Nwu

Dissolved oxygen concentration

Fig. 9.9. The occurrence of oxygen limitation during the dynamic gassing out of a fermentation.

Tuffile and Pinho (1970) compared a number of methods for the determination of Kta values in viscous streptomycete fermentations. The techniques used were static gassing-out, dynamic gassing-out and the oxygen-balance method. Tuffile and Pinho did not make it clear whether non-respiring mycelium was present during their static gassing-out procedure, but from their results it would appear that it was present in the vessel. Thus, the rheology of the fermenter contents would appear to have been similar for the different determinations. The KLa values, determined by the different techniques, for a 300-dm3 fermenter containing a 90-hour culture of Streptomyces aureofaciens are shown in Table 9.4.

From Table 9.4 it may be seen that the KLa values for the two gassing-out techniques were very similar but there was a considerable difference between the oxygen-uptake rates and the KLas determined by the dynamic method and the balance method. Tuffile and Pinho (1970) claimed that the low oxygen-uptake rate determined by the dynamic method was due to air bubbles remaining in suspension in the mash during the dynamic gassing-out period. Thus, the decline in oxygen concentration after the cessation of aeration was not a measure of the oxygen-uptake rate but the difference between oxygen uptake and the transfer of oxygen from entrapped bubbles. It was demonstrated that a large number of bubbles remained suspended in the medium 15 minutes after aeration had been stopped. The use of the low oxygen-uptake rate in the calculation of the KLa would result in an artificially low KLa being determined. Heijnen el al. (1980) also observed anomalies in determining KLa values in viscous systems due to the presence of very small bubbles having a much longer residence time than the more abundant large bubbles in the vessel.

Overall, it would appear that the balance method is the most desirable technique to use and the extra cost of the monitoring equipment involved should be a worthwhile investment.

Before considering the factors which may affect the

KLa of a fermenter it is necessary to consider the behaviour of fluids in agitated systems.

FLUID RHEOLO<;y

Fluids may be described as Newtonian or non-Newtonian depending on whether their rheology (flow) characteristics obey Newton's law of viscous flow. Consider a fluid contained between two parallel plates area A and distance x apart. If the lower plate is moved in one direction at a constant velocity, the fluid adjacent to the moving plate will move in the same direction and impart some of its momentum to the 'layer' of liquid directly above it causing it, also, to move in the same direction at a slightly lower velocity. Newton's law of viscous flow states that the viscous force, F, opposing motion at the interface between the two liquid layers, flowing with a velocity gradient of dv/dx, is given by the equation:

where fjt, is the fluid viscosity, which may be considered as the resistance of the fluid to flow.

Equation (9.7) may be written as:

F/A is termed the shear stress (r) and is the applied force per unit area, dv/dx is termed the shear rate (y) and is the velocity gradient. Thus:

Equation (9.8) conforms to the general relationship:

where K is the consistency coefficient and n is the flow behaviour index or power law index.

For a Newtonian fluid n is 1 and the consistency coefficient is the viscosity which is the ratio of shear stress to shear rate. Thus, a plot of shear stress against

Table 9.4. K ¡a values for a 300-dm? fermenter containing a 90-h culture of S. aureofaciens (Tuffile and Pinho, 1970)

Method of KLa

Measured oxygen

K,a

determination

uptake rate

(h-1)

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