3D and contour plots of k
a values with respect to stirrer speed and flow rate and k
a values obtained by headspace aeration with respect to stirrer speed. Blue dots in all plots indicate measured values. The impact of the stirring rate is much higher on kla then the influence of the flow-rate and. Higher flow rates result in a linear increase in kla whereas higher stirring rates increase kla exponential as illustrated in the 3D plot. High stirring rates and low flowrates deliver high kla values. A further increase of the flow rate has just a minor effect on kla as indicated in the contour plot. Low stirring rates have a minor effect when only headspace aeration is applied, while higher stirring rates cause an increase of kla which can be at 500 rpm around 10% of that achieved by bottom sparging.