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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.

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., (Copyright 1998 John Wiley & Sons, Inc.)