HOMEBREW Digest #5906 Thu 01 March 2012

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  Keg flushing (Paul Edwards)
  removing  oxygen from kegs ("Dave Burley")
  Flushing ("A. J. deLange")
  Keg Purging (Patrick Babcock)
  Evacuating a cornie keg (Patrick Babcock)
  Re: Flushing (mossview5)

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---------------------------------------------------------------------- Date: Thu, 1 Mar 2012 07:55:13 -0500 From: Paul Edwards <sdrawdep7821 at sbcglobal.net> Subject: Keg flushing Bill, you wrote to HBD: "yes, but CO2 is heavier than air, would sink to the bottom after you let it sit for a while and the lighter gases would purge out of the relief valve." That's not how gas mixtures work. Gases obey Dalton's law of partial pressure amongst other gas laws. The gases in a mixture mix uniformly (assuming the don't chemically react with each other), with each gas exerting it's pressure on the entire vessel. The total pressure in the vessel is equal to the sum of the individual partial pressure from each gas. http://chemistry.about.com/od/gas2/a/gasproperties.htm - --Paul Edwards Foam Blowers of Indiana (FBI) Central indiana Alliance of Beer Judges (CIA) "We tap kegs, not phones" Return to table of contents
Date: Thu, 1 Mar 2012 09:17:45 -0500 From: "Dave Burley" <Dave_Burley at charter.net> Subject: removing oxygen from kegs Brewsters, Devonna, I can imagine the keg might collapse. Flat sided paint thinner cans collapse when evacuated. - ------- Bill Keiser, One of those gas laws is that a gas occupies the entire volume of the container. It is a common misconception that gases will separate due to differrences in density. Doesn't happen. - ---------- Keep on Brewin' Dave Burley > Return to table of contents
Date: Thu, 01 Mar 2012 09:59:04 -0500 From: "A. J. deLange" <ajdel at cox.net> Subject: Flushing Dave B's whole point was that the gasses follow the gas laws so that CO2 will not 'settle to the bottom' (nor will the oxygen and nitrogen float to the top). Here's why that is the case. A Cornelius keg is about 19L. At STP a mole is 22.4 L so lets say a Cornelius keg holds about a mole at room temperature. A mole is 2E23 molecules. A liter is 1000 cc and a cc is 1cm x 1 cm x 1 cm = 1E8 x 1E8 x1E8 = 1E24 cubic A (Angstrom unit) so a liter is 1E27 cubic A and the keg about 2E28. Into that keg we put 1 mol of gas so each molecule has 2E28/2E23 = 1E5 cubic A allocated to it. This would be the volume of a cube 46.4 A on an edge. A carbon dioxide molecule is linear and with 2 bonds 1.16A in length we might say it is 4.6 A long so it would fit entirely in a 4.6A x 4.6A x 4.6A box. There are 1000 of these in a cube 46.6A on an edge. Thus the keg is empty for all intents and purposes. Less than 1% of it's volume is occupied. Looked at another way, it contains molecules 3 or 4 A long separated by an average distance of 10 times their length. This is at atmospheric. Go to 3 times atmospheric and the number of molecules trebles which reduces the linear separation by the cube root of 3 (1.44). From 10 times to 6.9 times. Three percent of the volume is taken up. The other part of the story is that the gas is at room temperature so these molecules aren't just sitting there. They are whizzing past one another and whacking into each other and the walls of the container. This is what is responsible for the pressure. According to the kinetic theory of gases, vrms, the root mean square velocity of a particle at temperature T (Kelvins) is vrms = sqrt(3*Kb*T/m) where Kb is Bolzman's contant (1.381E23 J/K ~ kg-m2/K-sec2) and m the mass of a carbon dioxide molecule (.044/2E23 kg). Thus vrms = sqrt(3*293*1.381/.044) = 190 m/sec ~ 424 mpH (T = 293K ~ 20 C). For a lighter oxygen molecule vrms = sqrt(3*293*1.381/.032) = 223 m/sec And for nitrogen vrms = sqrt(3*293*1.381/.028) = 238 m/sec For perspective note that a ball bearing released into the mouth of a Cornelius keg would be accelerated by gravity to about 1 m/sec by the time it hit the bottom. Also consider the case of a molecule traveling 200 m/s that hits the wall of the container dead on and bounces back at the same velocity. Assuming the interaction with the wall takes a microsecond that molecule has been subjected to an average force 4E8/9.8 ~ 4E7 times greater than the force of gravity. Carbon dioxide, nitrogen and oxygen are not ideal gasses at 1 - 3 atmospheres but they are pretty close. Non ideality would make the velocity calculations more complicated but the basic concept of molecules whizzing around in a nearly empty container should make it clear that the longer you wait, the better mixed the gasses will become. Return to table of contents
