FORUM ON BEER, HOMEBREWING, AND RELATED ISSUES
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Contents:
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"
------------------------------
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
>
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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.
------------------------------
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.
------------------------------
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
------------------------------
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
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End of HOMEBREW Digest #5906, 03/01/12
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