Can You Help Me Understand CO2 in Forced Carbing??

[doverox]

Hi,
So, I just got a job at a brewery. I am essentially a paid intern. I have homebrew experience but have only bottle conditioned. Also, I am horrible at science/math, I'm very much left-brained and have always struggled with these things.
I really don't have a grasp of CO2 and it's role in cleaning kegs, filling kegs, and force carbing in the tank.
I don't understand all the purging and refilling. For example today, I was purging clean kegs to prep them to be filled. And I guess I don't get the concept.
I go through the motions but I don't understand the science behind it. Can someone please help me?
And please don't say I'm in the wrong job, I'm just learning is all, I'm not expected to fully know these things.
Thanks in advance.

 

[Helloworld]

Purging the kegs is probably to get all the air in the keg out, thus reducing the risk of oxidation.

 

[scottballz]

Purging the kegs is probably to get all the air in the keg out, thus reducing the risk of oxidation.

Right on! you are prepping the clean kegs to be filled at this point. You are adding pressurized CO2 to the keg and this pressure forces out any oxygen inside the keg so when you go to fill it with delicious beer no oxygen gets introduced into the beer.

 

[Lbm]

The reason is CO2 is heavier than O2, so the CO2 stays on the bottom while you vent the O2 out.

 

[Lbm]

Also, CO2 is what you need to carbonate the beer. When you bottle condition, the yeast eats the extra sugar you add and poops CO2, naturally carbing the beer. If you force carb in the keg, that is just injecting CO2 (instead of adding sugar) that eventually will be assimilated into the beer. There are charts that show pressure vs temp for desired volumes of CO2. I'm sure they give you that info if you're doing that, though. You can search the web and find the charts. Different beers have different vol CO2 requirements/norms.

 

[doug293cz]

Purging the kegs is probably to get all the air in the keg out, thus reducing the risk of oxidation.

Getting the oxygen out is one reason, but you also want to get the nitrogen out.

The equilibrium carb level (volumes of CO2) is a function of temperature and the partial pressure of CO2 in the headspace above the beer, and having air in the keg will reduce the partial pressure of the CO2 thus affecting carbonation levels. In a mixture of gases, the partial pressure of one gas is the pressure that would exist if none of the other gases were present. So, if I have a fixed volume filled with air at atmospheric pressure (14.7 psi) and I add CO2 until the pressure on the gauge reads 12 psi, then the total pressure in the volume will be 14.7 + 12.0 = 26.7 psi. The partial pressure of O2 will be about 0.2 * 14.7 = 2.94 psi, of N2 about 0.8 * 14.7 = 11.76 psi, and the CO2 partial pressure will be 12.0 psi.

Now the carbonation charts and calculators are based on gauge pressure and assume that all of the gas present is CO2, so that the CO2 partial pressure is 14.7 + the gauge pressure. Let's do an example based on the chart below:



If I want 2.6 volumes of CO2 @ a beer temp of 45°F, then I need to have a CO2 partial pressure of 14.7 + 16.0 psig = 30.7 psia. If I don't purge the air then the total absolute pressure will be 30.7 psia, but 14.7 psi will be from the original air, and the partial pressure of the CO2 will only be 16 psi. If the air wasn't present, then my CO2 gauge pressure would only be 1.3 psi. This would only give a carb level of about 1.35 volumes (interpolating in the chart.) You need to purge the air in order to get the carb level specified by the chart when you can only measure the total pressure.

Getting rid of all the air isn't as simple as it might seem. If I have a volume filled with air at atmospheric pressure (14.7 psia), and add 14.7 psi worth of CO2, the gauge will read 14.7 psi. The gas in the volume will be 1/2 air and 1/2 CO2. If I then vent to the atmosphere the absolute pressure will drop back to 14.7 psi, but the gas will still be 1/2 air and 1/2 CO2. So, I haven't gotten rid of all the air (or O2) yet. If I repeat the process, the mixture after venting will be 1/4 air and 3/4 CO2. Each time I repeat the process, I cut the amount of remaining air in half. To reduce the number of steps required to reduce the air fraction in the volume, I can use higher CO2 pressures at each step. If I fill to 29.4 psig, then the mix is 1/3 air and 2/3 CO2, and after the second iteration it is 1/9 air and 8/9 CO2.

Hope this helps.

Brew on

 

[MagicMatt]

The reason is CO2 is heavier than O2, so the CO2 stays on the bottom while you vent the O2 out.

