rapid mixer coagulation

[Seda6334]

Hi
I am going to use a very small container (D is about 6 cm, with no baffles) with magnetic stirrer as a rapid in-line mixer after dosing the coagulant. the flow rate is in 60-100 l/hr range.
The technical data of stirrer:
motor rating output: 0.8 w
power input: 10w
speed range: 300-1800 rpm
magnet: 3 cm

I need to check if this mixer fine, considering the retention time and velocity gradient range. (it shouldnt be more than 10 s)

G= sqrt (P/viscosity*vol)

My main problem is that I do not know this P is which power. Is it the power drawn by the motor (10W) or power dissipated to water or the motor rating output?
Or shall I calculate the Re number (N*D^2/kinematic viscosity) and use a graph to get the power number (Np) and P accordingly? if yes, which graph ? since I am using the magnetic stirrer and I couldnt find any appropriate graph ...I think I should look for a 2- paddle mixer graph, but anyway still could not find it.

Any help would be appreciated!
Many thx

 

[ashtree]

It needs to be the mechanical energy supplied to the mixer, not the motor rated power.

However i would suggest that you may be over complicating this and the use of the magnetic stirrer will make it very difficult to calculate.

I would therefore use speed of rotation of the stirrer as a substitute for power. It will not give you the G number you may be looking for but if everything else remains constant you will at least be able to compare speed x with speed y and it should be a consistent relationship. Most likely the mixer driving energy will increase by the square of the speed of rotation so this may allow you to substitute in some values that will give you a representation of P for the calculation you are doing. Unless you can test otherwise you may have to assume that the actual mixing energy imparted to the solution by the mixer follows the same relationship.

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

 

[bimr]

Mixing in laboratory vessels is more thorough and occurs faster than is possible industrially. For that reason, it is not practical to model with full scale equations.

Magnetic stir bars are radial-flow mixers that induce solid body rotation in the fluid being mixed. This is acceptable on a small scale, since the vessels are small and mixing therefore occurs rapidly (short blend time). A peculiarity of laboratory mixing is that the mixer rests on the bottom of the vessel instead of being suspended near the center.

 

[Seda6334]

Thanks a lot ,

I agree!
I have been given this small model chamber with the magnetic stirrer to use in a pilot plant, and have to figure out if the G is in the range or not!
I finally found out very few papers (

https://doi.org/10.1002/bit.25575),

reporting the Np=1 or 1.2 for this kind of stirrer (but with no indication of physical characteristics of the magnet and chamber. It is said that although it is somehow similar to the paddle mixers (as you mentioned, it is not suspended), its Np should be lower than Np=2 (which is for paddle types)!

Anyway, magnetic stirrer aside, I think I should start looking for another appropriate mechanical mixer, IFM (instantaneous flash mixer), Inline-hydraulic jet, ...? any idea that where I can find lab/pilot scale mixers? not many choices on the web!


Thx

 

[bimr]

You really should consider a rapid mix container followed by a flocculation container.

There are a few companies that sell these:

Jar test

Search for a jar test device.

 

[Seda6334]

I do appreciate your advice
Yes, I do aim for that.
"jar test device": Great idea!
Difficulty: I should install it over a few different lines and I am sure it is not possible to make all the lines pass through this 1 apparatus with multiple containers and mixers. Am I right?

 

[ashtree]

Thats correct. The standard device is all about making test batches from grab samples. If you want to plumb it in it will require considerable modification.


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

 

[bimr]

Of course you do not have to flow through all of the containers. However, what you desire will have to be custom made. I believe that the linked firm will be willing to fabricate a pilot plant for you.

 

[Seda6334]

Hi guys

I am back. With another question regarding this tiny rapid mixing container. For now, I am gonna make use of this small container on a magnetic stirrer ... Theoretically, I used some formulas to design the chamber with the magnetic bar. But now I want to measure the real value of G (velocity gradient) in this mixing unit. On my way, I came across Stroboscope, Torque measurement (with for example torque meter or sensor) or electrical methods ...but mostly applied for mechanical agitators, not for a mixing unit with a magnetic bar!
I know that some of the magnetic stirrers on the market also display the torque...mine doesn't. and I just need to come into terms with it.

I wonder if anyone has any idea that how possibly one can experimentally measure G value in such a rapid mixing container.


Thanks a lot,