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Continuous culture fermentations of Escherichia coli W3110 have been carried out at controlled dissolved oxygen levels of 40% and 10% of saturation. Satisfactory and reproducible results were obtained. Agitation speeds of 400 and 1200 rpm at an aeration rate of 1 vvm have been used as well as an aeration rate of 3 vvm at 400 rpm. The upper levels of these variables represent much higher agitation and aeration intensities than those normally used in practical fermentations. The fermentations were monitored by mass spectrometry and optical density, and cell samples were studied by flow cytometry, SEM, and TEM. Protocols were developed so the state of both cell membranes and cell size could be measured by flow cytometry. Under all the conditions of agitation and aeration, flow cytometric analysis indicated that both cell membranes were intact and that a cytoplasmic membrane potential existed; also the cell size did not change, results confirmed by SEM and TEM. There were no detectable changes in off‐gas analysis or optical density during the continuous fermentation nor in the cell structure as revealed by SEM or TEM, except at the highest agitation intensity. Under the latter conditions, after 7 h, the outer polysaccharide layer on the cell was stripped away. It is concluded that any changes in biological performance of this E. coli cell line due to variations in agitation or aeration intensity or scale of operation cannot be attributed to fluid dynamic stresses associated with the turbulence generated by impellers or with bursting bubbles. © 1998 John Wiley & Sons, Inc. Biotechnol Bioeng 59:612–620, 1998.

Foam disruption by agitation-the stirring as foam disruption (SAFD) technique-was scaled up to pilot and production scale using Rushton turbines and an up-pumping hydrofoil impeller, the Scaba 3SHP1. The dominating mechanism behind SAFD-foam entrainment-was also demonstrated at production scale. The mechanistic model for SAFD defines a fictitious liquid velocity generated by the (upper) impeller near the dispersion surface, which is correlated with complete foam disruption. This model proved to be scalable, thus enabling the model to be used for the design of SAFD applications. Axial upward pumping impellers appeared to be more effective with respect to SAFD than Rushton turbines, as demonstrated by retrofitting a 12,000 l bioreactor, i.e. the triple Rushton configuration was compared with a mixed impeller configuration from Scaba with a 20% lower ungassed power draw. The retrofitted impeller configuration allowed 10% more broth without risking excessive foaming. In this way a substantial increase in the volumetric productivity of the bioreactor was achieved. Design recommendations for the application of SAFD are given in this paper. Using these recommendations for the design of a 30,000 l scale bioreactor, almost foamless Escherichia coli fermentations were realised.

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n the fermentation industry, the formation of foam during the fermentation process can lead to blockage of the outlets and reduce I the productive volume. Foam can be broken mechanically, through the installation of a foam disrupter, but the installation and operation of such a system can be costly. Another much used method is the addition of anti-foam, which however leads to a reduction in kp and consequently in oxygen transfer capacity (van't Riet and Tramper, 1991). A novel approach is the use of the upper impeller of a set of impellers for the elimination of foam. This technique has been described by Hoeks et al. (1997), who called it "stirring as foam disruption" (SAFD). Since the process of foam formation is a dynamic one (Pahl and Franke, 1999, the aim of SAFD is to use agitation to enhance foam disruption mechanisms. Hoeks et al. (1997) tested the concept by measuring, for a given combination of impellers, the thickness of the foam layer under different operating conditions and unaerated liquid level. They then gradually lowered the level until the agitation eliminated the foam layer. Altogether they tested five different upper impellers; two sizes of Rushton turbine, two downward pumping pitched blade impellers and a hydrofoil Lightnin' A31 5 pumping downwards, using a low viscosity fermentation broth. The "best" impeller is one that is able to disrupt foam at relatively high unaerated liquid volumes and low specific power inputs. Of the impellers tested, the Lightnin' A31 5 gave the best foam disruption performance since, although it did so with much the same amount of unaerated liquid volume as the other impellers at the same aeration rate, it consumed less power. However, all the impellers tested were of rather large impeller diameter (0) to tank diameter (T) ratio, i.e. 0.49Tto 0.64T. The Rushton turbines were of D/T = 0.49 and 0.62 whilst the "standard geometry" for Rushton turbines is a diameter of T/3 (Harnby et al., 1997; Oldshue, 1983). As SAFD is a new technique, its assessment with such "standard" impellers is important. Therefore, the use of the T/3 Rushton turbine for SAFD is the subject of this study.For multiple Rushton turbine systems, the impellers act independently in unaerated conditions provided that the inter-impeller clearance, AC > T (Hudcova et al., 1989). Under aerated conditions with such a spacing, the bottom impeller behaves like a single impeller whilst all the upper impellers behave the same but differently from the bottom one.The bottom impeller handles all of the sparged gas and gives the greatest power drop upon aeration provided N > N, , since the upper impellers handle less than the gas feed rate, as some gas will bypass them. This consequently leads to less drop in power in comparison to the bottom impeller (Hudcova et al., 1989; Abrardi et al., 1990 The concept of using the upper stirrer for foam disruption in a bioreactor agitated by multiple impellers has recently been published by Hoeks et al.(1 997). Thi s concept, stirring as foam disruption (SAFD), was shown...