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I’m trying to convince my boss that the mixers we are using are too much. I’m trying to prove that we are over-mixing our product. Our product is ink…just your basic ink found in your printer at home. We mix in a 23 inch diameter 50 gallon vessel using a 2.75 inch diameter axial flow impeller going at 1050 rpm with a 3 inch rotor stator 5 inches below it. The inks have a viscosity of around 3. What Reynolds number should be sufficient for this process or can you give me any further input? Any help would be GREATLY appreciated. Thanks.

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I believe someone solving the problem would need to know in what units the viscosity is 3 and the density of the ink. –  NowIGetToLearnWhatAHeadIs Apr 30 '13 at 13:42
    
The Reynolds number is not the only important parameter here. Time is definitely also one. Even at high Reynolds numbers if you mix for only a nanosecond your product will not be mixed. Moreover, mixing times are very dependent on the type of mixer you are using, just saying that it is an axial flow impeller is not nearly enough information to work out the necessary mixing time. –  Michiel Apr 30 '13 at 15:41
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At least in the industry I work in, sizing mixers is more an art than a science. If you are convinced the process works at too high a Reynolds Number, try if you can make an experiment:

  • find out why it is done the way it is done and how the mixer was sized
  • when you still think the Re is too high, convince your boss to make an experiment with one batch
  • mix a batch or two with lower speed (rpm) and see if the ink still has the required quality, maybe also try different mixing times
  • If it does not work, you should be able to achieve the required quality by continuing mixing, so no material is lost in your experiment

In general, you want to achieve two things with mixing: A global flow in the container, to homogenize the mixture, and high shear forces (associated with high turbulence) to disperse small droplets. If the ink is destined for a printer, I assume that small pieces of pigments and the like may pose a huge problem - so the process may designed like this to guarantee a minimum size of particle.

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