Pressure Increase 1

[BenGreenhalgh]

Hi all - I am new here, so hope I have found the right place to ask this question. I'm a new engineer and work in a biocontainment facility, but I am confused by the gas storage system we employ. It works of course, but my understanding has a gap that I hope you could fill.

We store liquid CO2 and LN2 in large cryocontainers (uninsulated apparently). Assume the system is sealed, so that it's just the tank and the low temp liquids inside. My understanding was that the temperature over time within the tank would reach ambient temperature (say 20 degrees C), and the vapour pressure within the cylinder would reach the pressure at that temperature, so for CO2 at that temperature, it would be over 70 bar??

The tank is rated for 23 bar, so why does the tank never reach 70 bar? I am aware that by venting the gas for use in the labs, the pressure inside the tank reduces and more of the liquid evaporates (lowering the liquid temp), but say the labs were shutdown...what does the liquid /gas do with nothing being used?

Many thanks -

 

[GBTorpenhow]

The tanks are probably internally insulated, with an inner cryogenic pressure vessel, carbon steel outer vessel, and vacuum/expanded perlite in between for insulation.

If there is no product withdrawal for long enough, one of the relief devices installed on the tank will open and vent some of the gas to atmosphere.

see e.g.

https://files.chartindustries.com/14388350_VS01.pdf

for the internals of a typical LN2 tank.

 

[BenGreenhalgh]

I am so glad to hear you say this. As I expected this would be the case. I would have found it very odd that cryogenic liquids were kept in a non insulated container. Thanks

 

[LittleInch]

If the containers were not insulated, they would show signs of large ice growth or ground freezing and would be dangerous to touch as the surface would be very very bad for your fingers to touch it.

"Large cryo containers" is very vague. Your large might be my small and cryo container is not a recognised term in my world.

Pictures, data sheets etc would tell us a lot.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Highly doubtful that there is no insulation. You are probably looking at a large "thermos bottle" type tank. Stainless steel on the outside, but with another insulation-wrapped, steel tank inside.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[georgeverghese]

Perlite in the annulus is purged with dry N2 under a slight positive pressure to prevent moisture ingress. There should be a breather valve to keep this pressure in the annulus.

 

[pierreick]

Hi,
A google search should be your ultimate resource:

https://www.rfcryogenic.com/product...MIqOiLxtyHiAMVrqVmAh2wFw31EAAYASAAEgLty_D_BwE

Good luck
Pierre

 

[BenGreenhalgh]

All very helpful there everyone. Many thanks!

 

[lucky-guesser]

This is one of the major challenges with hydrogen powered cars, as the fuel is constantly trying to expand in the fuel tank. Ideally, you would fill up right before you leave for work, do your driving and get back home with an empty tank, letting it sit empty overnight. A full tank sitting there all day will begin to vent the expanding gasses, not only wasting it but creating a huge explosion risk if you park in an enclosed garage.

 

[1503-44]

Look at it this way. The tank is never empty.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[satthepbienhoa]

In a sealed cryocontainer, the pressure of liquid CO2 or LN2 will not necessarily reach the vapor pressure at ambient temperature (e.g., 70 bar for CO2 at 20°C) due to the low temperatures maintained by the liquid. Even though the tank is uninsulated, the low temperature of the liquid helps keep the pressure lower. The pressure inside the tank will generally stabilize below the vapor pressure at ambient temperature because the system is designed to handle the pressure within its rated limit. If the labs are not using gas, the system will still maintain a balance between the liquid and vapor phases, preventing the pressure from exceeding the tank’s rating. The pressure remains stable as long as the system is properly sealed and no additional heat is transferred to significantly raise the temperature.

 

[BenGreenhalgh]

@satthepbienhoa, I am a bit confused by this if you don't mind explaining a little more?

You said it will not reach the vapor pressure at ambient temp. Is this purely because it takes the external environment a long time to heat the liquid to the point where it reaches equilibrium with its surroundings? Eventually it will though?

"The pressure inside the tank will generally stabilize below the vapor pressure at ambient temperature because the system is designed to handle the pressure within its rated limit." I don't understand this. If the temperature of the fluid eventually reaches the temperature of the environment the vapor pressure at that temperature must be within the tank, surely?

 

[LittleInch]

I think what satthe is saying is that if the heat ingress through the insulation is low enough and the vapour pressure is maintained then you will have a temperature differential.

Essentially as the temperature of the liquid increases the heat flow, such as it is, reduces even further and hence in reality, the fluid in the insualted vessel rarely reaches ambient, which is also rarely a steady temperature.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BenGreenhalgh]

Okay brilliant, that makes sense thanks.