Why is liquid nitrogen stored at low pressure in dewars?

Why is liquid nitrogen stored at low pressure in dewars? If stored at high pressure, without any thermal insulation, how much pressure could
it generate? Could a SCUBA tank (3000 psi) hold it? I searched for information on this but had no luck, everybody uses dewars.
And what about those trucks I see transporting it on highways, the ones with the long skinny horizontal tanks? Are they at low pressure and insulated like a dewar, or are they at high pressure?
Thanks for your help.
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The dewar in my lab has a safety release valve that's set to 20 psi. And that's generally the pressure within the dewar until it all gets used or boils off.
Ken Muldrew snipped-for-privacy@ucalgazry.ca (remove all letters after y in the alphabet)
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But I don't know if that helps retain the liquid, or if it's mostly just to keep some pressure inside to blow the liquid out.
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PV=nRT
what is your n? How many moles of liquid nitrogen do you want to start with and what is the volume of the tank you want to put it into?

Depends on how much liquid nitrogen you put in there.
# Critical Temperature: -232.5°F (-146.9°C) says that you can't keep it liquid above -146.9 degC, no matter how much pressure you apply.

yes, if you call 'about 1 atm' 'low pressure'.

# Molecular Weight: 28.01 # Boiling Point @ 1 atm: -320.5°F (-195.8°C, 77oK) # Freezing Point @ 1 atm: -346.0°F (-210.0°C, 63oK) # Density, Liquid @ BP, 1 atm: 50.45 lb/scf # Density, Gas @ 68°F (20°C), 1 atm: 0.0725 lb/scf # Specific Volume @ 68°F (20°C), 1 atm: 13.80 scf/lb # Latent Heat of Vaporization: 2399 BTU/lb mole # Expansion Ratio, Liquid to Gas, BP to 68°F (20°C): 1 to 694
So 1 ltr of liquid N2 expands to 694 ltr of N2 gas
--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.
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bz wrote:

======================================================= I don't buy that "can't keep it liquid above its critical temperature" stuff.
Say you poured a small amount (1 oz.) of liquid nitrogen into a tank with 5' thick steel walls, then heated it to 150°C.. What would happen? There's no way the tank would rupture, and not enough space for the nitrogen to become gas.
Either it has a measureable pressure or we just invented the next atomic bomb.
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I've seen a demonstration of critical temperature. A liquid half-filled a pressure cell with a window, and we could clearly see the liquid level. As it was heated, the line indicating liquid level disappeared. It didn't go up or down, it just faded away in place.
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Gregory L. Hansen wrote:

============================================== That's a demonstration I'd really like to see. It's hard to believe.
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Bruce W.1 wrote:
<snip>

temperature"
Above the critical temperature a fluid is called a "super-critical fluid".
There is no equilibrium between liquid and vapor phases as there is at lower temperatures. Below the critical temperature the gaseous phase is called a 'vapor' to distinguish it because it can exist in equilibrium with the solid or liquid at the 'vapor pressure.'
The diagrams that show the solid, liquid, and gas phases of a substance are called "phase diagrams."
As a gas is cooled below the critical point, small droplets of liquid spontaneously begin to form, making the mixture look milky. This is called 'critical opalescence': http://demo.physics.uiuc.edu/LectDemo/scripts/demo_descript.idc?DemoIDq4
Cool it further and the droplets combine and fall to the bottom, producing the familiar two phases of liquid and vapor.
Tom Davidson Richmond, VA
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All gases have a temperature, above which they can only exist as gases, not liquids, no matter how high the pressure is. For nitrogen, that temperature is a cryogenic temperature, so there is no advantage to trying to pressurize it.

If they are carrying liquid nitrogen, the tanks are either Dewars or they are actively being refrigerated.
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-Mike

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Bruce W.1 wrote:

If you had to store nitrogen in your child's bedroom, would you choose to store it at low pressure as a liquid in a dewar, or at high pressure as a gas in a very high pressure tank?
Here is some data.
A rough rule of thumb is that the liquid expands between 1,000 and 2,000 times as it converts to a gas.
So, to keep about the same volume tank as the liquid dewar, the high pressure gaseous tank would be operating at 1,000 atmosphere to about 2,000 atmpospheres.... or about 14,000 Psi to 28,000 Psi......
Really expensive and heavy tanks to store a gas that can easily be liquified.
You can understand why the hydrogen powered car will probably not use pressurized hydrogen in room temperature tanks..... The pressures required to store the equivalent of 20 gallons of H2 are enormous and expensive....
Expensive in the tank.... Expensive in the weight of the tank... Expensive in the time and work needed to compress the gas to such high pressures.......
The long skinny tanks you see are probably pressurized.... small long cylinders are the lower weight, lower cost higher safety form of storing highly pressurized gases.
You can look up calculations for the stresses in tank walls in terms of pressure, wall thickness and tank radius.
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2,000 times as it converts to a gas.
Usually less than 1000 times. It's 866 for nitrogen and not much different for helium.

atmosphere to about 2,000 atmpospheres.... or about 14,000 Psi to 28,000 Psi...... <<
Yes. More than 1000 atm since we're well away from the place nitrogen starts to depart from ideal gas law behavior (or Boyle's law behavior, which we're discussing here), which is somewhere around 200 to 300 atm (max scuba tank pressures are 240 atm in the US and 300 atm in Europe). And the departure from ideality is in the direction of needing more pressure to get the same stuff into the same volume, than Boyle's law would predict. For reference, pressure at deepest ocean depths is only a bit over 1000 atm.
SBH
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Sbharris[atsign]ix.netcom.com wrote:

LN2 has a different volume change on vaporization if the gas is cryogenic than if at STP from PV=nRT.
At toom temp, the volume change is more than twice that of 77K.
You can use the rult of thumb and modify it for the actual temperature, but that is more complicated to explain to the OP.

