Impact of surface finish on paste to powder drying rate?

[hollerg]

Anyone have experience with trading off dryer wall smoothness for heat transfer to paste/powder systems?

I have a paste that drys into a free flowing powder, if low enough in moisture. Too much moisture and the organic salt tends to coheasive/ adheasive. At higher temperature differentials and rates, the agitated LIST Discotherm dryer loses some heat transfer.

The fouling is periodic. Lowering the inventory restores the lost heat treansfer. That the fouling clears so easily makes me think that a packed solids layer forms against the cylindrical wall. Lowering the inventory allows material to be dislodged by the impeller and gravity.

So, to reduce adheasion/friction/surface energy of the wall, I was thinking of adding a diamond like coating. It's low friction, and has high thermal conductivity.

What are the heat transfer ramifications of doing this? Should heat transfer should go up because of less "fouling" rather than down because of a coating?

 

[coatingengineer]

Hi.

Heat transfer (i.e. removing trapped moisture from your powder) will be improved with a better micro finish. More surface area contact between your product and dryer wall will best improve heat transfer.

Keep in mind that moisture loss occurs in two stages: The second (and slower) stage relies on 'diffusion' for evaporative loss. Molecules of water or other carrier, regardless of flash point, must "work their way" through the polymeric structure, to outermost surfaces of the powder.

So, if moisture retention is an issue, consider these options:

1) Reduce paste load in the dryer.
2) Increase the amount of surface contact between dryer walls and paste product by polishing dryer walls.
3) Consider a no stick coating for your dryer walls (i.e. hydrophobic properties are key).

If you wish to pursue a coating, first, see this link:

<a href="

http://www.industrialcoatingsworld.com/Non-Stick-Coatings/index.html">Non

Stick Coatings</a>

Good luck!

William Gunnar

http://www.IndustrialCoatingsWorld.com