Synthesis gas fermentation at high cell density: How pH and hydrogen partial pressure affect productivity and product ratio in continuous fermentation

For the first time, syngas fermentation was operated continuously with total cell retention and process pressures up to 4 barg in long-term runs of up to 3000 hours. Throughout this time, the process was stable. The measured data have shown that hydrogen uptake and ethanol space-time yield are highest at a slightly reduced pH of 5.7 compared to pH 5.9. Even lower pH values lead to higher acetic acid to ethanol product ratios, while C2 space-time yields remain constant. Increasing the hydrogen partial pressure to 1.52 bar resulted in a significant increase in hydrogen uptake rate and ethanol formation. An ethanol space-time yield of 10 mmolL-1 h- 1 was short-term achieved, being the highest space-time yield measured to date for the wild type of C. ljungdahlii. Hydrogen uptake above a theoretical equilibrium concentration of 1.2 mmolH2 L- 1 is significantly reduced, indicating an inhibition of an enzymatic reaction.