Well I am just joining these forums having recently found them. I have read many many pages back and the amount of great reading is wonderful. So thank you all for making this community.
Anyways I came across a question at work today that I could not think of the answer to and goggling lead me in circles.
Looking at the regulator on a co2 tank we have I noticed that the pressure read something like 1500psi. From my understanding what happens inside a co2 tank is once the pressure reaches about 850 psi at room temp the co2 begins to form a liquid that settles at the bottom of them tank.
What I don’t understand is how the regulator can read a higher pressure if the maximum pressure of a gas form of co2 is about 850psi at room temp.
If I am missing something very fundamental I apologies but an answer to this would be most appreciated.
It also has to be above 31.1 degrees celsius though, which isn’t very hot but above room temperature. And since liquids aren’t compressible, I don’t see how you could have that pressure below 31.1 degrees in a sealed container like that.
Just an aside, our liquid oxygen tanks at work don’t have pressure guages. The guages just read the height of the liquid, like in a car’s fuel tank.
No it doesn’t. The phase chart shows that liquid and gas differ only up to 73 ATM and 31.1 C. Exceed either one and you end up with a supercritical fluid. That’s why the line dividing liquid from gas stops at that point instead of continuing straight up.
To be clear, it is very possible to have a room-temperature tank of CO2 in supercritical fluid state as long as the pressure is above 73 ATM.
As an aside, back when I was studying boilers in the Navy I found the thought of a “critical point” for water to be absolutely mystifying and tantalizing. Since steam plants operate well below that pressure, it was a moot point, but I wondered just what it would look like if you could see into a pressure vessel filled with supercritical water. I’m certain it would be very boring.
I eagerly await an answer. I sure can’t figure it out.
I haven’t seen a CO2 regulator in a long time though. I don’t know what type of system you are dealing with. The only CO2 tank I’ve used was a crude way to make dry ice for the lab. I can’t imagine carbonation systems getting to 1500 psi, but what do I know. If this is a lab setting, I might suggest you have the wrong regulator, but it should still give a correct pressure I think. Most regulators won’t fit the wrong gas cylinders, but some will. If it is an old regulator it could just be wrong. This is especially the case in a lab where co-workers have been jimmying the mechanisms trying to figure out why it won’t fit onto the reverse threaded CO tank.
Here is an example of the type of regulator. Not exactly the same but has the same gauge on it.
We have one that reads 0-2000 and one that reads 0-3000
Nothing special happens at 73 ATM though. A phase transition does occur at a somewhat lower pressure (the vapor pressure of C02 at this temperature), though the phases are increasingly alike as the temperature approaches the critical temperature.
By the above definition, you have supercritical CO2 at any pressure on a somewhat hot day. You could walk around with a balloon full of it in summertime. But nothing special happens at 31.1 C either.
Once it hits the critical point, CO2 gas and liquid behave the same. At critical temperature (a little lower than the critical point) the gas can no longer be liquified by pressure. I don’t claim to understand all the chemistry behind it, but as Uncertain put it, nothing special really happens.
It can, but since liquids aren’t compressible, you wont get more CO2 in the tank by doing so. Once you get the tank to 850 psi, the rest of the CO2 in the tank should turn liquid until it is full of liquid. So until the tank is full, the tank should have a pressure of 850 psi.
I also disagree with minor7flat5 that one only need exceed the critical pressure is to have a supercritical fluid. That is not how I interpret his/her phase diagram, and it is not how I remember supercritical fluids being described.
Any chance the tank was overfilled? My understanding is that a given tank will only be partly full of liquid when “filled” to its rated capacity. If it was actually topped off, and cold from the filling, it might be conceivable that the pressure could get out of control when it warms to room temperature. Especially if the cylinder was in a warmer area than the ambient room temperature. – was it tucked away in a hot cabinet, or in direct sunlight?
Google seems to confirm this for me. This link (warning: PDF) claims that a 20 lb cylinder, overfilled to 25 lbs, would reach 1430 psi when warmed to room temperature. I’m not quite sure how that works, thermodynamically…
Did you actually see the dial reading above 850 psi or are you assuming because the regulator has numbers that high that the pressure will read that high.
It is actually reading that high, when we get them in they are at around 1500psi.
I doubt they were overfilled as this is how we always get them in. Also they don’t go from 1500psi to 850psi as soon as we start to use them. The reading just drops gradually.
What grade of CO2 is it?
Who is your supplier?
What is it used for?
What does the product sheet say about it?
I’m wondering if they add some N2 to the tank. I don’t know how much N2 they would need to add, but for most applications N2 wont be an issue. If they also had a dipstick going to the bottom of the tank, then N2 wouldn’t be a major impurity until you were below 850 psi. I think the only type of tank with a dipstick are the ones that are used for making dry ice.
Compression would cause the temperature to rise, especially as the gas transitioned to a liquid. The tank would have to cool after filling, not warm. We get frost on our oxygen cylinders as we use them. In the winter, they can even be covered by a layer of solid ice which has to be broken to close the valve. And they can get cold enough that the pressure drops significantly.
If the tanks are completely full of liquid at a certain temperature, then any increase in temperature will be accompanied by an increase in volume, causing the tank to swell. This would cause the pressure to increase dramatically, opening the relief valve. I would expect any tank without a relief valve to rupture after a few degrees at most. That is unless it’s warm enough to form a supercritical fluid. I’m not sure how compressible they are.
Shinybrain, about what temperature are these tanks at when they read this pressure?
The main point is that it’s not physically impossible for the pressure to be higher.
I’m not sure what the liquid is like so close to the critical temperature. The liquid and gas phases are quite similar near the phase transition with pressure. The liquid might be much more compressible than liquids we’re used to.
A welding shop might label bottles of Argon/CO2 mixture (“Stargon”) used in mig welding steel as “CO2” to distinguish them from the pure argon used for TIG, and migging aluminum.
