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  • Tank purging with an orifice plate?

    Are you doing it? Care to share your SOP?

    I was reading this MBAA TechTip where the author suggests that running CO2 though a 0.2" orifice plate with a 5 psi differential pressure will deliver 20# of CO2 per hour and purge a 100bbl tank in 4 hrs. Our tanks aren't that big (~18bbl.) so if the method scaled linearly purging should take about 45 mins. Without an O2 meter we're riding blind (but we're doing so already with our current method.)

    I haven't seen reference to this technique before and Googling hasn't turned up anything too helpful.
    Clarke Pelz
    Cynosure Brewing

  • #2
    Not being a member of the MBAA, I can't access this. However, I know the basics of a system being used to remove CO2 from vessels, using an air purge, with either slight air top pressure on the tank, or with completely free access by external air to the top of the tank. They use it for degassing multi-thousand hectolitre FVs prior to cleaning, even though they use acid cleaning - the caustic preshots, as I understand it, work almost as well in a CO2 atmosphere as in a non CO2 atmosphere, so I am not sure why they use it routinely. Perhaps the preshot efficiency is not as effective under CO2 as I understood it to be (I accept there will be some deterioration in performance though).

    I would be grateful if someone would forward the article - PM please. Thanks.
    dick

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    • #3
      Thanks Dick. You've got mail.

      I *think* in this case it's a matter of quiescently introducing CO2 through the bottom of the tank. The flow restriction of the orifice and low differential pressure minimize mixing. I'd like to learn a bit more about the technique hence the question. I'm using the pressurize twice to 5 psi method of the oft cited (but never produced) Weihenstephan paper, but I'm losing faith.
      Clarke Pelz
      Cynosure Brewing

      Comment


      • #4
        Originally posted by cpelz View Post
        Thanks Dick. You've got mail.

        I *think* in this case it's a matter of quiescently introducing CO2 through the bottom of the tank. The flow restriction of the orifice and low differential pressure minimize mixing. I'd like to learn a bit more about the technique hence the question. I'm using the pressurize twice to 5 psi method of the oft cited (but never produced) Weihenstephan paper, but I'm losing faith.
        Two cycles to 5 psi? That seems awfully low. I don't have any published research on it, but when I worked at a company that made UHP gas purification equipment, we used cycle purging to remove atmospheric oxygen and CO2 from our nitrogen and argon purifiers before lighting them off. Too much O2 would cause the getter to melt down...a spectacular but undesirable outcome. Anyway, as I recall, 3 cycles to around 90 PSIG brought it down to a level that was essentially identical to the purge gas used. I'm sure you can't go that high in your tanks, but I would guess that it will take a lot more than 2 cycles at 5 PSIG to get it low.

        In theory, if you use cycle purging (e.g. dilution), the concentration of undesired gas after one cycle is:

        C1 = C0 ( Pv / Pi )

        Pv is the "vent pressure", typically atmospheric, but in really high purity applications we vented to vacuum. Pi is the purge gas pressure. The pressures are absolute, not gauge (e.g. PSIA, not PSIG). After n cycles, the theoretical concentration is

        Cn = C0 ( Pv / Pi )n

        This is theoretical; in practice you may not get good enough mixing to assume perfect dilution. You should be able to figure out how many cycles at a given pressure will be sufficient to reach your target.

        Continuous purge should work well in a piping system; I don't know about a CCV. But an orifice plate is just a simple device for regulating flow. You could use a rotameter calibrated for CO2 if you wanted to control your purge rate...It seems to me that the MBAA example is sized for the vessel.

        Edit: 2 cycles at 5PSIG is:

        Cn = .21 * (15/20)2 ~= 12%

        So, correct me if I'm wrong, but you're only going from 21% to 12%. Not very much at all!

        Regards,
        Mike Sharp
        Last edited by rdcpro; 02-23-2018, 12:30 PM.

