Fluid dynamics, heat transfer, and anaerobic exothermic processes question

[Descartes]

I'm going to attempt to articulate my question with appropriate lucidity, but please let me know if I fail to do so.

I am a vintner (a wine maker), so achieving a palatable batch of wine necessarily elicits some required understanding of chemistry and physics. I understand that the wine yeasts I use ferment by breaking down the sugar into ethyl alcohol and carbon dioxide gas. I also understand that the interaction of oxygen with the resultant alcohol produces acetaldehyde and acetic acid. During the first phase of the fermentation process the yeast are usually fervent (depending on temperature), and as a result there is a sufficient blanket of CO2 on the top of the wine to keep air out. When the fermentation slows down we usually put it in a container with little room for oxygen and attach an airlock. The CO2 is released by the airlock, but oxygen is not allowed in.

Ok, that was just to provide the background for my question. What precisely causes the CO2 to bubble up through the lighter air in the container and then through the airlock? Is it the heat from the exothermic reaction freeing some CO2 molecules thereby decreasing the density and creating a convection current? Is it simply more pressure of more CO2 molecules from the fermentation? Is there any point in really distinguishing between heat and pressure in this context?

I hope this makes sense. I have a friend who feels that temperature has nothing to do with it, and that if the CO2 were somehow able to keep constant with the temperature above, the increase in CO2 molecules would still cause some of the CO2 to percolate through the air in the container and out the airlock.

Thanks!

 

[Bystander]

I'm going to attempt to articulate my question with appropriate lucidity, but please let me know if I fail to do so.

Ok, that was just to provide the background for my question. What precisely causes the CO2 to bubble up through the lighter air in the container and then through the airlock? Is it the heat from the exothermic reaction freeing some CO2 molecules thereby decreasing the density and creating a convection current? Is it simply more pressure of more CO2 molecules from the fermentation? Is there any point in really distinguishing between heat and pressure in this context?

Thanks!

You are seeing the "diffusion process" in action --- component of a mixture, solution, system, moving from region of high concentration to region of low concentration.

 

[Descartes]

That's a bit of anti-intellectualism on my part for not realizing that. I think I was too busy concentrating on the argument.

At any rate, I thank you for your reply!

 

[Bystander]

"Anti-intellectualism?" Hardly --- someone says, "airlock," and the thought process has one direction to proceed.

 

[Gokul43201]

Actually, there's more than just diffusion happening.

When you put the wine into the container with the airlock (which is just a valve) the space above the liquid level has air in it. Very soon, the fermentation process results in a CO2 blanket over the wine, just like in the large vats. Since CO2 is about 50% denser than air, it mostly stays below the air, and there is not very much diffusion happening unless the container is disturbed. However, there will be some small amount of diffusion - in both directions. So you have a layer of predominantly CO2 (with a little air) over the wine, and above that is mostly air, with a little CO2 mixed in.

As fermentation proceeds, more and more CO2 is being produced, and there is only a fixed volume available for the gases. So something's got to leave. Since the airlock is near the top of the container (I'm guessing ...that would be sensible) the first gas that goes out is the mixture that is mostly air. There is however, some small amount of CO2 mixed in this air (due to some diffusion). As more CO2 is produced by fermentation, the boundary layer separating the predominantly CO2 phase from the mostly air phase keeps rising, till it reaches the top of the container. From this point on, additional production of CO2 causes this CO2 rich mixture to have to leave through the airlock.

Temperature too plays a role. If the fermentation is an exothermic process (which I imagine it is) the CO2 molecules get some of the energy that is given out. They eventually distribute this energy about by collisions with other CO2 or air molecules. But, for a short time the extra energy (heat/temperature) helps them diffuse into the lighter air. However, even without this extra energy there would still be some amount of mixing - only less.

 

[Descartes]

So, is it then most appropriate to say that it's the pressure from the increased CO2 density that forces it out through the airlock and not so much the increased kinetic energy of the CO2 from the exothermic process?

 

[Bystander]

Caught me in a "one-way" thought process. If you're considering the late stages of fermentation, heat production is slow enough that most is lost through the container rather than to evolving gas. But so long as the yeast are producing CO₂, it (CO₂) dissolves in the liquid phase, increases the pressure in the vapor phase, or escapes through the airlock (I've assumed you're talking about an "S"-trap rather than a check valve). Once the fermentation process ends, CO₂ will continue to move through such a trap by diffusion. That's the question I answered, but it certainly ain't the one you asked.

 

[Gokul43201]

So, is it then most appropriate to say that it's the pressure from the increased CO2 density that forces it out through the airlock and not so much the increased kinetic energy of the CO2 from the exothermic process?

Depends on which stage of the process you are talking about. Initially, CO2 comes out because of diffusion (and perhaps, with a little thermal help) . Eventually, CO2 comes out because it's the only gas around.

 

[Descartes]

Thanks very much to the both of you!