Equilibrium in carbonation

[mlachelt]

Hi all,

I'm a bit foggy on equilibrium. Is this the state reached when co2 dissolves into the liquid from the headspace at the same rate as it leaves the liquid and enters the headspace?

If this is correct, it would seem that equilibrium is reached, and carbonation complete, when the headspace pressure gauge stabilizes at the psi set by the regulator (ie when they are equal).

However, this contrasts with what I know about the process of carbonating: the head pressure quickly matches the psi set by the regulator, but isn't nearly finished.

I wonder where my understanding has gone astray. Is it my assumption that equilibrium is the state reached when the head pressure gauge reads the same as the regulator?

That must be my mistake: I guess the difference is that, while the head pressure quickly reads the same as the regulator, the co2 in the head is still dissolving into the liquid below at a faster rate than it is leaving the liquid. In this state, the regulator will continue to send co2 into the liquid and headspace, in order to keep the pressure at the target.

So, if this is right, equilibrium is reached when the concentration of c02 in the liquid is such that, given the pressure in the head, it comes out of solution as fast as it goes in. In that case, the regulator will not send additional c02 into the tank, and carbonation is complete. In order to carbonate further, one than has to increase the psi set by the regulator.

Is this a correct understanding of equilibrium? If so, my question is: how can you tell if equilibrium is reached? Do you hear a difference in the regulator no longer sending gas into the tank?

Thanks, and apologies for thinking out loud!

 

[day_trippr]

The answer is simple: the rate of absorption is far far lower than the rate of introduction.
Hence the low pressure gauge reflects the regulator setting - pretty much forever and ever, amen...

Cheers!

 

[Wynne-R]

You’re right. CO2 will diffuse from the head space until the partial pressure in the liquid equals the partial pressure in the head space. This takes a while, as gas diffusion into a liquid is a slow process. You’re also correct that you are supplying a bit more gas to maintain the pressure as the CO2 goes into solution. In a closed system that head space pressure would decrease as you approached equilibrium.

As this is a completely reversible process, it’s actually a dynamic equilibrium. Your intuition that gas in = gas out indicates equilibrium is spot on.

 

[doug293cz]

Hi all,

I'm a bit foggy on equilibrium. Is this the state reached when co2 dissolves into the liquid from the headspace at the same rate as it leaves the liquid and enters the headspace?

Correct.

If this is correct, it would seem that equilibrium is reached, and carbonation complete, when the headspace pressure gauge stabilizes at the psi set by the regulator (ie when they are equal).

This is where you go wrong. The regulator controls the pressure in the headspace, not the amount of CO2 entering or escaping from the beer. The regulator will keep letting more CO2 into the headspace as more CO2 dissolves into the beer, thus keeping the headspace pressure constant. Without the regulator (and constant CO2 supply), the pressure in the headspace would drop as more CO2 is dissolved into the beer, until equilibrium is reached, at a pressure lower than the originally set pressure.. The regulator is a device that selectively lets more CO2 into the headspace as needed to keep the headspace pressure constant (https://en.wikipedia.org/wiki/Pressure_regulator.) Therefore, equilibrium will eventually be reached at the pressure set by the regulator.

However, this contrasts with what I know about the process of carbonating: the head pressure quickly matches the psi set by the regulator, but isn't nearly finished.

See above.

I wonder where my understanding has gone astray. Is it my assumption that equilibrium is the state reached when the head pressure gauge reads the same as the regulator?

Yes, that assumption is incorrect. The regulator controls the pressure in the headspace, regardless of the equilibrium pressure for the currently dissolved amount of CO2. Also, see above.

That must be my mistake: I guess the difference is that, while the head pressure quickly reads the same as the regulator, the co2 in the head is still dissolving into the liquid below at a faster rate than it is leaving the liquid. In this state, the regulator will continue to send co2 into the liquid and headspace, in order to keep the pressure at the target.

Correct.

So, if this is right, equilibrium is reached when the concentration of c02 in the liquid is such that, given the pressure in the head, it comes out of solution as fast as it goes in. In that case, the regulator will not send additional c02 into the tank, and carbonation is complete. In order to carbonate further, one than has to increase the psi set by the regulator.

Correct. You finally talked yourself into the right answer.

Is this a correct understanding of equilibrium?

Yes

If so, my question is: how can you tell if equilibrium is reached? Do you hear a difference in the regulator no longer sending gas into the tank?

Thanks, and apologies for thinking out loud!

