Average shear rate for non-Newtonian fluids in a concentric-tube airlift bioreactor

In the present study, a simple methodology for evaluating the average shear rate ((gamma) ove dot(av)) in an internal concentric-tube airlift reactor has been developed for non-Newtonian systems. The volumetric oxygen transfer coefficient (k(L)a) was chosen as the appropriate characteristic parameter to evaluate the average shear rate ((gamma) ove dot(av)) because the oxygen transfer occurs through the interfacial area of the air bubbles distributed evenly throughout the bioreactor. The average shear rate ((gamma) ove dot(av)) values of up to 9300 s(-1) were estimated as a function of the gas velocity in the riser (0.0094 <= U-GR <= 0.0943 m s(-1)), of the rheological parameters of the fluid (0.180 <= K <= 1.833 Pa s(n) and 0.234 <= n <= 0.461), and of the volumetric oxygen transfer coefficient (0.0056 <= k(L)a <= 0.0609 s(-1)). Profiles of the (gamma) ove dot(av) as a function of the gas velocity in the riser (U-GR) displayed different behaviors from those of the literature since (gamma) ove dot(av) increased up to a maximum value and then decreased with an increase in the UGR. The shear rate estimated by the proposed methodology lay within the range of the correlation values in the literature. Unlike other correlations in the literature, the average shear rate in the present study (gamma(av)) is estimated considering the rheological parameters of the fluid, the volumetric oxygen transfer coefficient (k(L)a), and the mass transfer parameter, which is a function of the operating conditions and the geometry of the bioreactor. (C) 2007 Elsevier B.V. All rights reserved.