Mechanical aeration scaling

Constant Reynolds number is not used for fermentation scale-up it is only one factor in the aeration task. This is also true for considering the impeller as a pump and attempting scale-up by constant momentum. As mechanical mixing tends to predominate over bubble effects in improving aeration, scale-up equations including bubble effec ts have had httle use. 

Foaming is often a problem in large-scale aerated systems. Antifoam cannot always be added for the reduction of foam because it may inhibit the growth of the microorganisms. However, there are several mechanical methods by which the foam can be broken up. 

Scale-up to the industrial scale is only achievable and economical reasonable in stirred or airlift systems. To achieve high cell densities optimal nutrient supply is necessary. Therefore transport limitations have to be avoided by good mixing of cells (microcarrier) and medium. This movement caused by stirrer or aeration leads to mechanical forces evoking severe cell damage or cell death [3,4]. 

This process, according to the manufacturer,54 has been developed in such a way that space requirements are kept to a minimum. A BIOPAQ IC reactor is used as the initial step in the treatment process. The name of this anaerobic reactor is derived from the gas-lift driven internal circulation that is generated within a tall, cylindrical vessel. These reactors have been operational in the paper industry since 1996. The second step in the purification process is a mechanically mixed and aerated tank. The aerating injectors can be cleaned in a simple way without the need to empty the aeration tank. Potential scaling materials are combined into removable fine particles. At the same time, the materials that may cause an odor nuisance are oxidized into odorless components. The process can be completed by a third and a fourth step. The third step focuses on suspended solids recovery and removal. The fourth step is an additional water-softening step with lamella separation and continuous sand filters in order to produce fresh water substitute. The benefits claimed by the manufacturer are as follows54 ... 

For scale-up of inoculum conditions of hairy root cultivation, a 1-L bioreactor (working volume of 800 mL) was used. This bioreactor had a height/diameter aspect ratio of 7.14. The bubble bioreactors had no internal mechanical agitation parts. The supplied aeration rate was 0.1 wm at the bottom by sparger. Each bioreactor was inoculated with 0.2-2.0 % (w/v) g fresh weight of hairy roots and cultured for 32 d. 

Since in animal cell culture processes the effects of mechanical stress are much more relevant than in microbial fermentations (Chisti, 1993), it is quite common to adopt scale-up criteria that are associated with cell damage (Joshi et al., 1996), such as constant peripheral impeller velocity, constant aeration rate, and constant integrated shear stress (Croughan et al., 1987). 

It has already been pointed out that experiments in differently scaled models are inevitable if physical properties cannot be changed and therefore one has to use the same material system (Example 8 Mechanical foam breaker, section 6.1). The same is true if the acceleration due to gravity is influencing the process under investigation. However, as we all know, gravity cannot be varied on Earth (Example Surface aeration, section 10.3.2). 

Mixing by a laboratory shaker is adequate to cultivate microorganisms in flasks or test tubes. Rotary or reciprocating action of a shaker is effective to provide gentle mixing and surface aeration. For bench-, pilot-, and production-scale fermenters, the mixing is usually provided by mechanical agitation. 

Mechanically stirred vessels Mixing is promoted by mechanical stirring (occasionally magnetic in bench-scale vessels) using different impeller types. In addition, aeration is also employed to augment mixing. In this most commonly used type of bioreactor, the tank content is perfectly mixed and hence uniform in composition. 

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