Out-of-phase operating conditions, a hitherto unknown phenomenon in shaking bioreactors

One of the important parameters in characterising fermentations of aerobic microoganisms is the specific power consumption. A new method has been intr oduced which enables the accurate determination of the power consumption in shaking bioreactors. It is based on torque measurements in the drive and t he appropriate compensation of the friction losses. Measurements of the pow er consumption revealed the phenomenon of the liquid being 'out-of-phase' f or the first time for shaking bioreactors. This occurs at certain operating conditions and is characterised by an increasing amount of liquid not foll owing the rotating movement of the shaker table, thus reducing the specific power consumption, mixing and the gas/liquid mass transfer. With respect t o this, different hydrodynamic cases have to be distinguished. All these ca ses have in common, however, that the probability of 'out-of-phase' conditi ons increases with lower shaking diameters, lower filling volumes, larger n umber and sizes of baffles and higher viscosity. For unbaffled flasks with a nominal volume less than or equal to 11 the "out-of-phase" phenomenon is described in the form of a newly defined non-dimensional Phase number (Ph). To avoid the (unidentified) development of a screening project in unfavour able directions or even its complete failure, researchers must be aware of the 'out-of-phase' phenomenon. The experimental protocols have to be carefu lly selected so that the occurrence of such unwanted hydrodynamic condition s is not possible under all experimental circumstances. (C) 2001 Elsevier S cience B.V. All rights reserved.