Wednesday, January 30, 2013

Process Wednesday: sight glasses

A fairly standard sight glass
Credit: John C. Ernst
So we've covered problem separations here before, but this is a really neat little suggestion from Francis X. McConville's "The Pilot Plant Real Book" on problem liquid-liquid separations when the issue is that you can't see the phase split:
Occasionally, the interface is so clean that it is impossible to see. In such cases, a conductivity measurement can be most useful to distinguish between the aqueous and the organic phase. It is also useful to sprinkle a pinch of Celite (diatomaceous earth) into the batch. The Celite tends to collect at the interface and helps to accentuate it. The Celite of course must be removed later by polish filtration, and so this is not recommended during the final processing steps. 
One of the things that I've mentioned before is that sometimes, it's actually quite difficult to see the reaction in the plant, because you can't see through the reactor walls like you can with a separatory funnel. You have all sorts of visual cues for a separation through a all-glass separatory funnel: you can typically watch the little bubbles of phases, the vortices, you can shine a flashlight into it (great trick, works like a charm.)

But when you're working with a large enough reactor, the walls are not going to be made of glass -- then, you're going to have to rely on a sight glass.* Sight glasses are smallish glass cylinders attached to the bottom valve of the reactor that your solutions flow through. Experienced operators have told me that they've blinked as they're pumping solutions through the sight class, missed the phase cut and had to send the entire 2,000 gallon batch back around through the reactor (which, of course, costs time and money, etc.) That Celite trick might help a little -- I'll have to try it sometime.

*Unless, like Mr. McConville notes, you're relying on conductivity measurements or a density flow meter.

7 comments:

  1. Stewie Griffin:
    Why not just use ice cubes in place of celite?

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    1. aww... cute. I remember people using colored NMR tube plastic caps for the purpose - they float in water but sink in many organic solvents that form upper layer on water

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    2. Stewie Griffin:
      Thanks... I think??
      I was serious. Celite shows the phase separation but then has to be removed via polishing filter. Ice cubes should be easy to see in the sight glass, and ice doesn't have to be removed.

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    3. It sounds like an idea worth trying -- if I had concerns, it would be about the ice cubes melting by the time they got to the interface? I dunno. It's certainly worth a shot.

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  2. Here's a trick for "tricky" phase cuts we've used in the past:

    1) weigh bottom layer from lab scale reaction
    2) calculate predicted weight of bottom layer for scale up run
    3) separate abottom layer directly into drums on a balance, such that you have a good idea when you are nearing the phase cut

    Should work just as well using volume estimates if your reactors have level indicators, but not every reactor has them. A balance is also usually much more accurate than a level indicator.

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    1. This is a good idea. I like it.

      (Hey, Ken, thanks for all your comments on PW posts recently. They're high quality; I hope my posts continue to inspire them.)

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  3. I can verify Celite trick works great in real world. Rather than a "pinch" would use about a tablespoon worth.

    I swear some of the old operators could make the cut by ear as they would listen to the down flow and could tell when solvent changes pass through the valves.

    Ken what you state should be SOP/understanding for all process chemists to always extrapolate bench observation into the plant. Therefore can estimate where the phase cut should be and do an initial rapid transfer to get close then slow down to fine control for final cut.

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