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Old 09-08-2013, 03:32 PM   #1
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Default Stratification of gases in closed environments (Like fermentors and kegs)

I posted an unproven theory of mine in another thread. That theory was that if I filled a new keg with CO2 that the CO2 might settle to the bottom in a few hours and then push more of the O2 out the top when I purged. Another poster made the point that gases mix very quickly and so this theory has no merit. You can find the conversation starting on post 9 here:
http://www.homebrewtalk.com/f35/tran...g-kegs-395265/

This conversation got me thinking.

I know gases mix easily in open environments because of currents and convection. Any movement in an environment will facilitate mixing. Is this the case in a closed environment like a keg or fermentor? In a fermentor there are yeast producing CO2 during fermentation and the beer is off-gassing afterwards, so there is bound to be some type of convection going on for quite a while I would think. If that is the case and I displace some of that CO2 with a wine thief then the air that replaces it will mix very quickly with the CO2 in the headspace.

I have always thought of the CO2 in a fermentor or keg as a protective blanket or layer. If any introduced air is quickly mixed with this layer then it isn't a protective blanket at all. I have been browsing the web for a while trying to find research to back up either side of this argument and so far it looks like stratification of gases is pretty difficult to obtain to any meaningful degree unless you have a very tall column of gas (much taller than we are talking about with brewing equipment).

I was hoping to get input from others on this subject and hopefully come to a conclusion one way or another.

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Old 09-08-2013, 03:48 PM   #2
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Here is an interesting document on CO2 stratification in caves.

http://caves.org/pub/journal/PDF/v71/cave-71-01-100.pdf

The author postulates that CO2 is found in low places in caves mainly because that is where it is being created in the first place. It then takes a while to disperse into the environment. Because of this one might think that it had settled, when in actuality it just hasn't had a chance to go somewhere else yet.

I am a diver and I started thinking about air mixes in cylinders. If you make nitrox by mixing in cylinder you are supposed to tumble the cylinder to mix the gases. Once the gases are mixed they are considered stable, although you should always check with a meter before using the cylinder.
This would tend to show that if gases are separate in a closed environment they might not mix quickly or at all on their own, but once they are mixed they will stay mixed. Here is some supporting math and confirmation by a professor of science that gases don't un-mix:

http://www.newton.dep.anl.gov/askasc.../chem03325.htm

So the question becomes how quickly and easily do they mix in the first place? If they mix easily then the CO2 blanket theory is unsound.

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Old 09-08-2013, 03:55 PM   #3
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Quote:
Originally Posted by Setesh View Post
http://www.newton.dep.anl.gov/askasc.../chem03325.htm

So the question becomes how quickly and easily do they mix in the first place? If they mix easily then the CO2 blanket theory is unsound.
I'm NO scientist, so I'll look forward to a real scientist coming and and giving us the proper explanation.

I can give you my anecdotal experience, as well as my thoughts, though!

First, I'm always troubled by this "co2 blanket" theory, as even in an active fermentation the headspace is never 100% c02. It may be mostly co2, and during active fermentation this is not a concern. But after fermentation, this protective c02 blanket will not be there forever.

The way I think of it is this- we inhale air (using the oxygen in it) and exhale c02 (for the most part- again, never 100%). If this "c02 blanket" was as persistent as people seem to think, we'd all die in our sleep from co2 poisoning as the co2 would never rise.

I understand that in a carboy with an airlock, that there would be a significant amount of co2 in the carboy. But since it's never going to be 100% full of co2 (even with purging), that oxidation can occur in the long term. That's why winemakers top up, reducing any headspace, so that the area exposed to air is minimized. They also use antioxidants (sulfites).

