The Effect of Gas Pressure
The effect of partial pressure of gas components is another controlling factor that can affect the ethanol yield. In an early study with Clostridium ljungdahlii, varying initial CO partial pressures (up to 1 atm) in a closed bottle system was used to assess cell growth and product formation [26]. In this investigation it was shown that varying CO partial pressure did not affect the maximum cell growth. Additionally, ethanol production was similar for partial pressures up to 0.75 atm and acetate production had no apparent correlation with CO partial pressure. However, more recent work in this area was aimed at keeping partial pressure constant throughout the fermentation process. Since gas partial pressures do not remain constant in a closed bottle system as the metabolic process proceeds, later work utilized an experimental design to maintain constant CO and CO2 gas phase partial pressures during the entire experiment. In a 2010 investigation Hurst and Lewis have studied the effect of CO partial pressure effects on the metabolic process of syngas fermentation by keeping partial pressures constant throughout the process [46]. In this work they assessed the effects of constant CO partial pressure (PCO), ranging from 0.35 to 2.0 atm, on cell growth, acetic acid production, and ethanol production using Clostridium carbox- idivorans P7T. The key findings of this study could be summarized as follows: (a) the maximum cell concentration increased with increasing PCO, increasing 440% with a PCO increase from 0.35 to 2.0 atm, (b) ethanol production changed from non-growth-associated to growth-associated with increasing PCO, (c) acetic acid production (gram acetic acid per gram cells) decreased for PCO > 1.05 atm relative to PCO < 0.70 atm, and (d) acetic acid appeared to be converted in the latter growth stages for PCO of 1.35 and 2.0 atm.
The work of Hurst and Lewis as well as other groups have emphasized the importance of PCO and the PCO to PCO2 ratio on electron and ATP production. Since gasification processes that generate syngas could result in differing gas partial pressures, the process variations could significantly change growth and product formation as evidenced by these metabolic changes.