Mixer - Can invert the spinning direction cause serious trouble? 2

[guerky]

I have a reactor with a botton valve suffering with obstruction several times a week, thanks to some heavy solids in suspension depositing over it (excess of one reagent).
The mixer is a pitched turbine with four paddles that aparently forces the bulk of the mixture towards the bottom in the actual spinning direction.
I wish to invert this spinning direction, hoping to keep most of this solid particles in suspension by favoring an upwards axial flow. The reaction is very fast, and the mixing is actualy good. Also, there is no risk of unwanted parallel reactions.

Can this work?
Can i expect some heavy loss in agitation quality or in the worst case scenario i will have just an increased energy consumption?
I also thank for any hints, rules of thumb or materials on mixing issues.

 

[Latexman]

Not enough information to make a guess. I've heard "plant legends" that the direction of rotation doesn't make much difference, but never saw it myself. In the past, our agitator suppliers have convinced me it's an important parameter, so we have always verified the spinning direction prior to start-up.

Good luck,
Latexman

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

 

[bimr]

It may work. Reversing the direction will provide greater mixing efficiency above the mixer if you are adding the reagents from the top.

See the paper:

http://core.ac.uk/download/pdf/12039904.pdf

 

[guerky]

It is a batch reactor. The angle of the paddles is about 45°. I already expect a sharp rise in the eletricity consumption for this reactor. If it work well enough i will re-design the stirring system (maybe an impeller would do the same trick with less energy).

For now...let's try it out. Thanks guys.

 

[1gibson]

You'll reverse the direction of thrust for the shaft, make sure you consider how that might affect the bearing arrangement, and have some sort of plan. Bare minimum, keep a close eye on bearing temperature. Probably not a big deal if its low power.

 

[itdepends]

I can't recall seeing an agitator installation that is designed with the impellor rotating to give upward flow from the centre. They're always set with down flow. I would expect solids suspension to be a problem on the tank floor at the perimeter if set to up flow.

However Google is your friend- search for upflow agitation

http://oatao.univ-toulouse.fr/1406/

https://books.google.com.au/books?i...mCh2SlAsh#v=onepage&q=agitator upflow&f=false

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[Latexman]

Mixing is sometimes not intuitive. I have seen a video with a mixer suspending dissolving solids. At a somewhat high speed, a pile of solids accumulated directly under the shaft (probably over your valve). At a lower speed, it did not!

A single video is not representative of the world, so I'm not saying this is your problem, but . . . it's easy to test, right?

Good luck,
Latexman

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

 

[Compositepro]

Reversing rotation will not likely help your problem. Your intuition is that reversing the mixer will cause an upward flow. But upward and downward flow are always equal in a mixer. The flow patterns will be different but there is a reason that mixers almost always pump downward. When you add materials they will flow toward the shaft and down to the mixing blade where they are quickly dispersed and then circulated back up the walls. Reversing the shaft will pump material up in the center of the mixer and added material will flow to the walls where they may stick, and there is little shear.

Your problem is deposits in the valve area. This is naturally a low shear area. You can try to increase flow there, but your problem is more likely that you are simply adding materials too quickly to the mixer.

Another option is a different draw-off valve that doesn't get plugged. There are ram-type plug valves that are flush with the inside of the mixer wall when closed. You could also recirculate through bottom valve back into the mixer during the mix, to keep the valve clear. An air operated diaphragm pump works well for this, as it is easy to connect.

 

[bimr]

Upward and downward flow are not always equal in a mixed tank. The reason is that the mixer is generally located close to the bottom. The reason that the mixer blades are located near the bottom is to mix the bottom of the tank,

Mixers may generate foam and splash when mixing in the upward direction.

 

[guerky]

Here is a picture of the reactor internals (the plastic scrapp on the botton paddles was removed, but the problem persisted).
This particular reactor dont have a frequency inverter, so i cant play with the agitation speeds.
I could put a larger pulley and increase the overal speed, but since the botton exit dont have a ram-type plug valve the shear will stay low on this particular area (there is a 10 cm neck on the botton before the valve).

There is some angle, paddle configuration or agitator type that could cause a higher shear directly under the end of the shaft?

 

[bimr]

Have you considered revising the location/size of the outlet and valve? That may be an easier fix.

