Effects of Geometric Parameters in Viscous Fermentation Fluids

Steel and Maxon (1966) have attemped to solve the problem of supplying adequate oxygen through studies of impeller design, and found the advantages of the multiple-rod impeller shown in Fig. 5. It gave the same novobiocin yield and oxygen availability rate at about one-half of the power required by turbine impeller. 

Blakebrough and Sambamurthy (1966) carried out oxygen-transfer studies in gas-liquid and gas-liquid-solid systems, and found signiflcant differences in the operation of turbine impellers in the two systems. In the former system, at a given aeration rate, a limitation in the operation of the impellers was reached at a power input of 0.36 HP/m. Both below and above this level, the efficiency of oxygen transfer was independent of the manner in which power was transmitted, namely, irrespective of impeller speed, impeller diameter, blade dimensions, and 

The available literature contains very few references to studies of the power consumption for agitation and the volumetric oxygen transfer coefficients of multi-stage impeller fermenters. Takeda and Hoshino (1968) showed experimentally that closely spaced impellers caused serious interference between the flow streams from adjacent impellers and an overall reduction in power consumption. On the other hand, Oldshue (1966) has indicated that within fairly large ranges of geometric variables, a similar oxygen transfer coefficient is obtained if the power per unit volume is maintained at similar values under a constant aeration rate. 

Richards (1963) has reported qualitatively the effect of varying impeller spacing on flow patterns and power consumption in a fermenter as follows. 

In fully baffled tanks, the types of turbine impeller result normally in a flow of liquid from the central axis of the tank to the walls, then vertically up or down, being finally drawn back again into the eye of the impeller. If the impellers are too widely spaced, it becomes possible, especially in such liquids as viscous fermentation broths, to have regions unagitated by the action of the impellers. On the other hand if they are too close together, serious interference can occur between the How streams from two adjacent impellers. This can have the dual effect of reducing power input and inadequate mixing. 

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