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Nitrogen only expants about 175 times going from liquid to gas at its boiling point. It's a factor of between 700 and 900 going from liquid N2 to gas at STP which is 25 C (there are various figures given on the web for this, beware). Helium figures are in this range. You won't find any gas that gets close to a factor of 2000 going from liquid to STP. In fact, I doubt you'll find one that breaks 1000. But feel free to cite a cite to prove me wrong. But don't blather.
SBH
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Sbharris[atsign]ix.netcom.com wrote:

Standard molar volume of a gas is 22.4 liters per mole.
1 mole of H2O is about 18 grams or .018 liters
22.4 liters / .018 liters = 1,244 for an expansion ratio for water vaporization at STP.
Water breaks 1000....
Where are your cites.....
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the
Standard molar volume of a gas is 22.4 liters per mole. 1 mole of H2O is about 18 grams or .018 liters 22.4 liters / .018 liters = 1,244 for an expansion ratio for water vaporization at STP.
COMMENT:
Water is not a gas at STP. Nice try, though, since liquid water has an *unusually* low molar volume. But the reason it's so low is part of the reason why water isn't a gas at 1 atm until you reach much higher temps.

Nope.
Where are your cites.....
Here are a couple for He and N2
http://homepages.cae.wisc.edu/~helium/n2_he_data.htm
But you can do them yourself. You're going to need a molar volume less than 22.4 mL (or less than 24 mL if you use STP at 25 C rather than 0 C). Nitrogen liquid density is 0.8, so a mole of 28 g has volume 35 mL. Not even close. Hydrogen liquid density is about 0.07, so a mole is 28.5 mL. Still doesn't break 1000. Ammonia is 17/0.68 = 25 mL. No joy.
I'm waiting.
SBH
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Bruce W.1 wrote:

Because there is little need for *high pressure* liquid nitrogen. Commercially, LN2 is used primarily as a source of "cold". It allows you to cool things (like sensitive electronic detectors) quickly, efficiently, conveniently, and with little energy consumption or environmental impact.
Compressed nitrogen is cheap and has other uses. The 'cold' is the whole point of LN2.

could
LN2 at atmospheric pressure is about 700 times as dense as room temperature nitrogen gas at the same pressure. Letting LN2 warm up without letting it expand will give you 14.7*700 = about 10,000 psi.

No, not even close...
I have seen a section of a 9000 psi gas cylinder. It looks like a small cannon with steel walls about 2-inches thick for about a 6-inch bore. VERY heavy.

If a web search can't get you answers, find an expert - like here.

ones
I don't know exactly what you are seeing.
Liquid Nitrogen is transported in cryogenic tanks at pressures less than 40 psi. These tanks will usually be several feet in diameter and may be spherical or up to 40 feet long. There will be a placard on each side of the truck with the ID number 1977 (for 'Nitrogen, refrigerated liquid') and the container will be clearly labeled "Cryogenic Liquid".
Sometimes compressed gases are also transported in bulk containers (trailers). When compressed nitrogen is so transported, it will be in a 'tube trailer' (a trailer with an array of 12 to 30 tubes, each no more than a foot in diameter, stacked like logs). The placards will be a green diamond (for non-flammable compressed gas) and the ID number will be 1066 (for 'Nitrogen, compressed'). The placarding is for the convenience of emergency reponders and is standardized by the US Department of Transportation.
Most emergency responders (firemen, police, state troopers, highway patrol, EMTs) know about this placarding system and either have handy or quick access to a copy of the "North American Emergency Responcse Guidebook" which identifies all the hazardous materials those trucks you see are transporting on highways.

BTW, 'cryonics' is *not* the same thing as 'cryogenics'. Look them up.
Tom Davidson Richmond, VA
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tadchem wrote:

==================================================== If we're talking 30,000 psi for warm liquid nitrogen then I now see why it is not stored like this.
The transport trucks I'm talking about have cylinders maybe 18" wide and 30' long, and there's usually at least a half dozen of them on the trailer. And every one I've ever seen was painted white. Next time I see one I'll take note of the number on the sign. I've got a "North American Emergency Response" book, it's great fun on long road trips.
Anybody know how nitrogen is stored on spacecraft like the space shuttle? I've seen the spherical tanks. Is this high pressure 'warm' liquid nitrogen, or do they let it vent?
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When I took this Nitrogen in cylinders was only put in grey cylinders with a black stripe and hand a left hand thread so you couldn't hook it up accidentally.
A can't see a scuba tank ever being able to hold any amount for long max pressure I think is 3500 or 4000 psi. The cold liquid would make the cylinder very brittle. Just like liquid O2 it has to vent or the vessel will not hold the pressure and I would want to do trial and error to see how long it would take. There a movie out there on high pressure cylinders (1800 to 3000 psi) called the sleeping giant. bust the valve off one of those and the cylinder can go through concert walls

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At work, all the liquid nitrogen dewers had flare fittings on the liquid and vapor ports, which matched fittings on the fill station. Then one day all the vapor ports had swagelok fittings, matching a swagelok fitting on the gas line on the fill station. They must have been connected the wrong way just a few too many times.
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