Fluidized Bed Dryer Operation: What a Mess

[chemEcaleb]

Please refer to attached sketch.

Last June, I completed a project that eliminated a screw auger between a centrifuge and a fluidized bed dryer (not my idea for the record). Since then operators have not been happy with me because they had to wash out the dryer every 3 days due to chunks forming in the dryer. Prior to the elimination of the screw auger, the dryer ran beautifully and they washed it out maybe twice per year. I've made a massive amount of changes, and I have it to the point where they only have to wash it out every month but the operators are not happy so I'm not happy... but I'm at my wit's end with this project and it's been one hell of a wasted summer because of it.


From my daily observations I have found that the chunks build up on the chute between the dryer and the centrifuge, and the flow diffuser on the inlet to the dryer. They used to build up on the sides of the dryer, but I made a change to the flow diffuser that fixed the issue.


Here is a description of the process

Sodium chlorate slurry (35-45% solids) with an average particle size of 0.65mm feeds a centrifuge at a flow rate of 7.4 short tons per hour of dry sodium chlorate (roughly 40 gallons per minute). The centrifuge operates with a basket size of 450mm, at 900RPM (the velocity of the chlorate exiting the basket is nearly 100 mph). Two spray bars wash the slurry with chilled, condensed water at 2 gpm each. The sodium chlorate exiting the centrifuge is 1.5% moisture which is pretty typical for this kind of operation.


Here are my questions to all of you:

What is the difference between when we had a screw auger and without a screw auger? Why should we get more build up on the sides of this chute when we have a straight drop as opposed to having a screw auger feed the chute?

What is the best material for this chute application? I've tried mirror polished stainless steel, normal stainless steel, Teflon (abrasion destroyed this in a matter of days), and nylon hose. All of them either get build up, or get eroded to dust in a matter of days. The next thing I want to try is glass lined stainless steel.

Do you have any ideas for how to improve this system? I'm open to almost anything other than putting the auger back in...

I wish I could calculate my way out of every problem...

 

[itdepends]

Do you have before and after photos or GA's?
Was the screw feeder connected to the top of the dryer- providing an air lock to prevent air being drawn in from the centrifuge? I would expect that with the new system you would be sucking air, fine particles and saturated mist from the centrifuge through the dryer- that would encourage build up on the discharge chute.

The other possibility is that if your chute was previously sloped the bottom surface may have been self cleaning due to the action of the solids.

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

 

[MortenA]

So would it be possible to put the screw auger back in (i guess not since you havnt done that)?

 

[chemEcaleb]

It depends-
Was the screw feeder connected to the top of the dryer- providing an air lock to prevent air being drawn in from the centrifuge? The auger was connected to the dryer by a straight, rectangular chute. There wasn't actually an air lock on the old system. See attached drawing.

I would expect that with the new system you would be sucking air, fine particles and saturated mist from the centrifuge through the dryer- that would encourage build up on the discharge chute. The centrifuge does have an air inlet, but this new system is actually more sealed than the old system. You're right though, I agree that part of the problem is that mist/droplets of water end up feeding the dryer along with the dry chlorate. Back when we had the auger, all of the droplets were mixed in with the bulk of the feed, but now it has a tendency to drip down the sides of the chute. Maybe I could dramatically lower the flow rate of spray water... although product quality might be a concern at some point.

The other possibility is that if your chute was previously sloped the bottom surface may have been self cleaning due to the action of the solids. This is an interesting point. Initially I had a huge 12x18" chute and the chunk problem was way worse. When I changed the chute down to 6" and 8" the problem got better, and I think it's because of this "self cleaning" effect.

MortenA: I'm not putting that auger back in. We've spent way too much money on this. Not only did we eliminate an auger, but we also improved working conditions for operators and mechanics by relocating the centrifuge to a more accessible location.

 

[itdepends]

Looking at your before and after pictures- previously the dryer would have drawn fresh air in and mixed it with the damp solids coming out of the auger- it wasn't able to draw the moist droplets from the centrifuge.

Post mod the air is being drawn through the centrifuge- bringing the moist air/droplets with it.

As you've said above- you're stuck with it (not having the auguer).

The smaller you can make the chute the better- (squeeze the moist air out).

Some fresh air inlets on your new chute (near the top) may help as well- is the centrifuge running under vacuum? (Does it have a scrubber on it)

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

 

[chemEcaleb]

That's an interesting theory. I think I'll give it a shot next time we're shut down. There's no scrubber on the centrifuge, and the centrifuge is running under a vacuum; the air inlet is on the box where liquid drains from.

 

[controlnovice]

I read two things:

- you've improved working conditions by relocating the centrifuge
- You've made more work because they have to clean out the dryer more frequently.

Which of these is more important to the operators/maintenance/company?

______________________________________________________________________________
This is normally the space where people post something insightful.

 

[chemEcaleb]

They're both important, that's why I need to make this installation work. The problem is that I've probably worsened conditions because now operators go in to the dryer once or twice per shift (for maybe 3-5 minutes) while it's operating with a pitchfork to pull out chunks. I have to make it work.

 

[Compositepro]

The screw auger would have broken-up large chunks of wet feed material and made the feed more uniform. Dropping large chunks into a fluid bed dryer will result in them sinking to the bottom and touching hot surfaces where you will get a solids build-up. Wet material that touches a hot surface will leave a residue that will build-up continuously. You want to have wet feed material touch only dry powder.

 

[MJCronin]

Has the person who originally came up with this idea (client ?) now disavowed all knowledge of it ? Are you now the focus of all blame ?

Usually, that is one of the signs of a project about to end...

Don't park your car in the same lot as the operators.....

MJCronin
Sr. Process Engineer

 

[chemEcaleb]

The person who came up with the idea is still working with me to trouble shoot. I am generally the focus of the blame from the operators (or maybe they're just looking for someone to focus their complaints on, that's a definite possibility), but I'm the process engineer at the plant so I have to live with this.

 

[zelgar]

The problem seems to be caused by the water. Do you need the water? Could you cool the feed material in an alternate method from spraying water into the slurry? Is the problem of too much water, or is the water spray not diffuse enough (e.g., the sprayer/misters are clogged and not functioning properly)?

 

[itdepends]

zelgar it's a mixture of salt and saturated solution going into a dryer- the feed isn't cooled before feeding to the dryer and it there's any slurry outside of the centrifuge feed the OP has bigger problems.

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

 

[chemEcaleb]

UPDATE:

Changed operating parameters. Fluidized bed dryer has three chambers. The first two chambers are fed air from a hydrogen burner, and the last is fed with cold air. Chunks seem to be building up in the middle chamber, so I increased the air in the middle chamber and decreased the flow in the first chamber just a little bit. I'll let it run over the weekend and see if that helps. I'm optimistic that this will help.

 

[chemEcaleb]

UPDATE:
Increasing airflow in the center chamber helped, but still not fixed.
I'm trying an experiment today. Going to plug the air vent on the drain box and see if that will cause less water droplets to be entrained in discharging product... Centrifuge manufacturer was confused why air was being sucked in through the vent, it's supposed to do the opposite, air is supposed to be discharged through the vent and sucked in through the solids discharge which really doesn't make sense to me... but whatever. This should help.