title: Comments on the communication: on the calculation of shear rate and apparent viscosity in airlift and bubble column bioreactors.
abstract: The determination of the shear rate in bubble column and airlift bioreactors is an important question from both the perspective of cell damage and the correlation of hydrodynamic parameters in non-Newtonian fluids in these contractors. In the context of correlating hydrodynamic parameters in non-Newtonian fluids, a common approach involves assuming that there exists an average shear rate in the column that is proportional to the superficial gas velocity. This average shear rate is then used to evaluate an effective viscosity of the non-Newtonian fluid that is subsequently used to quantify the fluid's rheological behavior in correlation. Contrary to a recent communication, this report illustrates that this approach, which has mainly been applied to bubble columns, can also be applied to external loop airlift contractors, replacing the superficial gas velocity by the superficial gas velocity by the superficial gas velocity supplied to the riser of the contractor. This extension is based upon consideration of the relevant characteristic velocity in the active zone (i.e., the riser section) of the reactor.
authors.affiliation: Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada M5S 1A4.
authors.forename: D G
authors.initials: DG
authors.lastname: Allen
authors.forename: C W
authors.initials: CW
authors.lastname: Robinson