This is inaccurate for a couple of reasons. First, gases diffuse. The whole "CO2 blanket" is a complete myth. Carbon dioxide is not "heavier" than oxygen, but it is denser. So CO2 will, on average, equilibriate slightly lower than O2 in a gravitational field. But the difference in the force of gravity is very small compared to the random thermal motion of the molecules, thus the effect is negated in fermentation scenarios. Consider this:

You have an empty keg that you just sealed. The entire volume of the keg is filled with gas (atmospheric levels & pressure). Now if you pump in CO2 to say 15psi, the CO2 will mix with the gases inside. The gases are evenly mixed and as the pressure inside the keg is raised, it will become higher in CO2 concentration than other gases. If you then disconnected the CO2 and left the keg sitting for a week, you wouldn't find that the CO2 has "settled" out or anything of the sort. There will be next to no stratification of the diffused gases.

Just trying to help prevent the spread of misinformation. It's been discussed a lot around here though.

 

[Iseneye]

That's some good stuff Doug. I used to purge kegs with C02 before filling but based on some HBF advice I now fill the kegs with water / sanitizer and push the water out from one keg to another keg (or sink) via C02 tank. That leaves the keg with nearly pure C02.

 

[doug293cz]

That's some good stuff Doug. I used to purge kegs with C02 before filling but based on some HBF advice I now fill the kegs with water / sanitizer and push the water out from one keg to another keg (or sink) via C02 tank. That leaves the keg with nearly pure C02.

That method works, and will get a much lower residual air level in one iteration, but will use more gas than one iteration of gas purging only. If I get motivated, I might do some calculations to see which method uses less CO2 to achieve a particular level of residual air.

Brew on

 

[Hoppity]

If CO2 settled to the bottom, valley dwelling people would not exist. I do, however, purge kegs and bottling buckets with CO2 every time I move beer. Call me paranoid.

 

[Lbm]

I understand the whole mixing thing, but on a molecular level, a molecule of CO2 is heavier than a molecule of O2 so per equal volumes the weight of CO2 is more than O2.

I think it's more of a simplification to think of it that way (that the CO2 "pushes" the O2 out), even though not technically true. You're still effectively reducing the O2 in the keg, which is the goal.

I have been told, though, that over long periods of time, gases in a cylinder will stratify in the absence of external force. In a totally different application that doesn't apply to this at all, we either heat or roll gas cylinders to prevent stratification of the gases. Of course, the time period is much longer than it takes us to finish a keg!

 

[chisena]

Getting the oxygen out is one reason, but you also want to get the nitrogen out.



The equilibrium carb level (volumes of CO2) is a function of temperature and the partial pressure of CO2 in the headspace above the beer, and having air in the keg will reduce the partial pressure of the CO2 thus affecting carbonation levels. In a mixture of gases, the partial pressure of one gas is the pressure that would exist if none of the other gases were present. So, if I have a fixed volume filled with air at atmospheric pressure (14.7 psi) and I add CO2 until the pressure on the gauge reads 12 psi, then the total pressure in the volume will be 14.7 + 12.0 = 26.7 psi. The partial pressure of O2 will be about 0.2 * 14.7 = 2.94 psi, of N2 about 0.8 * 14.7 = 11.76 psi, and the CO2 partial pressure will be 12.0 psi.



Now the carbonation charts and calculators are based on gauge pressure and assume that all of the gas present is CO2, so that the CO2 partial pressure is 14.7 + the gauge pressure. Let's do an example based on the chart below:



View attachment 277877



If I want 2.6 volumes of CO2 @ a beer temp of 45°F, then I need to have a CO2 partial pressure of 14.7 + 16.0 psig = 30.7 psia. If I don't purge the air then the total absolute pressure will be 30.7 psia, but 14.7 psi will be from the original air, and the partial pressure of the CO2 will only be 16 psi. If the air wasn't present, then my CO2 gauge pressure would only be 1.3 psi. This would only give a carb level of about 1.35 volumes (interpolating in the chart.) You need to purge the air in order to get the carb level specified by the chart when you can only measure the total pressure.



Getting rid of all the air isn't as simple as it might seem. If I have a volume filled with air at atmospheric pressure (14.7 psia), and add 14.7 psi worth of CO2, the gauge will read 14.7 psi. The gas in the volume will be 1/2 air and 1/2 CO2. If I then vent to the atmosphere the absolute pressure will drop back to 14.7 psi, but the gas will still be 1/2 air and 1/2 CO2. So, I haven't gotten rid of all the air (or O2) yet. If I repeat the process, the mixture after venting will be 1/4 air and 3/4 CO2. Each time I repeat the process, I cut the amount of remaining air in half. To reduce the number of steps required to reduce the air fraction in the volume, I can use higher CO2 pressures at each step. If I fill to 29.4 psig, then the mix is 1/3 air and 2/3 CO2, and after the second iteration it is 1/9 air and 8/9 CO2.



