Mixing problems

[Roberttt]

Hi,

So I am optimising a mixing tank to mix a few compounds with each other. We only got some problems during the mixing;

For some reason the propeller only mixes the centre of the tank and not the sides of the tank as well as at the base of the tank what leads to a heterogeneous mixture. A heterogeneous mixture also leads to variable viscosity readings. The viscosity is between 5000 to 11500 cps. We are also using baffles in the mixing tank. For mixing we are using two pitched blade impellers on the same axes.

I checked all the sizes from the tank & the propeller and they seem to look good.

Does someone has any ideas how to fix this problem?

Thanks

 

[ashtree]

How big is the tank and what are the dimemsions?
How big is the mixer by comparison and is it turning fast enough?
What data do have on the mixer for example: motor hp, rpm, k value for the impeller?

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Roberttt]

Here some information:

Htank = 2.5m
Dtank = 1.8m
Vtank = 5.828 m3
Himpeller from bottom = 0.5m
Hsecond impeller from bottom = 1.5m
Dimpeller = 0.6m
D/T = 0.34 (looks correct to me!)

I do not have more data yet about the mixer. Will do some research tomorrow.

I did also some other research about maybe changing the impeller to a Rushton turbine or a Counter-flow impeller. They seem to give more turbulence in the vessel and might mixes the compounds better.

 

[LittleInch]

I think the answer lies in the 5000 to 11500 CP of your mixture. That is seriously stiff fluid so mechanical mixing is the only thing I can think of that would work.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Roberttt]

May I ask you what you mean with mechanical mixing?

 

[Latexman]

RPM of impellers?
Baffles - how many? Width? Distance off wall?
Bottom head - flat or curved?
Motor - hp/kW?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[RVAmeche]

Is it possible to heat the vessel so product viscosity drops during mixing?

 

[moltenmetal]

LittleInch means that you have the wrong design of impeller for the job of mixing something that viscous to the level of uniformity that you need. A helical or anchor impeller may be required. A larger diameter impeller might be an acceptable alternative, but that depends on whether or not those viscosities are Newtonian or the material is shear-thinning etc.

 

[don1980]

For water-like fluids, almost any pumping type agitator will do the job. For highly viscous fluids, and especially non-Newtonian fluids, a custom design is necessary. The mixing design needs to account for rheology of the particular fluid, the degree of mixing that is necessary, and the geometry of the mixing vessel. A generic (pumping type) agitator impeller won't do it for high-viscous non-Newtonian fluid. You'll end up with good mixing immediately around the agitator blade, but stagnant (unmixed) areas elsewhere in the tank.

Fortunately, your problem is a very solvable one. Contact one of the major agitator vendors (one like Ekato or Chemineer) that has experience and technology with this type of application. Once they understand the details of your application they will be able to provide options for solving the problem.

 

[Compositepro]

Your mix is pretty high in viscosity. Removing the baffles might allow the mixer to work adequately. Turbulent mixing is no longer an effective mechanism at high viscosity and wall wipers on an anchor blade are commonly used.

 

[georgeverghese]

Have a read of chapter 18 in Perry Chem Engg Handbook, 7th edn, subsection on high viscosity liquid-liquid system mixing - anchor or helical style impellers are recommended.