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Barometric Leg to drain a slurry from a system under vacuum

  1. Apr 12, 2015 #1

    rollingstein

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    Often to drain liquids from a vacuum a tall (32 feet) barometric leg is used. Can I use such a leg to drain a slurry?

    The vacuum is approx. 200 mm Hg abs & the slurry has 5% to 10% hard crystalline solids in water.

    In particular how do I design the liquid seal at the bottom of the barometric leg? I'm posting a sketch of a typical "hot well" seal but this won't work for slurries, right? The solids would settle & block the hotwell.

    Any ideas?

    ptZCNVG.jpg
     
  2. jcsd
  3. Apr 12, 2015 #2

    Baluncore

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    Maybe you need a diagonal screw auger or a narrow squirrel cage paddle of large diameter to remove the solids from the sump.
    Whatever form the mechanical sweeper takes, it needs to be very slow so it does not inject air bubbles into the water.
     
  4. Apr 12, 2015 #3

    rollingstein

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    You think a centrifugal pump rated for slurry duty would work? Might be cheaper / simpler than an auger. I could add a re-circulation line to ensure the seal liquid does not empty.
     
  5. Apr 13, 2015 #4

    Baluncore

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    I do not know the density or hardness of the solids expected, nor how many kg you expect per day.
    For minimum energy, I would select a low–speed, low–power mechanism that does not cause turbulence.
    Unless the solids are being transported in suspension for recycling, pick a method that tips the solid over the edge without too much water make-up being needed.
     
  6. Apr 13, 2015 #5

    rollingstein

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    The hardness, shape & density of these crystals are very close to common salt, NaCl.

    The quantity isn't large. Approx. 200 kg/hr.

    The final destination of the slurry is a Nutsche filter where the liquid gets sucked below the cloth filter by a vacuum & the solids get periodically raked out manually.
     
  7. Apr 15, 2015 #6

    Baluncore

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    Is it only slurry from the barometric pipe that goes to the Nutsche filter ?
    If so, why duplicate the vacuum systems when you could co-locate them.
    If not, circulate the slurry in the well to keep it in suspension while you circulate water between the filter and the well. If possible, to reduce wear of the pump, pump filtered water back to the well rather than slurry out of the well. The return flow of water to the well could drive the circulation in the well.
     
  8. Apr 16, 2015 #7

    rollingstein

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    Yes.

    How can I do that? Can you elaborate on this idea? Normally the Nutsche filter has a vacuum applied to the underside of the filter medium & the top of the filter medium is open to atmosphere. The difference provides the driving force for filtration. What do you mean by co-location?

    Agreed about the slurry circulation. Will need to do that.

    The idea about recirculating the filtered water is a good one. Though it will increase the apparant load on the filter, right? Because of the additional internal recycle.

    I was thinking of just adding a tee on the discharge of the slurry pump and circulating a part of that flow back into the well through a sparging ring in order to maintain the suspension.
     
  9. Apr 18, 2015 #8

    Baluncore

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    Sorry about the delay, it is a very busy month here, and I am now recovering from the flu. I need to think more clearly about the minimum possible system. It is easy to add more energy consuming equipment, but it is better to increase efficiency and eliminate or delay processes that are not essential. I am trying to see how to eliminate a pump and turbulence in the sump.

    What is special about the Nutsche filter at the end of the process ? Is it essential or could the product be dried by gravity drainage ?
     
  10. Apr 19, 2015 #9

    rollingstein

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    Indeed. I'm trying for the simplest system possible too. Thanks for helping out.

    What sort of gravity drainage? Nutsche seemed the simplest / fast system for seperating 5% solids from the mother liquor. Other option I considered was a filter press.

    But I'm open to sugesstions. Especially if they will lead to a simpler system.
     
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