The power of the mixing tank's agitator is no change with different fluid viscosity ?

[silebi66]

ear Sir :

I have two same size tank witch two kind of fluid's viscosity, one is 300cps, and the other is 30000cps.

I wonder why this two agitator(pitch paddle)'s power is constant with other condition is the same except the

fluid viscosity.

In other words, it is unrelated with fluid viscosity, but I am not quite sure it ture or not.

Could somebody give me hit or guide for this ?

Thank you very much.

 

[Latexman]

silebi66,

We need more information. We know you have two tanks of the same size. Do you mean same volume? What about diameter and height?

Are the two agitators identical? If not, how do they differ? What mixing parameters are the same or different between the two tanks? Number of impellers, number of impeller blades, paddle angle, type of impeller blades, RPM, impeller diameter, impeller width, baffling? Have you measured the power draw, or do they just have the same RPM and size motor (kW)?

Even if both systems are identical and they are drawing the same kW, the level of mixing will be less in the 30,000 cP one. I.e. the 99% homogeneous mixing time will be much less for the 300 cP than the 30,000 cP.

Good luck,
Latexman

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

 

[silebi66]

Dear Latexman

Thank you very much for your reply.

The tank parameter & fluid properties is following :

Fluid properties:
Fluid density : 1200kg/m3
Fluid viscosity : 30000 & 1000 cp

Tank dimension:
Tank capacity : 75m3
Top head : flat
Bottom head :10:1
Tank height(TTL-BTL) : 5500mm
Tank diameter : 4000mm

Impeller detail:
Impeller diameter : 2500mm
Impeller type : pitched blade( 4 blade 45degree), Np. power no. = 1.37
Impeller rotation speed : 50 rpm
Number of Impeller : 2
The spacing between top and bottom impeller : 3000mm
Select motor HP = 50 HP

Baffle detail
Number of Baffle : 4
Baffle width = 0.2*impeller dia= 500mm
Baffle height = Tank height*0.8= 4400mm
Offset from bottom = 0.2*baffle width=250mm
Spacing between baffle and tank wall = 1/24*Tank diameter = 166mm


Two tank design for the viscosity 30000cp & 1000 cp are the all same,

But, the fact is no need for the baffle to be installed for the high viscosity fluid (ex.30000cp) tank.

cause it won't be add much vortex formation near the top impeller shaft.

Also, the 4 baffle installation seems only for the low viscosity fluid (ex. 1000cap ) tank

, it can redirect the circular flow from bottom to top motion, and eliminate the center vortex formation.

Could you please give me a guide the motor HP (50HP) for this design is proper or not ? Since base on my calculation for this case (50rpm), the motor size

result to be larger such as 100 HP ?

Thank you so much.

 

[Latexman]

What is your mixing objective on the 1000 cP tank? 95% mixing time? Low, medium, intense mixing? Why is mixing needed? Liquid blending? Solids suspension/dissolving? Gas dissolving?

Ditto on the 30,000 cP tank?

Is there a perceived problem? Or, do you just want to do the best you can with what you have?

Good luck,
Latexman

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

 

[Latexman]

Is this existing and in operation, or is this a design basis?

Is the fluid very shear thinning? If not, it looks like the motor is way too small or the impellers are way too big!

Good luck,
Latexman

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

 

[silebi66]

Dear Laxtman

Thank you very much for your reply.


The tank is design for the Intermediate product tank. but I don't quite know the fluid content and shear thinning.

I guess it is like acrylic acid resin's intermediate product, and

it should be added other solvent or monomer to these tank to get the final product.

I am not sure our design basis now for the impeller is D/T=0.6 ,the 2 layer of pitch blade (4 blade 45 degree up),

and motor size is 40 HP is too small or not since I use other vendor's agitator calculator to get the motor size

to be as large as 100~150 HP.

Thank you very much for your bright thought.

 

[Latexman]

1000 cP ---> 236 HP per Aug. 29th post.

30,000 cP ---> 310 HP ditto.

Remember, there are two impellers in the soup, so 2X.

I recommend some expert counsel from a reputable agitation company or consultant. It could save a lot of energy $'s in the future.

Good luck,
Latexman

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

 

[silebi66]

Dear Sir

May bother you question about the fluid with high viscosity such as 30,000cp, is the power no. calculation for getting the power is still the same as turbulent flow ?

In turbulent region, the power can be get from P = Np*density*N^3*D^5/gc still can applied to the laminar region ? since I saw a different form.

Please help me out in this.

Thank you so much.

 

[Latexman]

Look at the attached. The 45[sup]o[/sup] P-4 goes by "A-2" in the graph. It is obvious the laminar regime cannot be represented by a simple, turbulent regime equation.

May I recommend that you contact your local Chemineer rep and ask them for their two Chemical Engineering magazine reprints titled, "Liquid Agitation" and "Advanced Liquid Agitation". When I first did this, I need to educate myself so I could communicate to them what help I needed. They should be happy to do this. If not, buy these two reprints from Chemical Engineering magazine. I don't know how expensive it is, but my recollection is it is reasonable.

Good luck,
Latexman

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

 

[silebi66]

Thank you Latexman, I really appreciate your experience's share.

It is just like you suggested that the power consumption for highly viscous fluid seems cant not be reached through a simplified

equation from the experiment.

I read a book written that the power input to anchor impellers for shear-thinning fluids showed that the clearance between the impeller

blades and the vessel wall was the most important geometrical factor: NP at constant NRe was proportional to the fourth power of the

clearance divided by tank diameter, but the other correlation equation I read from other book (Pls see the attached file in the post)

is in a different form, and I used this will get a unreasonable high power consumption.

Could you please kindly suggest any reference book or free or cost effective software for us to contact (I know the bioreactor software

seems cost effective but it can't be calculated in the high viscous condition?)

Thank you so much.