# Compressing nitrogen and its characteristics

Gold Member
Hey guys, i was wondering what would the graphs be of various attributes of a container of nitrogen while its being compressed. What would the Pressure graph look like, the volume graph, and the temperature graph look like? I was wondering about how they make liquid nitrogen and what the attributes would be and i came up to a problem. Im thinking "Ok, volume goes down, pressure must increase to make the ideal gas law make sense". Then i realized that liquid nitrogen is... well, really really cold so i was wondering what is actually going on.

Thanks for the help in understanding :).

Also, if you introduce liquid nitrogen into a vacuum (real world), wouldnt it start evaporating?

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pervect
Staff Emeritus
If you compressed nitrogen gas very slowly, giving it time to reach thermodynamic equilibrium during the compression process, you would compress it "isothermally" without changing its temperature.

However, that's typically not how you'd compress nitrogen as part of the liquification process. You wouldn't want to wait around that long. The gas actually heats up in the process. Rapid compression, where the gas does not have time or is not allowed to exchange energy with its surroundings, is "adiabatic" compression.

The next step is to cool the hot gas back down by running it through a heat exchanger - this cools it down to the surrounding temperature.

The final step is to allow the gas to expand - adiabatically. Just as compressing the gas adiabatically heated it up, allowing it to expand adiabatically cools it off. Part of the gas liquifies and is collected as it is allowed to expand.

If you control the temperature, you can liquify only nitrogen, which melts at 63K, and not oxygen, which melts at 54k

Gold Member
So lets see... according to PV=nRT... as you decrease the volume quickly, the temperature goes up. What does the pressure do in the process?

In the final step, why doesnt all of the gas liquify instead of just some of it?

Also, what would happen if you introduce some LN into a vacuum?

pervect
Staff Emeritus
Besides PV=nRT, you need to satisfy the adiabiatic condition

$P V^\gamma =$ constant

Take another look at the Wikipedia article I quoted. (Don't bother reading the detailed derivations unless you're really interested, but do read the basic adiabatic law).

If you know either the volume change or the pressure change, and the number of particles n remains constant, you can calculate all the other parameters given these two equations (the ideal gas law and the adiabatic law). Of course you need to know the value of the constant $\gamma$ to do this.

$\gamma$ is a constant that depends on the gas, it's 1.4 for diatomic gasses like oxygen and nitrogen. I think dry air is fairly close to this value too. CO2 and water vapor are a couple of tri-atomic gasses that affect the value of this parameter for air slightly.

As I mentioned before, nitrogen condenses out before oxygen (at a higher temperature), so with a little bit of process control you can keep the temperature low enough to condense out nitrogen, and not the oxygen.

Water would condense out first, and so would CO2 (dry ice), I'm not sure how these impurities are eliminated offhand, I think some sort of drying process is used (possibly a lower compression/expansion cycle first, to freeze out these two components, then a larger compression of the dried air to condense the nitrogen).

That's just a guess, you might want to do some more research on the www.

Gold Member
So if you quickly compress the volume, your pressure increases... but then why would the temperature increase? Is the change in volume not proportional to the change in pressure? (and whats left over is a temperature change?)   I wanna go back to school

pervect
Staff Emeritus
If you push down on a piston, compressing the air inside (like a bicyle pump, for instance), you are doing work.

Have you seen the formula work = force*distnace?

The work you do compressing the gas is what raises its temperature.

The ideal gas law, PV=nRT only works when a gas is behaving ideally. In reality particles even under ideal conditions experience forces of attraction. Under high pressure or low temperature these forces are significant enough to affect a gas in a not so ideal fashion.

as a gas is compressed if the temperature remains constant the pressure will rise until the gas liquifys. As the gas turns to liquid Volume decreases but pressure remains the same. This is the gas behaving as a real gas, not under ideal conditions.

What is really happening is far more complex. But in simple terms the relationship can be descibed by this mathmatical equation:

c^2 = (3RT/M)
c is the average velocity of the gas molecules in m/sec, and M is the molecular mass of the gas in gm/mol.

as the compressed gas particles escape through a small opening the are attracted to the particles remaining in the compressed gas, this attraction causes the particle to slow, thus the temperature(T) decreases. Eventually the temperature in the collection apparatus falls below the boiling point of the gas and it begins to condense.

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