Aeration And Agitation

Aeration and agitation will be considered in detail in Chapter 9. In this chapter it should be stated that the primary purpose of aeration is to provide microorganisms in submerged culture with sufficient oxygen for metabolic requirements, while agitation should ensure that a uniform suspension of microbial cells is achieved in a homogeneous nutrient medium. The type of aeration-agitation system used in a particular fermenter depends on the characteristics of the fermentation process under consideration. Although fine bubble aerators without mechanical agitation have the advantage of lower equipment and power costs, agitation may be dispensed with only when aeration provides sufficient agitation, i.e. in processes where broths of low viscosity and low total solids are used (Arnold and Steel, 1958). Thus, mechanical agitation is usually required in fungal and actinomycete fermentations. Non-agitated fermentations are normally carried out in vessels of a height/diameter ratio of 5:1. In such vessels aeration is sufficient to produce high turbulence, but a tall column of liquid does require greater energy input in the production of the compressed air (Muller and Kieslich, 1966; Solomons, 1980).

The structural components of the fermenter involved in aeration and agitation are:

(a) The agitator (impeller).

(b) Stirrer glands and bearings.

(d) The aeration system (sparger).

The agitator (impeller)

The agitator is required to achieve a number of mixing objectives, e.g. bulk fluid and gas-phase mixing, air dispersion, oxygen transfer, heat transfer, suspension of solid particles and maintaining a uniform environment throughout the vessel contents. It should be possible to design a fermenter to achieve these conditions; this will require knowledge of the most appropriate agitator, air sparger, baffles, the best positions for nutrient feeds, acid or alkali for pH control and anti-foam addition. There will also be a need to specify agitator size and number, speed and power imput (see also Chapter 9).

Agitators may be classified as disc turbines, vaned discs, open turbines of variable pitch and propellers, and are illustrated in Fig. 7.9. The disc turbine consists of a disc with a series of rectangular vanes set in a vertical plane around the circumference and the vaned disc has a series of rectangular vanes attached vertically to the underside. Air from the sparger hits the underside of the disc and is displaced towards the vanes where the air bubbles are broken up into smaller bubbles. The vanes of a variable pitch open turbine and

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