What amount of CO2 is reabsorbed back into the beer during cold crash ?

[meaulnes2]

I plan to build an airlock with a CO2 trap in order to reabsorb the CO2 trapped during a cold crash. I have a clear idea how to do it.

My question is: how much CO2 will be reabsorbed by the beer after fermentation during this cold crash?
Let's assume we have 20 liters of beer in a 30 liter container. 10 liters of CO2 will be already trapped in the fermentation tank and the capacity of my CO2 trap could be limited to the remaining.

Also assume that the temperature during the cold crash starts at 22°C and ends at 5°C.

According to Palmer's book "How to Brew", the volume of residual CO2 is 0.77 at 22°C and 1.32 at 5°C. The difference of 0.55 vol means that the maximum possible absorption is 0.55 * 20 l = 11 liters.
Therefore, the capacity of my trap may be limited to 1 liter. However, if my fermentation tank is closer to 20 liters, it will be more.
This calculation is based on total reabsorption. But is such a total reabsorption credible knowing that nothing comes to inject the CO2 into the beer as is the case when it is oxygenated with a pump and a stone or using pressure?

 

[hotbeer]

Assuming the beer is sitting there doing nothing and not moving, I wouldn't figure any. Unless perhaps you leave it there for months. You might have more or less just with the ambient atmospheric pressure changes.

I'd just worry about the Boyles Law stuff so you are certain CO2 is added to the headspace and not let air get sucked in.

 

[Dgallo]

.

 

[day_trippr]

The prudent minimal math answer is to simply use double the head space volume.
I wouldn't be concerned with CO2 absorption unless the vessel is going to be left alone for days after the crash has completed...

Cheers!

 

[meaulnes2]

Assuming the beer is sitting there doing nothing and not moving, I wouldn't figure any. Unless perhaps you leave it there for months. You might have more or less just with the ambient atmospheric pressure changes.

I'd just worry about the Boyles Law stuff so you are certain CO2 is added to the headspace and not let air get sucked in.

Thank you. Assuming always true the conditions I have stated above (30l fermenter with 10l head space)
Charles' law states that at constant pressure
V1/T1 = V2/T2 (T in Kelvin)
As 22°C = 295.15 K and 5°C =278.15

the contracted volume of the initial CO2 trapped in the headspace would be
V=10l *278.15 /295.15= 9.42l
To this contraction of 0.58 l must be added 0.17 l for the contraction of the beer, i.e. in total
0.75 l would be the new sucked in volume and this is not a problem.
But how to be sure that even in a quiet state (no move, no shaking) CO2 is not reabsorbed as the saturation at 5°C (1.32 vol) is very different from saturation at 22°C (0,77 vol)? Has any body an idea of the speed of this reabsorption if even it occurs ?

 

[doug293cz]

https://www.homebrewtalk.com/thread...-from-blowoff-tube.646327/page-2#post-8422519
Brew on

 

[meaulnes2]

https://www.homebrewtalk.com/thread...-from-blowoff-tube.646327/page-2#post-8422519
Brew on

Thank you doug293CZ. It's what I need. But would you allow me a question?
In the post you are pointing to I noticed the following assertions:

You would have to cold crash about 3 weeks, or more, to reach equilibrium?

Is this based on your own experience or does it come from an other source? In the second case, which one?

 

[doug293cz]

Thank you doug293CZ. It's what I need. But would you allow me a question?
In the post you are pointing to I noticed the following assertions:

Is this based on your own experience or does it come from an other source? In the second case, which one?

It's an educated guess, based on the conventional wisdom that it takes about 3 weeks for "set and forget" force carbing to "finish." About 63.2% of the carbonation takes place in the first week, 63.2% of the remaining carbonation takes place in the 2nd week, and then 63.2% of the remaining carbonation takes place in the 3rd week (86.5% done after 2 weeks, 95% after 3 weeks, and 98% after 4 weeks.) If you start with 0.8 volumes after fermentation, then you have to add 1.7 volumes to get to 2.5 volumes, and 0.8 + 1.7 * 0.95 = 2.4 volumes, which to most people is indistinguishable from 2.5 volumes, so essentially complete.

Brew on

 

[Ballaratguy]

I pressure ferment and scavenge the CO2 into 2 x corny kegs
When cold crash the pressure drops a little when the CO2 is sucked back into the fermenter
No oxygen problems and cheap to do