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Redesigning the impeller blade for a batch polyurethane tank

Q: I need to redesign the impeller blade of my batch polyurethane tank. I'm currently using a marine-type impeller, which incorporates a lot of air into the polymer polyol while mixing, causing a lot of bubbles. I need an impeller that mixes well but does not cause turbulence or a lot of bubbles. Please suggest the best option. Polyol has a viscosity of around 420-520 cSt. I'm mixing approximately 50 to 130 kilograms per batch.

A:

You have not given me enough information to really answer your question. The batch size and  viscosity only tell half of the story. The impeller size and rotational  speed are essential to understand the mixing intensity.

However, if you are pulling air into your polymer with a propeller, you are probably doing something else wrong. At those viscosities, you cannot use a center-mounted mixer without baffles. If you don't have baffles in the tank, then the mixer must be mounted at an angle and aimed so a vortex is not formed. If you have a vortex and drawing air in with a propeller, you are going to have the same problem with other impeller types, unless you just have too much mixing. A vortex on the  surface usually means that you have poor mixing without much top-to-bottom motion. Any air incorporation further hurts the mixing.

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