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Finding The Right Agitator Design for PVC Production

Q: We have a 20 m3 reactor for PVC production and H/D ratio of 6.0m/2.26m. The agitator consists of a 40kw motor, stirrer of 3 layer, 2 blades impeller of 1200mm x 200mm, and a paddle type at 120 rpm plus a single-blade baffle. The issue we are facing is a quality problem with the resins produced. Despite many trials in recipe changes, as it is lower in porosity and bulk density and consists of many odd shaped particles instead of round particles, some looked like tadpoles. Do you think we have the right agitator design and what can be done to improve it?

A:

I don't think that this problem has a simple "mixing" answer.  Without more details about impeller type and location, I can only generalize about the problem and possible solutions.  The mixer size and general description sounds appropriate for PVC.  However, PVC reactions are typically carried out with overhead condensation of monomer for heat removal.  The return and rapid mixing of the condensed monomer is critical for good polymer bead formation.  The high porosity and odd shaped particles sound as if the monomer is remaining on the liquid surface and not being quickly incorporated back into the batch.
 
The usual method for testing mixer performance is with a water batch and either adjusting the liquid level for the baffle effect to create a strong, but not too deep vortex on the surface.  Vortex characteristics are difficult to predict and must be tested in the specific reactor.  This problem is less about the reactor design and more about the operation of the reactor.  Formulation can only do so much to overcome mixing problems.

Here are more of the latest questions on: Mixing

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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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