If you look at the infections posted on this forum, they are almost always either in a bucket with wide headspace, or in secondary with headspace. Oxygen loving bacteria will take hold in an area with headspace, sooner or later, usually.
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Old 09-08-2013, 04:13 PM   #4
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OK, so gases mix MUCH faster than liquids. This process is called diffusion. Basically particles of gas naturally want to go from areas of higher concentration to areas of lower concentration. This naturally balances the mixture. The rate can be calculated by using Grahams Law:

http://library.thinkquest.org/12596/graham.html

So as far as I can see, there is no such thing as a CO2 'blanket'. I think of a blanket as a cover, something that is not easily passed through. A CO2 layer is present after fermentation, but it is a volatile layer, not a protective blanket. Any air that is introduced into the headspace will mix with the CO2, not lay on top of it.

So thank you very much day_trippr for starting me down this path of education. I love learning new things, and discovering I'm wrong about something is usually the first step in the process

I really hope a scientist or other big brained individual can come along and lay this out for us in lay terms because I can't seem to calculate how long diffusion takes. I tracked down the molecular weights of gases here:
http://www.engineeringtoolbox.com/mo...or-d_1156.html
and the calculator here:
http://www.1728.org/graham.htm
But the answer is still meaningless to me. What does a rate of .8 mean? Is that fast or slow? How many seconds/minutes is that?
Anyone?

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Old 09-08-2013, 04:35 PM   #5
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Quote:
Originally Posted by Yooper View Post
I'm NO scientist, so I'll look forward to a real scientist coming and and giving us the proper explanation.

I can give you my anecdotal experience, as well as my thoughts, though!

First, I'm always troubled by this "co2 blanket" theory, as even in an active fermentation the headspace is never 100% c02. It may be mostly co2, and during active fermentation this is not a concern. But after fermentation, this protective c02 blanket will not be there forever.

The way I think of it is this- we inhale air (using the oxygen in it) and exhale c02 (for the most part- again, never 100%). If this "c02 blanket" was as persistent as people seem to think, we'd all die in our sleep from co2 poisoning as the co2 would never rise.

I understand that in a carboy with an airlock, that there would be a significant amount of co2 in the carboy. But since it's never going to be 100% full of co2 (even with purging), that oxidation can occur in the long term. That's why winemakers top up, reducing any headspace, so that the area exposed to air is minimized. They also use antioxidants (sulfites).

If you look at the infections posted on this forum, they are almost always either in a bucket with wide headspace, or in secondary with headspace. Oxygen loving bacteria will take hold in an area with headspace, sooner or later, usually.
I wonder how permeable our rubber bungs and better bottles are? Gases can and will transfer through solids. Here is an excerpt taken from:
http://encyclopedia2.thefreedictiona...s+Permeability

The gas permeability of substances is defined in terms of the penetrability effect P, expressed in units of m4/(sec·N), or cm2/(sec·atm), where 1 cm2/(sec·atm) = 1.02 x 10-9 m4/(sec·N), and by the volume of gas that passes through a unit area (perpendicular to the gas flow) in the body in 1 sec with a pressure differential of 1. The coefficient P depends on the nature of the gas, and therefore the gas permeability of substances is usually compared on the basis of their hydrogen permeability. The values of P, in cm2/(sec·atm), of several materials at 20° C are presented in Table 1.

Table 1. Values of the penetrability effect (P) (20° C)
Metals ............... 10-18 to 10-12
Glass ............... 10-15 to 10-10
Polymers (films) ............... 10-12 to 10-5
Liquids ............... 10-7 to 10-5
Paper, skin ............... 10-5 to 10


So there will be some intrusion into a plastic fermentor by gasses, especially the lighter ones, through the plastic itself, the rubber bung, and the plastic airlock. Glass is going to block gas transport from what I can tell, and so will metal. So once your beer is in a keg it's all down to what's in the headspace. It seems like a bucket is going to be most permeable, then better bottles, and finally a glass carboy will be least permeable.
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Old 09-08-2013, 04:50 PM   #6
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If you want to see gas stratification, look for pictures of Chlorine gas leaks. Chlorine will pool in low lying areas until enough wind comes along to mix and dispurse it.