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        • #5
          Thanks for the notes

          This is not purging to remove CO2 as I was referring to, but pretty much exactly the opposite.

          They are simply using the orifice plate to control the flow into the tank to minimise mixing of the fresh CO2 and the residual gas, whatever that may consist of, in the tank. If it goes in too fast, the fountain of fresh CO2 creates so much turbulence in the tank that the fresh mixes pretty thoroughly with the residual gas, and does not form a boundary layer with a fairly clean interface - so you waste a lot of gas. Hence people water filling the tank and blowing out with CO2. However, you could use a gas flow control valve to achieve the same sort of flow rate.

          I believe what the MBAA are doing is showing that an orifice valve is a simpler, less cumbersome (less kit) and cheaper way of achieving the same result as a flow meter controlled flow control valve. It sounds simple, and providing you have a steady supply of fresh CO2 at constant pressure, and can control the pressure at which the excess gas is vented from the top of the vessel, then there should be little risk. You could use a simple manual flow control valve, but setting it so you get a consistent flow every time you purge a tank would require flow meters - so again, adding complexity.

          One thing I would add to that, if I were to use it, on the basis that they say you get a decent interface between fresh CO2 and the residual gas (mainly air), I would trial not purging with so much gas that the whole tank is "pure" CO2, but perhaps only the bottom 25 %. After all, as you fill the tank with liquid, the impure residual gas is going to be vented off anyway, so unless you have a CO2 recovery system, you would be wasting a lost of expensive CO2 if you fill the tank first with "pure" CO2.

          I know there have been discussions about tank purging methods, and not everyone will agree this is feasible, but the MBAA process appears to suggest this could be feasible.

          Don't forget the effectiveness of this will be vastly reduced, to the point of not really working if using nitrogen instead of CO2 as it doesn't layer at all due to having virtually the same density as air

          Having been around the brewing industry for probably too long, I have worked in places when CO2 purging was not practiced!! But generally, if there were a number of vessels to purge, we used process (but not usually deaerated) water fill and empty, but pumped the water from one tank to the next to be purged, so reused it a number of times for cost and time saving. So if you clean under acid, and only vent off every few months when carrying out a caustic clean, internal inspection etc, then reusing water becomes pretty cost effective.
          dick

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          • #6
            Two cycles to 5 psi? That seems awfully low.
            Agreed Mike. Without the referenced paper it's hard to say what the details are. The MBAA TechTip also discusses the approach you mention pressurizing the tank to 15 psi and venting once will halve the air content; twice will quarter it. That's kind of what got me thinking.

            Rotometer to the bottom of the tank seems like a better approach. Now I need to figure how to set it up.
            Clarke Pelz
            Cynosure Brewing

            Comment


            • #7
              Originally posted by dick murton View Post
              [...]
              One thing I would add to that, if I were to use it, on the basis that they say you get a decent interface between fresh CO2 and the residual gas (mainly air), I would trial not purging with so much gas that the whole tank is "pure" CO2, but perhaps only the bottom 25 %. After all, as you fill the tank with liquid, the impure residual gas is going to be vented off anyway, so unless you have a CO2 recovery system, you would be wasting a lost of expensive CO2 if you fill the tank first with "pure" CO2.
              [...]
              I've wondered about this for a long time. Does the CO2 actually maintain that interface for long? Conventional wisdom seems to say so, but...is that good enough for an oxygen-free transfer to a bright? And does the CO2 actually displace the oxygen, or is it simply that the CO2 pools in the low spot, along with the oxygen. I find it hard to reconcile with Dalton's law of partial pressures. When I would cycle purge a long piping system, I would pressurized from one end, and run to the other end to vent. But honestly, I have no idea if that helped or not.

              I imagine that any turbulence in the tank from the CO2 purge would mess the interface up, but a continuous purge while filling might work (similar to how you might bubble CO2 slowly during kettle souring).

              Regards,
              Mike

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