ON THE ESTIMATION OF EFFECTIVE SHEAR RATE IN EXTERNAL LOOP AIRLIFT REACTORS - NON-NEWTONIAN FLUIDS

The determination of shear rate in column bioreactors is an important step to estimate the cell damage rate in shear-sensitive biosystems as well as for correlating hydrodynamic and mass transfer parameters. Du e to the complexity of local shear rate measurement, it has been widel y assumed that an average shear rate exists and is proportional to sup erficial gas velocity. The most serious shortcoming in the analysis of non-Newtonian behaviour in airlift reactors is the lack of a reliable method for determining a shear rate and an effective viscosity approp riate to the airlift geometry. Recently Shi et al. (Chem. Eng. Commun. 1990; 89: 22-35) presented an empirical correlation of effective shea r rate as a function of superficial gas velocity and based on downcome r liquid circulation velocity. The external loop airlift reactor used by Shi et al. was 1.4 m in height, 0.194 m in riser diameter and 0.06 m in downcomer diameter. To validate Shi's et al. promising approach, this work has been carried our in a larger reactor of 6.5 m in height and 0.225 m in both riser and downcomer diameter. However, the rheolog ical properties of the studied liquids and the superficial gas velocit ies were similar to those used by Shi et al. Copyright (C) 1996 Elsevi er Science B.V.