Design of a mixing tank
Design of a mixing tank
(OP)
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!
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!





RE: Design of a mixing tank
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.
RE: Design of a mixing tank
RE: Design of a mixing tank
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
RE: Design of a mixing tank
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.