 600 m³ bubble column operating for Escherichia coli based l-phenylalanine fed-batch production. Microbial kinetics are derived from experimental data. Advanced Euler-Lagrange (EL) computational fluid dynamics (CFD) simulations are applied to track individual bubble dynamics that result from a recently developed bubble breakage model. Thereon, the complex nonlinear characteristics of hydrodynamics, mass transfer, and microbial activities are simulated for large scale and compared with real data. As a key characteristic, zones for upriser, downcomer, and circulation cells were identified that dominate mixing and mass transfer. This results in complex gradients of glucose, dissolved oxygen, and microbial rates dividing the bioreactor into sections. Consequently, alternate feed designs are evaluated splitting real feed rates in two feeds at different locations. The opposite reversed installation of feed spots and spargers improved the product synthesis by 6.24% while alternate scenarios increased the growth rate by 11.05%. The results demonstrate how sophisticated, spatially resolved simulations of hydrodynamics, mass transfer, and microbial kinetics help to optimize bioreactors in silico.

Continuous H-B-E fermentation by Clostridium carboxidivorans: CO vs syngas

Leveraging renewable carbon-based resources for energy and chemical production is a promising approach to decrease reliance on fossil fuels. This entails a thermo/biotechnological procedure wherein bacteria, notably Clostridia, ferment syngas, converting CO or CO2 + H2 into Hexanol, Butanol and Ethanol (H-B-E fermentation). This work reports of Clostridium carboxidivorans performance in a stirred tank reactor continuously operated with respect to the gas and the cell/liquid phases. The primary objective was to assess acid and solvent production at pH 5.6 by feeding pure CO or synthetic syngas under gas flow differential conditions. Fermentation tests were conducted at four different dilution rates (DL) of the fresh medium in the range 0.034-0.25 h-1. The fermentation pathways of C. carboxidivorans were found to be nearly identical for both CO and syngas, with consistent growth and metabolite production at pH 5.6 within a range of dilution rates. Wash-out conditions were observed at a DL of 0.25 h-1 regardless of the carbon source. Ethanol was the predominant solvent produced, but a shift towards butanol production was observed with CO as the substrate and towards hexanol production with synthetic syngas. In particular, the maximum cell concentration (0.5 gDM/L) was obtained with pure CO at DL 0.05 h-1; the highest solvent productivity (60 mg/L*h of total solvent) was obtained at DL 0.17 h-1 by using synthetic syngas as C-source. The findings highlight the importance of substrate composition and operating conditions in syngas fermentation processes. These insights contribute to the optimization of syngas fermentation processes for biofuel and chemical production.

A novel experimental method to determine substrate uptake kinetics of gaseous substrates applied to the carbon monoxide-fermenting Clostridium autoethanogenum

Syngas fermentation has gained momentum over the last decades. The cost-efficient design of industrial-scale bioprocesses is highly dependent on quantitative microbial growth data. Kinetic and stoichiometric models for syngas-converting microbes exist, but accurate experimental validation of the derived parameters is lacking. Here, we describe a novel experimental approach for measuring substrate uptake kinetics of gas-fermenting microbes using the model microorganism Clostridium autoethanogenum. One-hour disturbances of a steady-state chemostat bioreactor with increased CO partial pressures (up to 1.2 bar) allowed for measurement of biomass-specific CO uptake- and CO2 production rates (q CO ${q}_{{CO}}$ ,q CO 2 ${q}_{{{CO}}_{2}}$ ) using off-gas analysis. At a pCO of 1.2 bar, aq CO ${q}_{{CO}}$ of -119 ± 1 mmol g-1 X h-1 was measured. This value is 1.8-3.5-fold higher than previously reported experimental and kinetic modeling results for syngas fermenters. Analysis of the catabolic flux distribution reveals a metabolic shift towards ethanol production at the expense of acetate at pCO ≥ $\ge $  0.6 atm, likely to be mediated by acetate availability and cellular redox state. We characterized this metabolic shift as acetogenic overflow metabolism. These results provide key mechanistic understanding of the factors steering the product spectrum of CO fermentation in C. autoethanogenum and emphasize the importance of dedicated experimental validation of kinetic parameters.