Читать книгу Agitator Design for Gas-Liquid Fermenters and Bioreactors - Gregory T. Benz - Страница 72

By k_la, we mean the overall mass transfer coefficient, as defined by:

(4.1)

Figure 4.1 Flooding determination by using k_la.

Imagine an agitator operating at zero rpm, with gas being bubbled below the impeller. Although the impeller is not dispersing gas, there will still be some mass transfer possible, and therefore, the value of k_la will be non‐zero. As the shaft speed increases, the k_la value will remain essentially constant, until a certain threshold speed is reached, at which point the k_la value will start increasing rapidly, as shown in Figure 4.1. We would call that threshold speed the flooding emergence point, by one definition. By inspection of Figure 4.1, the flooding emergence point is about 58 rpm.

One could also use mass transfer rate as the y‐axis if the driving force remained constant.

While this definition of flooding has the advantage of clearly defining the point at which the agitator actually begins to influence mass transfer and thus relates to the main purpose for the use of the agitator, it is experimentally difficult to carry out. Most published definitions and correlations of flooding do not use this method, as we will see. This method does, however, have the advantage of being possible to carry out in an opaque vessel.