CHARACTERIZATION OF SHEAR RATES IN AIRLIFT BIOREACTORS FOR ANIMAL-CELL CULTURE

A well established analysis of energy dissipation in the riser, downco mer and the bottom sections of split-cylinder airlift bioreactors (asp ect ratio = 7.6 and 14.5; equal riser-to-downcomer cross-sectional are a ratios of 1.0) was used to characterize shear rates in those systems for application to animal cell culture. Shear rates were evaluated fo r suspensions of typical microcarriers (loading = 0-30 kg m(-3), parti cle diameter = (150-300) x 10(-6) m; density 1030-1050 kg m(-3)) encou ntered in anchorage-dependent cell culture and for microcarrier-free l iquids. For the reactors tested, the highest shear rates were encounte red in the bottom zone; the riser had lower shear rate values, while t he downcomer was the most quiescent. The shear rates in various zones ranged over 0-12 000 s(-1) for a riser superficial gas velocity range of 0-6.7 x 10(-3) m s(-1) which is typical for cell culture. In all zo nes, the shear rates increased with increasing aeration rate. Shear ra tes declined with increasing loading of microcarriers, but were not su bstantially affected by the carrier diameter or density. Relative to t he microcarrier free system, even small amounts of carriers (6 kg m(-3 )) lowered the maximum prevailing shear rate to about 4000 s(-1). The shear rates were extremely sensitive to the length scale of the fluid eddies when the eddy length-to-carrier diameter ratio was less than or equal to unity. The results showed quantitatively how the shear rate in various zones of airlift reactors may be manipulated by modificatio ns to operational and geometric parameters. The methodology presented allowed for characterization of shear rates in the bulk flow, unlike e xisting studies that provide information only on wall shear rates whic h are not particularly relevant to shear sensitive bioprocesses. (C) 1 997 Elsevier Science B.V.