Figure 10

State of severe hydrogen inhibition as a function of hydrogen productivity, $P_{H_2}$ and $k_l{a}_{H_2}$. The straight lines show the combinations of H₂ productivity, $P_{H_2}$ and $K_l{a}_{H_2}$, resulting in a state of severe H₂ inhibition. The state of severe H₂ inhibition is assumed to occur at a value of H_2aq equal to 80% of H_2aqcrit (corresponding to a reduction in the growth and H₂ production rate of 37%). The further away from, and above, the lines the state of a fermentation experiment is, the lower the inhibition by H₂. An increase in H₂ productivity and a decrease in $K_l{a}_{H_2}$ move the state of the fermentation to the right and down, respectively, on the graph, while an increase in $P_{H_2}$ increases the slope of the lines. These changes all increase the dissolved H₂ concentration and move the state of a fermentation experiment closer to a state of H₂ inhibition. The open squares show the state of each experiment at the time of maximum productivity. An additional point is included for experiment 1 (12 hours), showing the state four hours before maximum productivity occurred. The curved line shows the relation between the stripping rate and $K_l{a}_{H_2}$ (the relation given by equation 27). The experimental $K_l{a}_{H_2}$ values (calculated from experimental values of $K_l{a}_{C{O}_2}$ using equation 25) are also included (open circles).