Sciencemadness Discussion Board

liquid oxygen evaporation

smeesh - 15-3-2012 at 16:29

hello there guys.
i had a question concerning liquid oxygen in a pressure vessel.
lets say, i dont use the "proper" vessel. lets say i use a good old fashioned gas cylinder that will allow the liquid oxygen to be heated by surrounding air that is coming into contact with the walls of the container. if i had filled the container to the brim and then i let this happen, is an explosion likely?
ive been going over it in my head and it seems quite possible. the change from liquid to gas could perhaps increase the pressure just like an old-timey steam boiler...right?

i could use some info. thank ya sirs

watson.fawkes - 15-3-2012 at 17:22

Quote: Originally posted by smeesh  
i could use some info.
What's the vapor pressure of O2 at 40 °C?

neptunium - 15-3-2012 at 18:05

oxygen, like a few other, gas were thought to be permanent gas. meaning during the mid 1800's ( the thermodynamic age),these gases could not be liquified by preassure alone.

O2 has a critical point above witch it will not be condensed into a liquide. water's critical temperature is arround 374C.
O2 critical point is at -118C, well below zero.
so unless you fill a steel cylinder with liquid ox and keep it below this temperature, your tank will explode.

franklyn - 16-3-2012 at 00:08

Liquid oxygen is kept in a dewer at atmospheric pressure , and slowly evaporates away.


Without refrigeration ordinary gas cylinders cannot hold up to the pressure of so much weight
of oxygen as would be initially present in the liquid state , without a valve to bleed the excess
pressure that would accumulate as it becomes a supercritical (gas / fluid) when it heats up to
room temperature. Something around 25000 pounds per square inch. What amounts to the
isothermal compression of oxygen gas in the ratio of 860 to one at room temperature.

http://community.discovery.com/eve/forums/a/tpc/f/7501919888...

http://navyaviation.tpub.com/14019/css/14019_15.htm

http://tinyurl.com/7fo5eb6


Phase diagrams are your friend
http://www.chemguide.co.uk/physical/phaseeqia/phasediags.htm...

Critical Temperature : Temperature above which liquid cannot exist at any pressure
Critical temperature of oxygen, -181.43 °F, (154.6 Kelvin)

Critical pressure : of oxygen (minimal pressure necessary at the critical temperature)
731.4 pounds per square inch absolute (psia)
or 50.43 bar (atmospheres)
or 5043 kiloPascals

Here is a demonstration of liquid chlorine undergoing transition from liquid to supercritical fluid
http://www.youtube.com/watch?v=Y6S7bZbx4-s
The meniscus slowly vanishes , then as it cools off to the lower transition temperature the
meniscus again appears signifying separate gas and liquid phases are again present.


Design Guide for High Pressure Oxygen Systems NASA RP 1113, 1983.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/1983002...

Cryogenic Solid Oxygen Storage & Sublimation Investigation
http://www.dtic.mil/dtic/tr/fulltext/u2/687852.pdf

P.S.
are you designing a nefarious device ?

.

Ral123 - 16-3-2012 at 10:30

So what are these big cylinders that welders use full of? So if they are filled with oxygen, some of it in liquid phase, the pressure is like 2000 atmospheres? And is the same with nitrogen?

franklyn - 16-3-2012 at 16:47

@ Ral123

So what are these big cylinders that welders use full of ?

They're full of whatever they are filled with. Otherwise
they are just full of air space. Note * Acetylene cylinders
are filled with porous rock and acetone , the solvent in
which the acetylene content is dissolved. This is done to
obviate detonation resulting from runaway polymerization .

So if they are filled with oxygen, some of it in liquid phase,
the pressure is like 2000 atmospheres ?

No.

If there is liquid phase present , it is because the bottle and
the contents are below the critcal temperature. The critical
pressure to maintain it liquid is just about 50 atmospheres.

To have oxygen contained at 2000 atmospheres, it will be
hotter than the critical temperature by some amount , and
determined by the weight of oxygen in the volume of the
cylinder. This is readily figured by the combined gas law
formula

P V = n R T , solving for the pressure P = nRT / V

P = Pressure
n = number of mols ( total weight of content divided by
. . . gram molecular weight of oxygen)
R = constant .0821 ( it is the same for all common gases)
T = ambient temperature ºC + 273.15
V = volume of cylinder

A minor point , the assumption is that only oxygen is present
inside the cylinder. One can have liquid oxygen at 2000 atm.
by having the unfilled space above the lquid phase filled with
some other gas having a much lower critical temperature ,
usually somthing like helium.

And is the same with nitrogen ?

It is also one of the so called permanent gases , so yes.

.

Ral123 - 17-3-2012 at 01:30

Forgive me for being so dump, but I don't understand a thing. If I wana have oxygen at near "1.141 kg/L" in my room with no cooling stuff, whatsoever, how may bars is that? For example if I'm out of ammonium nitrate and want to blow sh*t up and have only and old compressor. What pressure do I need to liquefie air?

unionised - 17-3-2012 at 05:25

Quote: Originally posted by Ral123  
Forgive me for being so dump, but I don't understand a thing. If I wana have oxygen at near "1.141 kg/L" in my room with no cooling stuff, whatsoever, how may bars is that? For example if I'm out of ammonium nitrate and want to blow sh*t up and have only and old compressor. What pressure do I need to liquefie air?


You can't.
as has been pointed out.
Critical Temperature : Temperature above which liquid cannot exist at any pressure
Critical temperature of oxygen, -181.43 °F, (154.6 Kelvin)

bquirky - 17-3-2012 at 06:33

I think welders oxygen is actually just compressed oxygen it is still in the gas state


watson.fawkes - 17-3-2012 at 07:01

Quote: Originally posted by Ral123  
So what are these big cylinders that welders use full of? So if they are filled with oxygen, some of it in liquid phase, the pressure is like 2000 atmospheres? And is the same with nitrogen?
Let me guess. After three response all having to do with phase diagrams, did you go look up the phase diagram of oxygen? It doesn't seem so.

My question was a trick one, because, as others have pointed out, at room temperature it's far above the critical point, and so "vapor pressure" isn't a well-defined term. You would have understood a lot to have figured that out for yourself.

Instead, go do some reading and come back with better questions.