Mechanically Agitated Reactors

The effect of air flow rate on KLa values in conventional agitated systems is illustrated in Fig. 9.12. The quantitative relationships between aeration and KLa for agitated vessels are considered in the subsequent

Volumetric air flow rate (volume of air volume"1 medium min~1)

Fig. 9.12. The effect of air-flow rate on the KLa of an agitated, aerated vessel.

Volumetric air flow rate (volume of air volume"1 medium min~1)

Fig. 9.12. The effect of air-flow rate on the KLa of an agitated, aerated vessel.

section on power consumption. The air-flow rate employed rarely falls outside the range of 0.5-1.5 volumes of air per volume of medium per minute and this tends to be maintained constant on scale-up. If the impeller is unable to disperse the incoming air then extremely low oxygen transfer rates may be achieved due to the impeller becoming 'flooded'. Flooding is the phenomenon where the air-flow dominates the flow pattern and is due to an inappropriate combination of air flow rate and speed of agitation (see also Chapter 7). Nienow et al. (1977) categorized the different flow patterns produced by a disc turbine that occur under a range of aeration and agitation conditions (Fig. 9.13) and these have been discussed further by Van't Riet and Tramper (1991). Figure 9.13 A shows the flow profile of a non-aerated vessel and Figs 9.13 B to F the profiles with increasing air flow rate. As air-flow rate increases the flow profile changes from one dominated by agitation (Fig. 9.13 B) to one dominated by air flow (Figs

9.13 D to F) until finally the air flow rate is such that the air escapes without being distributed by the agitator (Fig. 9.13 F). Different workers have used different criteria to define the onset of flooding with Nienow et al. (1977) claiming it to be represented by Fig. 9.13 D whereas Biesecker (1972) suggested Fig. 9.13 F. However, the desired pattern is represented by Fig. 9.13 C.

Several workers have produced empirical quantitative descriptions of flooding systems which may assist in avoiding the phenomenon:

(i) Westerterp et al. (1963) calculated that the minimum impeller tip speed to avoid flooding should be between 1.5 and 2.5 m second

(ii) Biesecker (1972) claimed that flooding occurs when the energy dissipated by the air flow is greater than that dissipated by the agitator. Van't Riet and Tramper (1991) modified this approach to consider the balance between the two energy dissipating systems in the lower compartment of the vessel because the energy dissipated by the agitator in the upper compartment of a large vessel is not related to gas dispersion.

(iii) Feijen et al. (1987) claimed that flooding could be avoided if:

where Fs is the volumetric air flow rate at the pressure conditions of the lower stirrer (m3 second N is the stirrer speed (second 1), D is the stirrer diameter (m), g is the gravitational acceleration (m second ~ 2).

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