Determining equilibrium is difficult to do without a separate pressure gauge attached to the keg. If you have a separate gauge connected to a keg gas QD, you disconnect the QD from the regulator, and connect the QD with just the pressure gauge. If the pressure stays stable for several hours (at constant temp of course.) Then you are in, or close to, equilibrium. The longer you wait without seeing a pressure drop, the more confident you can be on how close to equilibrium you are. This is because, if you are far from equilibrium, the pressure in the head space will drop fairly rapidly as CO2 is absorbed into the beer. If you are close to equilibrium, the rate of CO2 absorption goes way down (CO2 absorption rate is roughly proportional to the headspace pressure minus the equilibrium pressure for the amount of CO2 currently dissolved in the beer), so it will take much longer to see a perceptible pressure drop.

If you have a regulator that allows you to bleed the pressure from the high pressure side of the regulator you can monitor equilibrium as above, by turning the tank valve off. You need to vent the high pressure side, or it will take a while for the CO2 on the high pressure side to work its way into the keg. But if the keg is already at equilibrium that will never happen. You would have to wait a very long time to determine if you were not in equilibrium.

Hope it all makes sense now.

Brew on

 

[DrunkleJon]

To try to simplify and be clear at the same time. You have reached equalibrium once the beer has dissolved as much CO2 as it can at the current temperature and pressure.

The pressure of the headspace will not really be a good measurement of the process as long as the keg is hooked up to the regulator which will supply more CO2 as it dissolves into solution.

When it comes to force carbonating beers via equalibrium most here just set the pressure to the desired serving pressure that coincides with the desired volumes of carbonation for the serving pressure. This can be found in tables all over the internet and I am too lazy to look one up right now. Then it is left that way for 1.5-2 weeks (to condition as well) before serving. I have noticed less than perfect, but sufficient carbonation at a little over a week.
Otherwise some burst carbonate to speed the process which involves higher pressures and shaking to allow for more speedy dissolving of the CO2 into the beer. It is easier to overcarbonate this way and results vary depending on pressure, temperature and how much or hard it is shaken.

To answer your last question you can tell, so long as you dont have a leak, by shutting off the gas to the keg for several hours or a day or so and then reconnect it or turn it back on (same pressure) if you dont hear any gas go into the keg you should be there or close.

 

[mlachelt]

Thanks so much for these very helpful replies! I'm presently cooling down my tank to 39 degrees in preparation for carbonation. I'm looking to carbonate 1000 liters of cider to around 2.5 volumes. ("around" 2.5 volumes, because I plan to sample in order to select an appropriate level of carbonation).

Looking at all the charts out there suggests that I'll want to sample what equilibrium is like from about 9 psi (2.25 volumes) to about 12 psi (2.52 volumes).

I guess if I "set and forget" the regulator at 9 psi and wait for equilibrium (sampling along the way, I suppose), that will likely take a fairly long time? (A week, maybe?). My thought is therefore to set the regulator at a higher pressure initially (20?), and then after a period of time bring it down to 9, bleeding the head pressure accordingly (so that it reads 9 as well).

Any practical advice out there for this method? When increasing the regulator setting to speed things up, should I vent the head so that the head remains at 9, or around there? Or should I let the head pressure build up to 20, and vent it down later, when I decide to lower the regulator to 9, in order to reach equilibrium at 9? Also, is there a way that I can assess, or roughly assess, the amount of time I should keep the regulator and tank at the higher pressure, so that I don't end up over-carbonating? My only other question (for now!!) is whether speeding up the process in this way will have any negative effects on the resulting carbonation?

Thanks again for all the replies. This forum has been great.

Mike

 

[doug293cz]

Thanks so much for these very helpful replies! I'm presently cooling down my tank to 39 degrees in preparation for carbonation. I'm looking to carbonate 1000 liters of cider to around 2.5 volumes. ("around" 2.5 volumes, because I plan to sample in order to select an appropriate level of carbonation).

Looking at all the charts out there suggests that I'll want to sample what equilibrium is like from about 9 psi (2.25 volumes) to about 12 psi (2.52 volumes).

I guess if I "set and forget" the regulator at 9 psi and wait for equilibrium (sampling along the way, I suppose), that will likely take a fairly long time? (A week, maybe?). My thought is therefore to set the regulator at a higher pressure initially (20?), and then after a period of time bring it down to 9, bleeding the head pressure accordingly (so that it reads 9 as well).

Any practical advice out there for this method? When increasing the regulator setting to speed things up, should I vent the head so that the head remains at 9, or around there? Or should I let the head pressure build up to 20, and vent it down later, when I decide to lower the regulator to 9, in order to reach equilibrium at 9? Also, is there a way that I can assess, or roughly assess, the amount of time I should keep the regulator and tank at the higher pressure, so that I don't end up over-carbonating? My only other question (for now!!) is whether speeding up the process in this way will have any negative effects on the resulting carbonation?