As for the breathing thing, you don't inhale 100% O2 and what you do inhale, you don't completely convert to CO2.

This is why mouth-to-mouth works. 20% is normal oxygen in air. 15 - 19% decreases your ability to work. 10 - 12% is about as low as you want to go because your lips will turn blue. Below 10% and you'll pass out. Below 6% is fatal.

(These are among the reasons I won't do confined space entry.)

Gases in a 100% closed environment will eventually become completely mixed. But with out agitation, it occurs slowly through random Brownian motion and the Ideal Gas law.

A fermenter is a partially enclosed environment. Gas can get out but can't get in. Plus you have a source of CO2 gas generation at the wort surface. As CO2 is generated at the surface, it displaces the air closest to the air lock. Therefore, my reasoning is that the headspace of a fermenter is like a plug flow reactor. The CO2 generated by the fermentation will eventually push everything that was in the headspace up and out. There will be some mixing along the interface, but not so much to overcome the gradient of CO2 production during the most active phase of fermentation.

Now remember when you pitched the yeast and then aerated the hell out of the wort? You supersaturated the wort with O2. So some O2 will also come out of the wort as it ferments. But it's a small amount compared to the CO2 being produced.

When you rack a beer to a secondary, even though fermentation has stopped, the beer is supersaturated with CO2. So when you slap the airlock on it and see it bubbling, you are seeing plug flow working again. However, in this situation, there probably isn't enough CO2 coming out of solution to completely displace everything that was in the headspace.

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Old 09-08-2013, 04:56 PM   #7
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Above absolute zero all matter has vibrational energy. This will cause gasses to diffuse into one another(mixing). They will diffuse until the mix is homogeneous throughout. Room temp is considerably warmer than abs. zero, so mixing is assured.
A 'blanket' could develop if CO2 were continuously evolved from the wort, and it helps that the wort is on the bottom of the vessel. It would help if the evolved CO2 were cooler than the headspace.
Instead of invoking complex phys. chem. theory, try these: Introduce CO2 from your cylinder; introduce CO2 from an active batch; Fill headspace with a clean, thin, vented(up) plastic bag. As CO2 is generated, bag is collapsed and you eventually remove it; use N2, as I do, if you have it; use(at your own risk) freon from a can. didn't think of that one, did ya? Do that at your own risk, as I cannot be responsible for, well, whatever.
As for permeability through vessel walls, I think it would occur in both directions, and you would smell at least some of the small molecules from the ferment.

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Old 09-08-2013, 05:09 PM   #8
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Quote:
Originally Posted by Beernik View Post
If you want to see gas stratification, look for pictures of Chlorine gas leaks. Chlorine will pool in low lying areas until enough wind comes along to mix and dispurse it.
Chlorine has a molecular weight of 70 so it's very heavy, I assume that is why it settles so easily and is so hard to displace?

Those pictures remind me of dry ice fog on halloween
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Old 09-08-2013, 06:03 PM   #9
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N2 is ~ 28
O2 is ~ 32
CO2 is ~ 44
Cl2 is ~ 71

Chlorine gas leaks are one of those things that make my butt pucker. I've met guys that have been exposed. It's the safety aspect that has driven water suppliers to go to the chloroamines we hate so much in our brew water.

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Old 09-09-2013, 04:15 AM   #10
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Quote:
Originally Posted by Beernik View Post
Now remember when you pitched the yeast and then aerated the hell out of the wort? You supersaturated the wort with O2. So some O2 will also come out of the wort as it ferments. But it's a small amount compared to the CO2 being produced.
I understood that yeast will consume all of the oxygen in suspension: http://www.wyeastlab.com/comm_b_oxygenation.cfm

Interesting thread. I wonder if gasses of similar weights mix more than those of differing weights, or ultimately (like a container with washed yeast) will they all separate into layers.

I know you don't want to have Carbon monoxide in an unventilated basement, for an example similar to the cited Chlorine Gas one.
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