Dear Forum Members,

I am operating an LDPE plant.

In more than a decade on various plants I have never seen so much polymer dust generation.

I believe most of the dust is generated at the extruder, but also during futher conveying and blending.

The conveying is initially lean phase from extrusion to blending silos - about 60 meters.

After a number of circulations through blending silos, the polymer is conveyed again with lean phase about 80 meters to storage silos via an elutriator.

From storage to bagging the polymer is conveyed about 50m by dense phase.

The problem is worst with low MI products, and products having higher levels of "slip-additive."

Watching the dust outlet from the elutriator, one can see a dense dust cloud passing into the collection bins.

It has been suggested that a number of extraction units be installed, such as cyclones on top of each silo / blender, with dust extraction on the vents, dust extractors on the rotary feeder balancing lines and many more.

I feel this is over-reacting to a problem that could easier be adressed.

Something I am looking for is a rig to analyse dust content in a sample of pellets. Preferably a "wet analysis"

Any suggestions to either reduce the dust, or at least determine more accurately the source - ie analysis of samples taken from various points on the plant?

Any help would be appreciated,

Thanks,

Dave.

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Dave,

The sources of dust should be 1) insufficient mixing/homogenization of additives in virgin LDPE in your twin screw extruder, and 2) attrition of pellets in pneumatic conveying lines.

The first problem can be reduced by using an additives/resin preblender before the extruder, and by improving the mixing in the extruder by changing its screws or by changing the barrel temperatures to increase head pressure.

The second problem can be helped by reducing the conveying velocities, assuming that your conveying lines are internally roughened, particularly the long radius bends. You should inspect the lines to make sure they are still properly roughened.

Typical product quality standards call for a maximum of 50 PPM dust in the shipped product. I am sure your lab runs this test. I used to keep the dust at 15ppm.

Have you taken a sample of the dust to determine its composition?

In most plants dust is removed at the final shipping point.

Hope the above helps.

Amrit Agarwal

Consulting Engineer

Pneumatic Conveying Consulting

Email: polypcc@aol.com

Ph and Fax: 304 346 5125 ■

Dear Dave,

I calculated a number of PVC powder pneumatic conveying installations of various designs and tried to correlate the forming of streamers with the pneumatic conveying parameters s.a. material velocity.

That theoretical research resulted in the following document for the Bulk-Blog:

Influence of electro static charge on pneumatic conveying.

https://news.bulk-online.com/?author=15

The relation of velocity losses causing streamers was not found.

Therefore I inserted in my program the calculation of the energy, generated by the particle friction and velocity losses.

If that energy was fully absorbed by the particles, the resulting particle temperature could rise in total just a few degrees. In reality, this temperature increase is reduced by the cooling effect of the expanding air and heat losses to the surroundings.

Whether the dissipated energy per particle collision is high enough to break smaller particles loose from the original particle is not known.

As far as I know, the application of shot-peened pipes is based on the theory that the particles bounce on the pipe surface material, rather than slide and the assumption that sliding causes more wear than bouncing.

I think that this theory is too simple to conclude that it works that way.

The ratio of bouncing and sliding of a particle impacting on a wall is in fact the angle of attack.

In average, I suspect that the angle of attack of the particles is the same in normal “smooth” pipe and shot-peened pipe.

In addition, particles collide also with each other, not only in mid air but also against the wall and how the impact forces lead to the formation of dust is related to the strength of the particles.

The particles, therefore, should be “smooth” and free from weak bonded extensions.

Have you investigated the particle “resistance” against rubbing off?

Are you sure that there is dust generated in the dense phase conveying system as the velocities are low in such a system?

Is the static electricity not the disturbing factor in the detection of the dust?

Your “Sticky arm” could be a perfect electro static precipitator?

The elutriator is in fact a kind of “pneumatic conveying system”, whereby small particles are separated by an upward airflow using the terminal velocity as the separation property.

Static electricity can stick the dust to the main particles and that dust is detected further in the process.

If you are considering installing a system to blow ionized air through the product, have a look at:

http://simco.nl/

My working life was always with the rough, heavy, pneumatic ship unloading industry, where the solution for every problem was “more power”.

Unloading cement is just unloading cement, but in polymer conveying, physics play a much greater role.

Therefore, I might have missed some arguments, which could be important for your installation.

All the best

Teus ■

Dear Dave,

Dropping pellets on a rotating disk (at various rpm’s) and measuring the deterioration by sieving could indicate the sensitivity of the pellets for “rubbing off”.

The conditions in your pneumatic conveying system can be calculated.

If it is “plug” conveying, then the plugs are separated by pockets of compressed air.

These pockets of compressed air expand along the pipeline, whereby the velocity is increased and the “plugs” are blown up and the conveying becomes normal pneumatic conveying with an evenly spread mixture.

Are the pellets fed into the system in plugs?

If you give the pneumatic conveying installation data (horizontal length, vertical length, number of bends, compressor air displacement, real capacity at measured pressure) and the material data (size, particle density, bulk density), then I can set up a calculation.

I do not expect to find the cause of the generation of the dust there.

When it comes to electro statics and bonding energy and the air forces on the dust and the pellets, the knowledge of physicist and chemists is needed.

In cases like this one, many disciplines come together and that makes the cooperation between them so interesting.

Have a nice day

Teus ■

Dave,

Sorry for the delay in giving my responses to your questions. These are:

1. Could you suggest an alternative to dust measurement?

For measurement of dust, a lab size elutriator is a good choice. I have used it for many years. I can send its info to you if you like.

2. Could shot-peened lines in dense phase system create dust?

Definitely yes. Shot peening is not needed in dense phase conveying. Its only purpose is to reduce streamers.

3. As the bends wear smooth, would this result in more dust or more streamers?

Worn-out bends will result in more streamers.

4. Will use of ionized air help in de-dusting?

Tests and data I have show that ionizing does not help. Pelletron uses a magnetic field for de-dusting and their method does work.

Best regards,

Amrit Agarwal

Consulting Engineer

Pneumatic Conveying Consulting

Email: polypcc@aol.com

Ph and Fax: 304 346 5125 ■

Dear Dave,

What is the EVA content of your product? Based on this information some details of your problem could be explained. Rubber-like materials do not get well cleaned in a counter-flow elutriator as they bounce around, but do not get the necessary kinetic energy for proper cleaning.

Please advise.

Best regards,

Sven ■

Dear Dave -

I am sorry, I do not have the answers to the dust measurement questions. I do however know of a way to sell the LDPE dust.

Rob Starr

St Joseph Plastics

rob@stjosephplastics.com

816-279-1415 ■