Impellers baffled

As described in U.S. Patent 2,916,485 12 liters of a nutrient medium having the following composition is placed in a 30 liter fermentor equipped with stainless steel fittings including sparger, impeller, baffles and sampling lines and the medium is sterilized by heating at 121°C for two hours. 

The effect on the process of a change in operation of the mixer system (impeller, baffles, etc.) is the final measurement of performance. Thus, operations such as blending, uniform particle suspension, reaction, gas absorption, etc., may be acceptable under one physical system and not so to the same degree under a slightly modified one. The ratio per unit volume on scale-up must be determined experimentally. 

Circulation and shear of the liquid in a vessel can be accomplished with external pumps and appropriate location of suction and discharge nozzles, but a satisfactory combination of vertical and lateral flows is obtained more economically by internal impellers, baffles, and draft tubes. Some general statements about dimensions, proportions, and internals of a liquid mixing vessel can be made. 

Figure 10.1. Agitated vessel standard geometry showing impeller, baffles, and heat transfer surfaces.

In a tank with radial impellers, suitable baffles will produce strong top-to-bottom currents from the radial discharge. The installation of baffles generally increase the power consumption [65]. For axial flow impellers, the need for baffling is not as great as for radial flow impellers, thus axial flow impellers also consume less power than radial impellers. Baffles are normally used in turbulent mixing only. 

To prevent gross vortexing behaviour when low-viscosity liquids are agitated in a vertical cylindrical tank with a centrally mounted impeller, baffles are often fitted to the walls of the vessel. Typically four baffles will be used, each having a width equal to about one-tenth of the tank diameter. In some cases the baffles are mounted Hush with the wall, although occasionally a small clearance is left between the baffle and the wall. 

The Pb-free alloy compositions are more prone to erode the wave machine s structures, such as the impeller, baffles, and pot walls. New wave-soldering machines address this problem through the use of alternative steel alloys and ceramic coatings. 

The sliding mesh model is the most rigorous and informative solution method for stirred tank simulations. Transient simulations using this model can capture low-frequency (well below the blade passing frequency) oscillations in the flow field (Bakker et ak, 2000 Roussinova et al., 2000) in addition to those that result from the periodic impeller-baffle interaction. 

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