How to prevent solids build-up in a dryer?

[nbog]

Respected community members, I have one (only?) problem with my powder processing system and I hope someone will be willing to share his/her knowledge/experience. Here it is:
Experimentsl reactor is 12" ID, 3 ft long (with a cone at the end) and filter/fan downstream. Powder is fed from the top of the reactor into a flame (burner mounted on the top). Particles generated are 0.5-1 micron. After running the system for 1/2 an hour, most of the powder is deposited on the walls of the reactor instead of being conveyed. Gas velocity is enough to keep particles entrained but why they are sticking to the wall? Reactor T is 500 C, gas flow rate 350 slpm.
Would decreasing ID/length help and how much? Or injecting air along the walls (sweeping)?

Thanks

 

[unclesyd]

What is the product?
Is your particle water wettable or react with water?
What is the reactor made of?
How are you adding the raw material to the flame?

If you can comeback with a little more information about your process.

One possibility is that you could have to have incipient melting or wetting of the particles to cause the sticking. Flame temperature is considerably high than your stated 500°C. The skin temperature of the reactor may be higher or cooler that the stated temperature of the gas stream.


We have the essentially the same problem with polymer pellets. We went from fluidized bed type dryers to a Wyssmount Turbo Dryer or a tumbling dryer. For our highest temperature tumbling dryer we added inert pellets that can be easily separated.

 

[nbog]

Wall temperature is about 500. Flame is quite short (6") so the particle exits it and gets quenched, but it is not melted.
Reactor is SS (not polished enogh though). One of my speculations was that the rason may be the roughness of the wall. Water is actual media (suspension) but at a such high T of the wall I don't think it affects deposition.

 

[appy1]

We had same problem of cake formation on the wall of the tumbling dryer leading to depoists on the wall. but its jacketed dryer, so by incresing temperature of jacket fluid, this problem soved upto certain extent

 

[lattar]

One reason for solids build-up is small particle size. Small particles have greater tendency for this kind of buil-up as surface interactions (attractive/repulsive forces) means so much.

 

[nbog]

Frankly, my goal is to get the smallest particle size possible. That's why I am atomizing my slurry.
Would bigger chamber diameter help? I am also thinking about adding sweeping gas along the chamber wall to wash away all particles that come close to it.

 

[cub3bead]

A larger diameter should help.

You could verify this by heating the dryer up to operating T and then slowly introducing your slurry to the atomizer. Ramp the slurry fed rate up until the deposition starts to occur.

Can you raise your drying T above 500 C?

 

[Compositepro]

Static electricity is probably a significant factor in your problem. Sprayed particles will pick-up some charge and be attracted to any grounded surface. Intentionally charging the spray and the vessel walls to the same polarity would keep the particles from agglommerating and from sticking to the walls.

 

[nbog]

cub3bead, flame temperature is far above 500 C, wall T is 500 C.

Compositepro, thanks for the tip. When discussing the issue here with my coworkers, static electricity was mentioned but we actually didn't explore it further.

 

[m777182]

I had similar problem at 150 C and a glass reactor of slightly smaller dimensions. I coated the inner wall with shoe polisher and it worked to some extent. Then I add some tangentially directed nozzles to enhance rotary flow and it was still better. The problem with static charge still remains but there is an idea to make the whole reactor charged as a capacitor with a guard shield to focus particels(like electron beam in a cathode oscilloscope tube) but then the advantage of homogeniosly distributed particles is lost.
m777182

 

[elguero]

Larger diameter will definitely help. Don't think small; go to at least 8 feet diameter -- 16 feet would be better. Even after you go from a pilot operation to full scale, the larger diameter might be adequate. Consider better atomization (smaller particle size) too. High pressure drop across the atomizer and small amounts of compressed air can help too.

 

[prasanna2211]

Can we think of giving a slippery coat to the reactor internal. Probably reducing the friction may help solve the problem of particle agglomeration to a certain extent.
Ofcourse, the coat should be non-reactive to the reactor conditions.

I also have a question.
Is the reactor being uniformly heated up or are there some regions getting more heat and some getting lesser heat ?
Is is the particle agglomeration happening uniformly throughout the length of the reactor or is it restricted to top, middle or bottom ?

 

[nbog]

Coating is evenly distributed although bottom section gets hotter than the top one.