Mixing Slurry 1

[rolands1]

Hello,

I am having trouble figuring out what is typically done in my situation (FYI: I am a new graduate in chemical engineering). I have a cylindrical baffled tank (approximately 60" in height, 30" in diameter) with an agitator (2 impellers). I am mixing a slurry which has a strong desire to separate on me. The system is also equipped with a recirculation pump (whose capacity seems to be nowhere near the appropriate capacity).

The tank takes approximately 8 hours to empty. As long as the fluid is in contact with the agitator, the slurry is well mixed and no problems appear. However, the last 10" or so of the tank, the slurry level is below the agitator and the recirculation pump is the only mixing device in effect. Unfortunately, the last 10" takes about 1 hour to drain and due to the poor mixing, the slurry separates on me. I was looking for some solutions from people with mixing experience... here is what I am thinking and I would appreciate any advice...

1. increase the capacity of the recirculation pump, however, the capacity would have to be increased by a very large amount and the space/cost may be an issue
2. add an air sparging system to bubble gas through the bottom of the tank (is this typically done? could it be capable of creating the turbulence required to keep this slurry in tact?)
3. redesign the tank... a coned bottom would reduce the amount of "dead" material to an acceptable level (which would be discarded as waste. however, i fear a new tank would also require a redesigned agitator to ensure appropriate mixing.

Thank you all in advance.

Roland

 

[jmw]

Before bubbling air through the slurry, you need to advise how you are measuring the slurry % solids. Many of the methods used are vulnerable to entrained air.


JMW

http://www.ViscoAnalyser.com

 

[ash9144]

One possibility, highly dependent on specifics of your process, is to not initially mix in your total solvent amount. Use 3/4 of solvent to blend slurry. Then when you start pumping and get down to a low level near the bottom agitator start adding the last of your solvent. This will dilute the remaining solid to a pumpable level while maintaining your overall composition.

 

[JimCasey]

Sometimes fluidizing a mixture with air is not acceptable because gases in the air can degrade the mixture. Oxygen is a frequent culprit, but sometimes it's the water vapor or CO2 that is unacceptable.

If air is OK, you might inject it at the top to force-drain the tank quickly instead of having it trickle out over hours

Unrelated idea: You may wish to throw in an eductor connected to your recirculation pump and mounted to the lowest point in the tank. An eductor uses the energy of the pumped stream to move a lot of flow.

 

[jmw]

Eductors are used by offshore support vessels to keep drilling muds from settling out.

JMW

http://www.ViscoAnalyser.com

 

[itdepends]

Whats the slurry in the tank?

Why are you draining the tank all the time?- is it a batch make up process or is it just a normal shutdown situation

What stops you draining the tank faster once the level is below the blades

Cheers.

 

[cawse001]

rolands1,

You could install a false floor in the tank that slopes towards the drain nozzle. It should'nt affect your mixing dramatically, although you would need to monitor the new tank geometry for wear.

If your looking into air agitation, bear in mind that you will need to keep the slurry out of the air distribution system, which requires careful design.

NB

 

[rolands1]

Thanks to everyone for the suggestions so far.

The tank is used for batch mixing. It drains into another piece of processing equipment, which has a limited capacity... hence the slow drainage.

 

[itdepends]

If it takes so long to drain- do you need to batch mix it?

With an 8 hour drain time is the holding time important- or do you need to clean the tank out between batches.

Can you change your batching process to make a new batch up once you reach the minimum level?