Prior investigations comparing radial¯ow Rushton impellers with axial¯ow hydrofoil impellers (Max¯o T and A315) were extended at the pilot scale. Six types of impellers (disk-style Rushton, Prochem Max¯o T hydrofoils of three diameters pumping downwards and A315 hydrofoils pumping upwards and downwards) were compared for qualitative differences in power number behavior with Reynolds' number, single versus double impeller power draw, gassed power reduction with aeration number and gas hold-up. Power measurements were obtained using watt transducers which, although limited in accuracy and prone to interferences, were able to provide useful qualitative monitoring results. Measurements were conducted using three model liquid systems: water, glycerol and Melojel (soluble starch). Apparent viscosities for actual Streptomyces cultivations were estimated using measured gassed power values and the experimental relationships obtained for gassed/ungassed power to aeration number and power number to Reynolds' number for the glycerol model system. Results con®rmed the lower power number and lower shear environment for hydrofoil impellers, yet suggested useful trends for various process parameters and process¯uids.List of symbols c constant relating average tank shear rate to impeller speed g gravitational acceleration, 9X807 m/s 2 g c gravitational acceleration conversion factor, 1 kg Á m/s 2 Á N k¯uid consistency index n power law index n I number of impellers p T pressure at tank top, mPa v S super®cial velocity of sparged air based on tank diameter, m/s D I impeller diameter (tip to tip), m D T fermenter vessel diameter, m H tank hold up based on dispersion volume H L height of liquid in tank excluding bottom dish, m H T total height of tank, m IS impeller shear, s À1 ITS impeller tip speed, pD I N, m/s K constant relating N A to P g aP 0 N impeller speed, s À1 N A aeration or¯ow number, QaND 3 I N Fr Froude number, N 2 D I ag N P Newton or power number, P 0 aqN 3 D 5 I N Re Reynolds' number for impeller, ND 2 I qal N We Weber number for impeller P exp gas expansion power, m 3 as P 0 ungassed power draw, kW P g gassed power draw, kW P L power loss, kW Q volumetric gas¯owrate, m 3 as S impeller spacing, m V L ungassed liquid volume of tank, m 3 V T total volume of tank, m 3 W width of tank baf¯es, cm q liquid density, g/cm 3 q g gas 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 À1 c V average shear rate in viscometer, s À1 r surface tension, dynes/cm 1 Introduction Substantial research has been conducted at Merck Research Laboratories comparing radial¯ow Rushton impellers with axial¯ow Prochem Max¯o T and Lightnin A315 impellers [4, 10, 15±17]. The main characteristics of these impellers are summarized in Table 1. It has been well-demonstrated that Rushton turbines lose up to 70% of their power draw when aerated due to the formation of air ®lled ventilated cavities behind the blades [17]. In high viscosity broths, these air cavities become stabilized [18] alth...

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...