Hope this helps.



Brew on

Listen to this guy! Love it.

 

[doug293cz]

I understand the whole mixing thing, but on a molecular level, a molecule of CO2 is heavier than a molecule of O2 so per equal volumes the weight of CO2 is more than O2.

CO2 is about 1.5 times as heavy as air (similar to the NO2 shown later in the video.) Watch the mixing in the video. Bromine gas is 5.7 times heavier than air, and they mix spontaneously due to diffusion.

[ame]https://www.youtube.com/watch?v=_oLPBnhOCjM[/ame]

I think it's more of a simplification to think of it that way (that the CO2 "pushes" the O2 out), even though not technically true. You're still effectively reducing the O2 in the keg, which is the goal.

Thinking of it this way can lead you to false conclusions about how to most effectively remove O2. To understand what is going on you must think in terms of mixing and dilution, as discussed in my previous example.

I have been told, though, that over long periods of time, gases in a cylinder will stratify in the absence of external force. In a totally different application that doesn't apply to this at all, we either heat or roll gas cylinders to prevent stratification of the gases. Of course, the time period is much longer than it takes us to finish a keg!

Can you provide more specifics? Based on the spontaneous interdiffusion of bromine and air (video above), I can't see where spontaneous stratification could occur. If there really is something causing stratification in your application, I would like to understand it.

Brew on

 

[MagicMatt]

I have been told, though, that over long periods of time, gases in a cylinder will stratify in the absence of external force.

I'd have to disagree. At the very least, you'd need some force to cause the stratification (namely gravity). But it'd have to be some serious gravitational effort, if it could be done at all. Much more than anything we'd experience here on our planet.

That's a great video, Doug!

 

[Lbm]

I was just trying not to get too "science-y" about it. IME a lot of people tend to just zone out and eyes glass over if an explanation is too technical. Plus I'm sure I'm not as smart as a lot of the people on these boards and someone else who has a better background can give the nitty gritty details if need be.

I'm glad there are so many folks here who do have the knowledge and background to give a more detailed explanation. I pretty much knew when I posted that someone would correct the post LOL. I look forward to it. I always appreciate getting a deeper understanding.

I just recall doing those experiments where you pour CO2 gas from one container to another to put out a flame, like this:

http://youtu.be/2Mgvt20LOOg

So I just assume (<- yeah, I know lol) that the diffusion of CO2 and O2 would take a little time.


The other application is cylinders of gas to calibrate equipment and the stratification isn't spontaneous. Supposedly the gases stratify after sitting undisturbed for a year or so. I have no proof of this, just what I was told by the experts.

 

[doug293cz]

I just recall doing those experiments where you pour CO2 gas from one container to another to put out a flame, like this:

http://youtu.be/2Mgvt20LOOg

So I just assume (<- yeah, I know lol) that diffusion of CO2 and air takes a little time.

Yeah, the diffusive mixing does take time. That's why you can take the airlock off your fermenter for a few minutes and not get a lot of O2 in it, or lose much of the CO2. So, you can have a CO2 blanket for minutes, but not hours or days (unless you are continuously regenerating the CO2.) If the kids had waited half an hour or so after putting CO2 in the measuring cup before pouring the cup over the candle, it would have been a pretty boring video.

Brew on

 

[Lbm]

I'd have to disagree. At the very least, you'd need some force to cause the stratification (namely gravity). But it'd have to be some serious gravitational effort, if it could be done at all. Much more than anything we'd experience here on our planet.

That's a great video, Doug!

I tend to doubt the statement, too. But then, I'm not the expert.

I suspect that's what they tell us because when they mix the gas to specs, it is stratified and they have to roll it to mix it properly. Initially, the gases under high pressure going into the cylinder act like liquids in that they first stratify and will mix very slowly. Once all the gases have mixed, though (via rolling the cylinder or time passing), I don't really understand the concept that they would again stratify unless you drop the temp enough (and pressure is high enough) that you get state changes of some of the components and they liquefy. I guess maybe they tell us that because the gas cylinders are under high pressure and they don't know the storage conditions.

 

[rijnswand]

I just recall doing those experiments where you pour CO2 gas from one container to another to put out a flame, like this:

http://youtu.be/2Mgvt20LOOg

So I just assume (<- yeah, I know lol) that the diffusion of CO2 and O2 would take a little time.

There's another problem with that experiement: when you release gas from high pressure container, the temperature of the gas drops considerably. Cold gas is much heavier, so it naturally sinks down. That experiment might even work with helium.