Influence of impeller type on mass transfer in fermentation vessels

Abstract

Radial¯ow Rushton impellers were compared qualitatively with axial¯ow hydrofoil impellers (Max¯o T and A315) at the pilot scale. Six types of impellers were compared for qualitative differences in mass transfer. Measurements were conducted using three model systems: water, glycerol and Melojel (soluble starch). Power measurements were obtained using watt transducers, which although limited in accuracy and prone to interferences, were able to provide useful qualitative monitoring results. While there was little effect of impeller type on mass transfer as measured by the rapid pressure increase technique, signi®cant qualitative differences were observed using the rapid temperature increase technique speci®cally for the Melojel and glycerol model systems. The Miller correlation, relating gassed-to-ungassed power, was used effectively to qualitatively evaluate the power drop upon gassing for both the model systems and a Streptomyces fermentation for the various impeller types.A high oxygen demand Streptomcyes fermentation then was conducted in fermenters possessing each type of impeller. Performance was not adequate with the A315 impellers pumping upwards and the small diameter Max¯o T impellers. Peak titers and pro®les of the estimated apparent broth viscosity varied depending upon the impeller type. Mass transfer rates generally declined with higher viscosities when other fermentation operating conditions where held constant. Overall, values for OUR, k L a, P g /V L and other calculated mass transfer and power input quantities for the A315 pumping upwards and undersized Max¯o T (D T /D I 2.3) impellers were at the lower end of the range obtained for the larger Max¯o T (D T /D I 1.8± 2.0) and A315 impellers pumping downwards. Rushton impellers generally behaved qualitatively similar to hydrofoil impellers based on these calculated quantities. List of symbolsa surface area per unit volume of bubbles, cm A1 g gravitational constant, m/s 2 k¯uid consistency index k L mass transfer coef®cient n power law index t m mixing time, s v b bulk velocity, m/s v s super®cial velocity of sparged air based on tank diameter, m/s A cross-sectional area of the tank, m 2 D I impeller diameter (tip to tip), m D T fermenter vessel diameter, m D oi diffusivity of oxygen in water (w), glycerol (g) andMelojel (m), cm 2 /s N impeller speed, s A1 N A aeration or¯ow number, Q/ND 3 I N Fr Froude number, N 2 /D I g N m Mixing number, 1/t m N N P Newton or power number, P 0 /(qN 3 D 5 I ) N Q generalized pumping number, Q P /ND 3 I N Re Reynold's number for impeller, ND 2 I q/l N Sc Schmidt number, l/qD oi P 0 ungassed power draw, kW P g gassed power draw, kW Q volumetric gas¯owrate, m 3 /s Q P volumetric pumping rate, v b A, m 3 /s V L ungassed liquid volume of tank, m 3 q liquid density, g/cm 3 l viscosity, mPa á s l a apparent viscosity, mPa á s c T average shear rate in stirred tank, s A1 r surface tension, dynes/cm 1 Introduction For aerobic fermentations, oxygen management is about 15±20% of all operating costs. Consequently, a modest impr...

Oxygen transfer and mixing in mechanically agitated airlift bioreactors

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Glutaminase Production using Zygosaccharomyces rouxii NRRL‐Y 2547: Effect of Aeration, Agitation Regimes and Feeding Strategies

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Hydrodynamic and Mass Transfer Study in a Mechanically Stirred Hybrid Airlift Bioreactor Based on Impeller Type

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