Liquids, gases and pressure

[mistermack]

I'd be interested in what physics says about the following situation. You want to fill a small gas bottle from a big one. Can you fill the smaller one from a half empty big one? 

I'm intending to buy the kit to do it, but I'm not sure if it's worth it, if filling gets less and less complete as the big one empties. 

 

The picture doesn't really cover what I mean. The big bottle would be about twenty times the capacity of the small one, so I'm wondering if you can still fill the small one right up, if the big one is half empty. 

I'm talking about liquid CO2.

[exchemist]

1 hour ago, mistermack said:

I'd be interested in what physics says about the following situation. You want to fill a small gas bottle from a big one. Can you fill the smaller one from a half empty big one? 

I'm intending to buy the kit to do it, but I'm not sure if it's worth it, if filling gets less and less complete as the big one empties. 

 

The picture doesn't really cover what I mean. The big bottle would be about twenty times the capacity of the small one, so I'm wondering if you can still fill the small one right up, if the big one is half empty. 

I'm talking about liquid CO2.

The critical point for CO2 seems to be at ~30C and at a pressure of ~74bar. So you will not have any liquid above that temperature. At 20C, say, the minimum pressure to keep it liquid is about 65bar. Phase diagram here:  https://www.engineeringtoolbox.com/CO2-carbon-dioxide-properties-d_2017.html#phases

So I suppose if you are supplied with a "liquid CO2" cylinder, it must have a pressure when full of 70bar or so. But it won't take much of a drop in pressure before what you have is a gas, at which point the degree to which the smaller cylinder is "filled" will just be a function of the total volume of both cylinders, when you equalise the pressure between the two. 

At least, that's how I read the phase diagram. 

 

[mistermack]

Thanks. I just googled CO2 cylinder pressure, and this came up on a safety page :

CO2 cylinder pressure is about 860 psi at normal room temperature. Typical CO2 cylinders store about 50 lbs. of liquid CO2. Two pounds of liquid CO2 expands to about 20 cubic feet of pure CO2 at atmospheric pressure, or expands at a rate of 535:1. 

https://www.co2meter.com/en-uk/blogs/news/co2-tank-safety-precautions#:~:text=CO2 cylinder pressure is about,a rate of 535%3A1. 

 Some tanks are fitted with a "siphon" tube, that take liquid CO2 from the bottom of the tank, and some don't, and take gas from the top. 

I would like to completely fill the smaller tank with liquid CO2 from the big one, even when the big one is getting close to empty, so that I can use it all up. But I don't know if that is possible, or if the big tank is only 1/10 full, will I only be able to fill the small tank to 1/10 full ?                                  

It's my suspicion that once the small cylinder is 1/10 full, it will produce a back pressure that's equal to that in the big cylinder, and stop any more flow into it. But I don't know the fundamentals to calculate if that is so or not.

[sethoflagos]

5 hours ago, mistermack said:

I'd be interested in what physics says about the following situation. You want to fill a small gas bottle from a big one. Can you fill the smaller one from a half empty big one? 

'Half-empty' is a misleading concept when your gas density is >80% of the liquid density as in this case. Even if the receiving cylinder were free of liquid, at pressure it's still > 80% full.

A syphon discharge from below the liquid level of the main storage vessel will definitely get a few more pounds delivery, though some of this will evaporate on transit.

If the deficit is so important, then you could consider placing the receiving cylinder in a bath of iced water both prior to and during filling. This will condense everything coming in from the supply vessel. Even if the supply vessel pressure falls to 40 bar or so, the receiving cylinder will repressurise when returned to ambient so maybe think twice before exceeding the cylinder's rated CO₂ weight capacity.  

[mistermack]

2 hours ago, sethoflagos said:

'Half-empty' is a misleading concept when your gas density is >80% of the liquid density as in this case. Even if the receiving cylinder were free of liquid, at pressure it's still > 80% full.

Yes, thanks, I take your point about the density of the gas under pressure. But when I said half-empty, I meant half used-up, rather than referring to the liquid level, although I hadn't considered the implications of the difference. 

So I'm still wondering if when you've only got 10% of the mass of CO2 left in the big bottle, does that mean you can only charge the small one to 10% of it's capacity by weight? I agree about chilling the small bottle. I've seen online that it's best to put it in a freezer for a couple of hours. 

Going by what you said about the gas density being >80% of the liquid density, It's looking like the big bottle will be all gas, long before it's used up half of its full charge of CO2, so you will just be transferring gas, not liquid, once the liquid level drops to the bottom of the big tank.

[John Cuthber]

Do not fill a gas cylinder with liquid.

When it warms up, and the liquid expands, it will burst the tank (or the pressure relief if you are lucky).

https://www.phmsa.dot.gov/sites/phmsa.dot.gov/files/docs/technical-resources/55771/calculation-and-verification-filling-ratios-liquified-gases.pdf

 

[mistermack]

1 hour ago, John Cuthber said:

Do not fill a gas cylinder with liquid.

Thanks. What you're talking about there is over-filling. The cylinders have a rated weight of contents printed on the outside. What people do is to weigh the cylinder empty, and then refill it, while it is suspended from a spring balance, and stop when the target weight is reached. You have to let some CO2 out, if you accidentally exceed  the target full weight, although with care, you should be able to stop when just under. 

[John Cuthber]

4 hours ago, mistermack said:

Thanks. What you're talking about there is over-filling. The cylinders have a rated weight of contents printed on the outside. What people do is to weigh the cylinder empty, and then refill it, while it is suspended from a spring balance, and stop when the target weight is reached. You have to let some CO2 out, if you accidentally exceed  the target full weight, although with care, you should be able to stop when just under. 

I know that.
You know that.
The next guy reading the thread may not know that.

 

 

12 hours ago, mistermack said:

So I'm still wondering if when you've only got 10% of the mass of CO2 left in the big bottle, does that mean you can only charge the small one to 10% of it's capacity by weight?

No
You can essentially "distill" the CO2 into the smaller bottle if you cool it and/ or warm the big one.
In principle you can shift almost all the CO2 that way.

[mistermack]

On 6/9/2023 at 6:25 PM, John Cuthber said:

No
You can essentially "distill" the CO2 into the smaller bottle if you cool it and/ or warm the big one.
In principle you can shift almost all the CO2 that way.

Thanks, yes, I think that's right. I had trouble getting my head around it, because I didn't appreciate how big an effect the cooling would have. I think without cooling, you would not get much gas to transfer from a bottle that's nearly empty, but the cooling will maintain a pressure difference so long as it's in effect, so the flow will continue till you shut it off, or till the temperatures equalise.

[John Cuthber]

Eventually, you will end up with the big tank full of gas at the vapour pressure of the liquid in the small tank.
So you can't transfer all of it, but you can get very close.

On the other hand, getting very close would need you to use liquid nitrogen or something to cool the small tank.
Not only is that uneconomical,but you risk embrittling the steel.