keep TFE for feed plate or use?? Nyloil??, UHMWPE ?

[hollerg]

I have a wet filter cake that will stack up on the entrance to a fluidized/vibratory bed dryer. The falling granulated wet cake doesn’t always slide off the inclined (~25 degrees) plate into the fluidized bed.

The original bright 304 plate would develop a mound the diameter of a bowling ball, eventually plugging the 10" inlet. Switching to Teflon limited the size of the mound to a thick silver dollar sized pancake. Unfortunately the pancake is stable & thick enough to slide off & obstruct airflow screwing up fluidization. When agglomerated granules grow no larger than quarters before breaking loose, the fluid bed works fine.

I tried to pick out materials myself, but it's not clear to me what basis to use for evaluating what is better. I see surface energy, surface roughness, water absorption, water wetting/contact angle, and friction as possible factors.

So in my situation (conditions below) what else makes sense to try? (ie.UHMWPE or Nyloil, or ??) Your reasoning would be appreciated.

-- The Wet cake is pH 3 -4 due to the solids,
-- has a wide particle size distribution, w/ a significant fraction of < 40 micron, & the mean is 10x larger,
-- MP is ~340 F,
-- dryer pressure is atmospheric, & temperature is controlled @ 140 up to 200 F
-- the wet cake enters at ~ 100 F.

 

[Demon3]

That's one of the best framed questions I've seen.

If you've used PTFE that's about as low as you'll get in surface energy.

UHMWPE is not as slippery.

Nylon even less so and won't like that pH much.

I'd work more on vibrating that slide and perhaps adding some spike on it to break up the surface.

Could you perforate the surface and apply compressed air like an air hockey table?

Chris DeArmitt PhD FRSC CChem

 

[patprimmer]

I agree with Chris.

Nylon will certainly not like that ph.

UHMWPE will be quite soft at 200F.

PTFE will take some beating.

You might get a slightly better result if any grain effect is on the surface due to making the PTFE part, that the grain direction is smooth downhill and rough uphill.

Shaking or vibration and maybe breaker bars that block oversize lumps from progressing down the chute unless they break up to smaller pieces first.



Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[hollerg]

In addition to orienting the grain of the TFE sheet, is there any advantage to buffing, wiping with dry graphite or switching to a ptfe coated smooth metal to further reduce surface imperfections?

This plate already shakes with the fluidized bed, so not obvious to me how I get more vibration, unless I can make the plate bounce or or flex more.

 

[patprimmer]

I don't think so. It depends on what you already have.

A coated metal plate might shake with a sharper action whereas a full plastic plate might dampen some of the vibration. Maybe, but I am clutching at straws.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[btrueblood]

Have you tried a pebble-like coarse finish? I'm thinking something like you might find on commercial cutting boards, the intent is to break up the smooth surface and reduce the hydraulic "stiction" or suction effect. Something like these:

http://www.woodworkersauction.com/cookware/cookiesheet1.htm

You could add an electric motor with an off-center weight to vibrate your intake plate. With a DC motor and a variable voltage supply, you could vary the speed to figure out what frequency works the best.

Lastly, and Chris could tell you if this is a nutso idea or not, but what about a hydrophobic coating, e.g. silicone oil? Downside is that it may contaminate your cake, and would need to be periodically replenished.

I like the rock-catcher grate idea of Pat's, though it may be just another place for material to hang up.

Like Chris said, nicely formed question.

 

[hollerg]

If there were enough force to break the chunks against the pins, I would do the rock grate, but by the time they break loose they are too rigid.

The pebble course finsh is counter intuitive but unfortunately I have not found pebble grained PTFE.

 

[patprimmer]

Pebble grain PTFE would not be that hard to make with the correct equipment. Simply heat and press.

Coating pebble grained metal will also work.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[patprimmer]

Oh. One word of caution.

If you heat fluorocarbons, or chlorofluorocarbons to the point they degrade, they produce some VERY nasty gasses. Take care to exceed the softening point, BUT DO NOT EXCEED THE DEGRADATION POINT. This should be on the manufactures MSDS and the processing guides.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[Demon3]

Not many people know that PTFE becomes almost transparent when you heat it up to around 200-220C. Try it with a PTFE coated magnetic stirrer bar and you can see the magnet inside.

Chris DeArmitt PhD FRSC CChem

 

[btrueblood]

One final dumb question - why 25 degrees incline? No way to make this steeper? Even if only steeper at the initial contact point, it could help in maintaining momentum of the falling cake. A tautochrone shape (cycloid) comes to mind.

http://en.wikipedia.org/wiki/Tautochrone_curve

 

[hollerg]

The material is discharged toward the incline by a delumper mounted ~ 3 ft up, which sprays the small particles out in a number of directions. The discharged solids hit the housing and vertical duct, resulting in them falling with those that rain straight down onto the inclined plate.

Unfortunately there is a 16" slide gate valve mounted <12" above the plate. Short term ($$) restrictions prevent moving equipment to install a greater slope.

A curved duct to guide the particles to the bed doesn't fit the round hole very well, and I imagine the curve must stay steeper within 25 degrees of horizontal to minimize agglomeration. I will try to incorporate such a slope when relocating the delumper and valve is approved.