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Efficient Impeller Blade or Mixer

Q: We make a solution of water & benzoyl peroxide (in various percentages - 1%, 4%, 8%, etc...) in a mix tank & transfer it into several poly lined 55 gallon drums to store the solution until we are ready to fill from each drum - one at a time - until empty - into finished product containers. We have to stir each drum for approx. 2 hrs prior to beginning filling to get a homogenous solution in the drum - to get the BP back in suspension. Do you have any suggestions as to a more efficient, specific type of impeller blade or mixer to keep the BP in suspension - so we can keep the drum of solution as homogenous as possible in each drum? We currently have air mixers with a stirrer type 3 blades at the bottom of the shaft. Drum dimensions: int.hgt.= 37 1/16", int.diam. = 21.8 with 2 " Bung/opening with removable lid. Must be kept covered.

A:

You did not tell me much about the 3-blade impeller you are using, but almost any properly sized marine propeller or hydrofoil impeller should give you good solids suspension.  A 1/4 hp mixer running at 1750 rpm should have a 3.5" or 4" diameter hydrofoil impeller for best results.  You should be able to feed the mixer shaft from inside the bung hole and into the mixer drive before replacing the drum lid.  Trying to fit anything other than a collapsible impeller through a 2" opening is not likely to give you much mixing.  I would suggest a 1/4 hp electric motor instead of the air motor, unless you are in a Div. 1 hazard situation.  The electric motor will be quieter and more efficient.

Unless your benzoyl peroxide particles tend to stick together or dissolve slowly, a good mixer should take no more than five (5) minutes to uniformly suspend solids in a 55 gallon drum.  Actually, suspension should occur in less than 30 seconds, but just for good measure try five (5) minutes.

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We would like to have a general idea of pumping number for our reactors. Can you help us with blend-time calculations?
To characterize one of the reactors at plant scale, we use the discoloration method. The parameters for the mixing time are calculated based according to the following formula:

t_mix= K/(aN(D/T)^b (T/Z)^0.5 )
N = impeller speed
D = diameter stirrer
T = diameter tank
Z = liquid height
The divisor is known as Kmix

Because we don't really know the uniformity (U) reached with this method we don't replace K with

K = -ln (1-U)

But get the best fit for K, a and b by means of the least square method. It is known that the pumping number can be determined by

N_Q= (Vk_mix)/(ND^3 )

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N_Q= (VaN(D/T)^b)/(ND^3 )

Working in a turbulent regime, this result in a constant pumping number related to the tank geometry. Would the above approach be correct to compare pumping and mixing time capabilities of different reactor set ups? I think that not working at a fixed uniformity results in a gap in the above approach.

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