Barometric Leg to drain a slurry from a system under vacuum

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Discussion Overview

The discussion revolves around the feasibility and design considerations of using a barometric leg to drain a slurry from a vacuum system. Participants explore the challenges associated with the presence of solid particles in the slurry and the implications for maintaining a liquid seal at the bottom of the barometric leg. The conversation includes technical design aspects, potential mechanical solutions, and the overall efficiency of the system.

Discussion Character

  • Technical explanation
  • Exploratory
  • Debate/contested

Main Points Raised

  • One participant questions the suitability of a typical "hot well" seal for slurries, suggesting that solids may settle and block it.
  • Another participant proposes using a diagonal screw auger or a narrow squirrel cage paddle to remove solids from the sump, emphasizing the need for slow operation to avoid air bubble injection.
  • There is a suggestion that a centrifugal pump rated for slurry duty might be a simpler alternative to an auger, with the addition of a recirculation line to maintain the liquid seal.
  • Concerns are raised about the density and hardness of the solids, with one participant noting that the solids are similar to common salt and estimating a flow rate of approximately 200 kg/hr.
  • Participants discuss the final destination of the slurry being a Nutsche filter and question whether it is efficient to duplicate vacuum systems or if co-location could be beneficial.
  • One participant suggests circulating the slurry in the well to keep it in suspension and proposes returning filtered water to the well to aid in this process.
  • There is a consideration of minimizing energy consumption and eliminating unnecessary equipment, with questions about the essential nature of the Nutsche filter versus gravity drainage options.
  • Participants express openness to suggestions for simplifying the system, including alternatives to the Nutsche filter.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the design and efficiency of the system, particularly concerning the use of mechanical devices for solid removal and the necessity of the Nutsche filter. The discussion remains unresolved with no consensus on the best approach.

Contextual Notes

Participants acknowledge limitations in their understanding of the solid properties and the specific requirements for the system, which may affect the proposed solutions. There are also unresolved questions about the implications of recirculating water and the overall system efficiency.

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?

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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.
 
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Baluncore said:
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.

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.
 
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.
 
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Baluncore said:
I do not know the density or hardness of the solids expected, nor how many kg you expect per day.

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.
 
rollingstein said:
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.
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.
 
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Baluncore said:
Is it only slurry from the barometric pipe that goes to the N
utsche filter ?

Yes.

Baluncore said:
If so, why duplicate the vacuum systems when you could co-locate them.

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?

Baluncore said:
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.

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 apparent 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.
 
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 ?
 
Baluncore said:
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.

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

Baluncore said:
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 ?

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.