8 replies to this topic

[curious_cat]

There's a manually operated batch filter I'm trying to troubleshoot: This filter (sketch below) receives a hot (approx. 100 C), saturated slurry (4% w/w) of NaCl and the filter medium is a Nylon woven fabric. 

 

The  fabric rests on a perforated metal backing plate and a vacuum is applied to the chamber below the plate. The problem is that every day or two the holes in the metal plate end up being totally blocked with hard, salt deposits which need to be manually chipped out / pressure washed. The holes are approx. 5 mm in dia. 

 

There's two such filters and they operate alternately on production & cleaning out cycles. 

 

Any ideas what might be the cause / solution to this problem?  

 

Edited by curious_cat, 03 December 2015 - 01:14 AM.

[breizh]

Hi ,

Seems to be a crystallization problem . Why 5 mm hole not bigger ? What is the quantity of product  recovered on the filter cloth ?

 

Breizh

Edited by breizh, 03 December 2015 - 07:37 AM.

[curious_cat]

Hi ,

Seems to be a crystallization problem . Why 5 mm hole not bigger ? What is the quantity of product  recovered on the filter clothe ?

 

Thanks@Breizh

 

Legacy equipment. Came with 5 mm holes. I cannot find a design reason for this size selection.

 

If going for larger holes might fix the problem we can try larger holes. Would be an easy modification. Any idea what are typical hole sizes on such equipment? 10 mm?  

Edited by curious_cat, 03 December 2015 - 01:17 AM.

[curious_cat]

Quantity of product crystals recovered on filter cloth is approx. 80 kg /hr (current load; both filters combined; wet basis). 

 

At peak load we hope to recover 200 kg/hr. 

 

The filtration area is approx. 1.8 m2. (per filter)

Edited by curious_cat, 03 December 2015 - 01:29 AM.

[breizh]

Hi ,

To me should be trial and error , let you start with 10 mmm , I guess you can prepare different screens , not so expensive .

Good luck

Breizh

Edited by breizh, 03 December 2015 - 07:38 AM.

[breizh]

Hi ,

why don't you   consider to have a crystallizer upstream to cool down your solution prior to filtration ?

 

Breizh

[curious_cat]

 

why don't you   consider to have a crystallizer upstream to cool down your solution prior to filtration ?

 

I guess I could try that but then the filtered solution would have to be re-heated again. This filtration is a part of an evaporator. 

 

That's why I think cooling the solution does not make sense. What do you think?

 

Basically, starting with a NaCl solution the objective is to end up with only reasonably pure water & NaCl-solids (wetness OK). 

Edited by curious_cat, 03 December 2015 - 02:05 PM.

[PingPong]

This filter (sketch below) receives a hot (approx. 100 C), saturated slurry (4% w/w) of NaCl and the filter medium is a Nylon woven fabric.

It is not clear to me what you mean with Vacuum (300 - 500 mm Hg). Does that mean that the absolute pressure is 300 - 500 mm Hg? Or that the pressure is 300 - 500 mm Hg below atmospheric?

The boiling point of saturated NaCl solution at 500 mm Hg absolute pressure is less than 100 ^oC, so if the solution is 100 ^oC it is to be expected that at the outlet of the filter part of the water will vaporize and consequently some NaCl will crystallize on the backup plate. The deeper the vacuum, the more vaporization and crystallization.

So maybe the choice of vacuum pressure and/or feed temperature could be adjusted to reduce, or eliminate, vaporization and crystallization at the plate holes.

 

Making the hole diameters bigger will allow longer operation between cleanings, but if the holes are too big the pressure drop over the nylon cloth may push it into the holes and possibly cause cloth tear due to stretching.

Maybe instead of making the hole diameter bigger, it could be better to increase the open area by making new support plates with more 5 mm holes at a smaller pitch than in the present plates.

Or increase the open area by modifying the existing plates to create long rectangular slots, 5 mm wide, X mm long, by cutting away the metal between multiple holes. So two holes plus intermediate distance would become one slot. Or, if hole pitch is small, combine three holes into one slot.

I trust you understand what I mean without a drawing.

Edited by PingPong, 03 December 2015 - 03:23 PM.

[PingPong]

To maximise the open area of the supports you could also consider replacing the existing support plates by a woven wire mesh grid with openings of say 5 mm (or whatever you feel the nylon filter cloth can bridge):

 

 

This will however require support beams underneath the wire mesh grid as such grid will easily bend under the pressure difference over the filter cloth due to the vacuum underneath.