Date: Thu, 1 Mar 2012 10:27:44 -0500 From: Patrick Babcock <patrick.babcock at gmail.com> Subject: Keg Purging Greetings, Beerlings! Take me to your lager... I've been enjoying the discussion regarding CO2 purging. My approach has always been to remove the bung and purge by attaching my CO2 tank to the liquid-out connector at relatively low pressure until I can detect CO2 at the opening (sniff gently - CO2 will combine with the H2O in your nose to form carbonic acid. You'll definitely feel the burn. Sniff too deeply, and you'll feel the effects of a CO2-rich environment in your lungs...). At this point, I put the bung back in, open the vent, and increase the pressure and run it for about a minute. ***Note that this does not eliminate air in the keg, but makes it so rich in CO2 that the remaining oxygen is significantly reduced from that of the air originally contained. It is pretty much impossible to totally eliminate oxygen under the conditions most of us operate under. In terms of densities of gases, though they will not spontaneously separate, a high density gas can exist separated from a lower density gas. This is why propane-fueled appliances in basements basements comes with such dire warnings. In any case, for our purposes, the dynamics involved are not dissimilar to the pouring of liquids of varied densities into parfait drinks such as black & tans. Note that such separation is *NOT* typically steady state, and mixing, where chemically possible, will immediately begin to occur at the interface of the two. The rate of this mixing is dependent on many factors, including environment boundaries, turbulence, temperature, density disparity, and reactivity between the two - but it is, in the integral sense, *not* instantaneous. The time-bound nature of this mixing is what we depend on to create our CO2 rich, captive environment. See ya! Pat Babcock Chief of HBD Janitorial Services. Return to table of contents
Date: Thu, 1 Mar 2012 13:28:22 -0500 From: Patrick Babcock <patrick.babcock at gmail.com> Subject: Evacuating a cornie keg Greetings, Beerlings! Take me to your lager... On evacuating cornies, the cylinder is the strongest container shape, next to a sphere - it's just that spheres of any volume are so dagnabbit inconvenient to move about, store, or, for that matter, manufacture. In any case, the cornelius bung is effectively a spring-loaded negative pressure relief valve, and would begin to draw air long before there'd be enough negative pressure to cause the walls to collapse. See ya! Pat Babcock HBD Chief of Janitorial Services Return to table of contents
Date: Thu, 1 Mar 2012 15:17:04 -0500 From: mossview5 <mossview5 at gmail.com> Subject: Re: Flushing I'm pretty sure its a myth that CO2 will stay separated from air and settle to the bottom of any vessel. Brownian Motion and Diffusion theory suggest that once the gas is introduced into the vessel, that eventually the concentration of all of the gases in the vessel will reach equilibrium at their constant concentration across the entire vessel. There is no reason that CO2 would separate into its own separate layer, especially since it was jetted into the keg, which pretty well mixes all the gases together in the first place. By this mythical contention, we should find pockets of CO2 in low lying areas of the world all the time. I'm pretty sure that doesn't happen too much. The only reason we have a 'blanket' of CO2 over a fermenter is that the gas is actively evolving from the beer during fermentation. If the fermenter is open, even that blanket of CO2 will diffuse into the rest of the atmosphere. Martin Brungard Return to table of contents
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