Design of a mixing tank

[cal6404]

Hi,

On our site we have an existing mixing tank. At the moment, the impeller is too aggressive and breaks downn the suspended solids into a size that is too fine for downstream processing. The mechanism for reduction is most affected by macromixing (overal mixing in the tank and not shear stress due to impeller blades hitting particles).

Does anybody know how to calculate the minimum speed of an impeller in a mixing tank that will give just enough mixing required? Laminar mixing would be great. I have the specs of the tank and impeller, I'm just not quite sure how to apply them to any usefull equation.

Cheers!

 

[Latexman]

Was the existing agitator purchased from an agitator company? If so, and you can characterize how laminar the mixing needs to be, they can help. If the agitator is "homemade", then you will have to figure it out. There are many references that will help. Oldshue's book is a good one. Contact one of the big agitator companies and ask for a free reference that will introduce you to mixing so you can learn how to ask them how to help. Lightnin has a little book available; written by Oldshue and others. Lightnin gave me one. Chemineer has two reprints from Chemical Engineering magazine they may give you. That's how I got mine, from Chemineer.

How about installing a VFD and running at a lower speed? How about mixing for less time? How about trimming the impellers? There are many ways to get to a solution.

The hard part is characterizing how laminar the mixing needs to be. Do you have access to a lab or lab scale equipment?

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[bimr]

The best advice is to contact the supplier of the mixing equipment.

http://www.eng-tips.com/

 

[StoneCold]

The cheapest thing to do is to put the agitator on a VFD, Turn it down until you get the result you want.
If you are piloting this operation and you want to scale it up later, then you probably need to characterize the fluid velocity in the tank.

Regards
StoneCold

 

[rkg1]

Are the particles actually single crystals or agglomerates? Breaking up crystals takes a lot more energy than breaking up agglomerates.

What do you mean by laminar? Do you mean gentle?

What type of impeller is installed in the vessel?

What is the brake horsepower (not the motor rated power)?

What is the volume of the slurry?

There are calculations that can be done to assess the performance of this agitator and answering these questions would be the starting point. If you do not have time to do a detailed analysis of the problem, I agree that installing a VFD and tuning the agitator speed would be the best option.

Finally, the Chemineer articles and Oldshue's book were published in the late 70's - early 80's and we have come a long way in our understanding of mixing processes and equipment design since then. The Handbook of Industrial Mixing was published in 2004 and has the best (although not perfect in my opinion) collection of chapters on a variety of mixing operations.