The Colorful Fluid Mixing Gallery

Large scale fermenters are used to make such products as yeast, vitamin C, xantham-gum, citric acid, penicillin, and other products. Fermentations are usually carried out in tall vessels with multiple impeller systems. Air is sparged in at the bottom, to provide the micro-organisms with oxygen. It is important that the mixer disperse the gas into fine bubbles, to ensure good mass transfer from the air to the broth. The example here shows a drawing of a fermenter, the velocity magnitude around the cooling coils, and finally the results of a gas dispersion simulation, including mass transfer.

This fermenter is equipped with a radial flow CD-6 impeller with concave blades at the bottom, and three down pumping HE-3 impellers on top. The vessel has no baffles, but is equipped with twelve sets of eight cooling coils, that also act as swirl reducing baffles.

This image shows the velocity magnitude in four cross sections. From top left to bottom right, at 0.8, 0.6, 0.4, and 0.2 fraction of the liquid level off the bottom. Simulations like this can be used to design the impeller system such that there is sufficient liquid movement around the cooling coils.

The simulations shown here were performed with proprietary Ghost! software from Chemineer, Inc. The picture on the left shows the local gas volume fraction, the picture in the center shows the local mass transfer coefficient kla, and the picture on the right shows the local bubble size. The bubble size is smallest near the impellers (blue) and increases away from the impellers due to coalescence. The mass transfer coefficient is highest near the impellers, because this is where the bubble size is small (leading to a large interfacial area), and where the turbulence intensity is high (leading to fast surface renewal around the bubbles).

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Last Updated March 14, 1998 by André Bakker
© André Bakker 1998