 

[guerky]

bimr,

Cutting and welding would be necessary to do a decent job there. This kind of intervention require a complete plant stop and descontamination. Since this reation is not a bottleneck, i intend to minimize the operational stress in a way that i could push it until January, when we have a maintenance stop scheduled and more agressive changes could be made (the ram-type valve is the best candidate). What i could do for now is change the pulleys (up to 20% more agitation speed, 100 rpm to 120 rpm), flow mode (downflow to upflow) or change the paddles. The temperature is also adjustable, but its already at its limit.

 

[Compositepro]

The design of your paddles is creating the dead zone over the bottom center. The flange at the base of the paddle is not a good feature, as it blocks flow to the paddle. Just replace the bottom agitator with one the has 90 degree paddles, connected directly to a hub, that clamps to the mixer shaft.

 

[guerky]

Compositepro,

I like the idea of removing that flange. Welding a 45º paddle directly to the hub is already on the list o possible solutions.
But a 90° paddle has a radial flow pattern, so there is one more problem: the botton have a bushing to fix the end of the shaft, just over the exit line. An hydrofoil would not be a better attempt? It would be necessary a very vigorous axial downflow to keep the shear on the stagnant region under the bushing.

OBS.: the solid is KOH. It's heavy and goes straight to the botton.

OBS.2: I was able to make a quickfix to clear the plugging with a nitrogen nozzle pointing directly to the neck where the solids deposits. But the plugging itself is still a problem that i would like to get rid of.

 

[bimr]

Have you discussed this process with a mixing specialist or is this a homemade design? You realize that you have posted exactly what you are trying to mix or the fluid properties.

The rule of thumb is that the bottom mixer should be installed off the bottom a distance equal to the diameter of the mixer. From the image, the mixer looks to be much closer than that.

Also the baffles should not extend to the tank bottom, and be installed 4-6 inches off bottom.

 

[guerky]

bimr,

This is a vessel that was adapted from another process a few years ago (i have just a few months of real working experience here). The agitation system wasnt developed for this specific function, since this was a simple pre-mix step. The soluble fraction dissolves readily; the insoluble dont mix at all.

Another shot: I have barelly 2 m of liquid, a second mixer on the shaft is not really necessary. Since the reaction itself is pretty fast and the problem is just the heavy solids, i think about changing the botton mixer for a simpler 45º paddles (without those screwed baffles) and remove the upper mixer; that, acording to some estimations, would allow me to almost double the stirring speed without increasing the power requirements to a level that could damage the bearing or the motor. This single mixer can be brought up to 0,6 m of the botton, obeying the Ludwigs's guideline (a mixer too close to the botton acts like a radial impeller).

 

[gerhardl]

I have limited experience with mixer construction, but have operated several in painting, plastic and glue-producing industries.

Looking at the picture I imagine that with the short mixer-blades given, you will necessarily have a relatively high rotating speed.

The flow pattern in the tank, including the centrifugal force bringing the heavier particles to be sent in direction of the outer walls, will be dependent on how well the whole mass rotates and flow.

Could it be that even if the whole mass rotates, you have a heavier flow downstream near the axis than necessary, and a lesser rotation of the whole mass near the walls? This could be the reason that you do not have sufficient laminar stream at the bottom in the direction of the outer wall and then up to transport the heavier particles away from the center.

For all the mixers I have seen for fluid mixing with particles, the mixer blades have extended almost to the wall and rotated with a relatively slow movement.

In addition a slow-start/stop and speed control is a must for all mixers in my opinion.

 

[Compositepro]

Gerhardl, what you describe applies to mixers used for high viscosity fluids. They are unnecessary and often undesirable for watery fluids, where turbulence is a major component of the mixing process.

 

[gerhardl]

Hi Compositepro, Thank you, point taken, and yes, my experience is from higher viscosity mixtures!

(Only able to read the first picture/reference on my PC)

My additional question is even then that I cannot see how the total picture of flow in the tank, turbulent or partly laminar does not contribute to how the particles are distributed when you empty the tank. How is the procedure, stopping the mixing before drainage, or continuing or lower the rotation before emptying.

Anything to gain here in addition to alter the form of the mixer blades?

 

[itdepends]

What - exactly- are you adding to the tank? If you're just dissolving solids KOH pellets in water as a preparation step for a further reaction- slow down the rate at which you add it.

Is there any carbonate in your water source? NaOH pellets have a nasty habit of fusing together if you add them too fast to a tank that isn't agitated sufficiently. Does the tank have a dished bottom (looks like it does). That will make it harder to suspend the solids.

If you're trying to suspend solids- high flow axial agitator blades will do a better job than the ones you have. Those blades are designed for high shear- not solids suspension.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"