Thanks again for all the replies. This forum has been great.

Mike

1000 liters is a lot of cider to be experimenting with. How is it packaged? Most of the homebrew accelerated carbing methods are based on 5 gal corny kegs. The rate of carbonation is highly dependent on the surface area (in contact with CO2 gas) and the column height of the liquid. If your cider is packaged differently, not sure how well the guidelines will work for you. Reaching "true" equilibrium with set and forget actually takes about 3 weeks. The rate of carbonation starts out faster and gets slower as the difference between the current carb level and equilibrium level gets smaller. It's roughly an exponential decay curve, having a time constant τ. 63% of the carbonation happens in τ amount of time. After 2*τ amount of time, you are at 86% complete. 3*τ => 95%, 4*τ => 98%, 5*τ => 99%, etc. It's complicated by the fact that you don't start at 0 volumes of CO2, but closer to 1 volume, so if the final carb level is 2.5 volumes, you would only be adding 1.5 volumes. A common acceleration method is to put on 30 psi for 36 hours, and then set to equilibrium pressure. So, 30 psi for 36 hours must be adding something close to 1.5 volumes to liquid already containing 1 volume.

I know this probably doesn't answer your question, but it might help you think about how you want to approach this. Do you have a deadline to have it all carbed? Can you work with 19 L to start, and determine your optimal carb level, before trying to carb all of the cider?

Brew on

 

[DrunkleJon]

If you are carbing that much cider it may be advantageous to inject your co2 at the bottom of the pressure vessel so as it bubbles up it dissolves and stirs the cider.

Also I have found that I like cider carbonated higher than I do beer. The ciders I make is so much drier and the higher carbonation level helps.

 

[mlachelt]

Thanks guys,

The cider is in a bright tank, chilled to 39 degrees, and I'm hoping to carb it within a few days. It has a carb stone. Doug: Likely a stupid question but why do you say the cider will already contain 1 volume of co2? Also, when you say the common method is 30 psi for 36 hours, is this applied while venting? Or, is all that pressure held in the headspace? I've practiced with a full tank of water but the stakes are certainly higher now! Mike

 

[doug293cz]

Thanks guys,

The cider is in a bright tank, chilled to 39 degrees, and I'm hoping to carb it within a few days. It has a carb stone. Doug: Likely a stupid question but why do you say the cider will already contain 1 volume of co2? Also, when you say the common method is 30 psi for 36 hours, is this applied while venting? Or, is all that pressure held in the headspace? I've practiced with a full tank of water but the stakes are certainly higher now! Mike

During active fermentation the air in the fermenter headspace is displaced by CO2, so the headspace contains CO2 at 0 psi gauge or 14.7 psi absolute. The carbonation charts and calculators all use gauge pressure. The absolute pressure will be in equilibrium with some level of CO2 in the fermented liquid. If fermentation was at 55°F, then the carb level will be 1.00 volumes. You can use the calculator here to figure out the carb level for your fermentation temp (actually the highest temp the liquid sat at during or after fermentation in a vented vessel.) Put in your temperature, and play with the carb level until it tells you you need 0.0 psi to achieve that carb level at your temp.

The common accelerated carbonation of beer in 5 gal corny kegs is to pressurize the keg to 30 psi and hold it at that pressure for 36 hours, at temps in the range of 38° - 40°F. Then vent and set the pressure to something like 10 - 12 psi.

What is the liquid depth in your tank of cider? Might be able to do some back of envelop calcs that could get you close to 2.5 volumes. But I don't have any suggestions for test carbing to a bunch of different levels, and getting it done in a few days.

Brew on

 

[Brewmegoodbeer]

Cant you see if your keg is in equilibrium by shutting off the gas, purging the keg to 0 psi and waiting a few hours to see if the carbonation within liquid fills the head space and reads a goal psi on the regulator ( for example: 12 psi)? If its less than your target psi, then you are not at your goal yet.

 

[doug293cz]

Cant you see if your keg is in equilibrium by shutting off the gas, purging the keg to 0 psi and waiting a few hours to see if the carbonation within liquid fills the head space and reads a goal psi on the regulator ( for example: 12 psi)? If its less than your target psi, then you are not at your goal yet.

That will work, but you don't have to purge to zero psi, just a little below your target psi. If it doesn't come back to target you are not at equilibrium. You could also just shut off the gas to the regulator, and see if the pressure drops over time.

Brew on