Читать книгу Agitator Design for Gas-Liquid Fermenters and Bioreactors - Gregory T. Benz - Страница 74

The most common method of defining and determining flooding is the visual method. It is the method upon which all flooding equations in this book are based. Referring to Figure 4.3, flooding emergence occurs when the bubble flow and dispersion pattern changes from condition (a) to condition (b). This is easily observed in a transparent vessel and is a very repeatable condition. In condition (b), the bubbles are driven to the vessel wall, more or less in the plane of the impeller. This a somewhat conservative definition of flooding, as mass transfer enhancement actually begins before the bubbles are fully driven to the wall. It actually begins when the bubbles are driven even a short distance off a vertical path, as long as the impeller Froude number is at least 0.03 for Rushton turbines (the minimum Froude number may be somewhat lower for concave and deeply concave impellers, but the author is unaware of studies conducted below a Froude number of 0.03.)

Condition (c) in Figure 4.3 is called complete dispersion. It occurs when gas is driven to the bottom of the tank and some of it is recirculated into the impeller. It is not a design condition per se, but usually occurs when agitation is sufficient to support medium and high mass transfer rates (for example, above 80 mmol/l‐h for an air‐based fermentation at a back pressure of 1 bar‐g or less.)

The transitions from condition (a) through condition (c) can be thought of as either increasing shaft speed at constant airflow or decreasing airflow at constant shaft speed.

Figure 4.3 Visual flooding.

The above definitions work well for radial flow impellers, which are most commonly used as the primary disperser. Some systems use only axial flow impellers. There is no standard definition of flooding for these. The k_la method could be used, whether the impellers are up or down pumping. It would be difficult to use the visual method for an up‐pumping system. For down pumping, there is a transition from direct loading (the gas goes through the impeller on the way up) to indirect loading (the gas is driven out of the impeller’s path by its discharge velocity, and some gas is recirculated into the impeller from above). We do not have a figure to illustrate this. But this transition could be a sort of flooding definition.