[sadara]

I'm looking for a relatively common gas (co2/methane/n2 etc) with a vapour pressure that stays between ~200bar/3000psi and ~300bar/4500psi within the temperature range of 0-75 degrees Celcius. (I'm not sure if this is the correct terminology, I mean that a container containing both the liquid and gas phase will be at an absolute pressure of more than 200bar at zero degrees C and less than 300 bar at 75 degrees.

This is for powering an air rifle. CO2 is commonly used, at a pressure of around 60 bar, I'm looking for something with a bit more power.

Is there an online database where I can search for compounds sortable by vapour pressure?

EDIT:
Just to be clear, I'm not modifying an air pistol, I'm building one from scratch, and the tank I plan to use has a rating of 4500 psi with minimum burst pressure of 15'000 psi

[Arkcon]

In general, we don't give "how to" advice for any sorts of explosives, even "gun"-type.

However, you seem to know what you're doing in vessel construction, so I think we can give you a pass.

Somehow, however, I seem to be missing your question.  You want more pressure than 60 barr of CO₂.  So you just want more pressure, you simply want to put more "stuff" into a stronger container? 

Have you heard of PV=nRT?  That's the ideal gas law -- for a given amount of gas, the pressure, volume and temperature are all related by a constant.  Now, no real world gas is ideal, and under high pressure, they behave less and less ideally.   However, all gasses behave very similarly.  Some may condense into a liquid or solid more easily, but otherwise no gas give much more pressure than another.

However, vapor pressure is a different term, not related to "pressure you'll get in a closed container."  If you look up the definition, you'll see that.

[sadara]

Yes, I'm familiar with the ideal gas law, that is not what I'm asking.
At higher pressures, storing the working fluid (gas) of an air pistol in a gaseous phase is very bulky, liquid phases are generally much, much denser, and therefor require less material to contain. Also it could be much safer, mechanical failure will be limited by some of the working fluid remaining a (very, very cold) liquid (or in some cases, snap freezing into a solid)
What I'm looking for is a gas that has a liquid to gas phase transition (the blue line in the attached picture), at the target pressure/temperatures from the first post.

[sealed] container containing both the liquid and gas phase

What do you call the pressure inside of a half full butane cigarette lighter at room temperature?

[Borek]

One thing that comes to mind: liquid gases require heat to boil off. They will cool down on boiling and then the pressure drops down (not because of PV=nRT, rather boiling slows down and no new gas is introduced into the tank).

IMHO gas that doesn't liquefy will give more umpf.

[Arkcon]

Like Borek: said, you don't want a liquid, by definition, that gives no pressure, unless there's heat added to vaporize it.  I know a frozen or liquified gas doesn't need much temperature to vaporize it, but it still needs a finite amount of heat.

What i fail to understand is:

You have x amount of CO₂ -- that's n, moles.  In a vessel of some size V.  At ambient temperature, T and you have 60 barr of pressure.

You put twice as much N₂ gas, that's 2 n, into the same size vessel at the same temperature.  Don't you get something like 120 barr of pressure?  Can't you get more pressure with a smaller (hopefully stronger) container holding more (not easily liquefied, for example nitrogen) gas?

[sadara]

I'll give some examples :
Storage vessel volume : 1 liter
Storage vessel Max Pressure: 30Mpa (~4500psi)

CO2 Liquid @ 6Mpa (60bar):
1.1 kg/litre
44 g/mol
= 25 mole of gas

N2 gas * 30mpa (25DegC)
34 grams / litre
28 g/mol
= 1.21 mols of gas


The problem with CO2 is at room temperature I can't get more than 6Mpa, as if the gas is compressed more than this, it will liquefy, and the pressure will never go over 6Mpa.
Methane seems to be a possibility, but I cant seem to find accurate phase diagrams.

[billnotgatez]

I stumbled on this article which might relate to this discussion.

Why is propane stored in household tanks but natural gas is not?

http://wtamu.edu/~cbaird/sq/2013/05/02/why-is-propane-stored-in-household-tanks-but-natural-gas-is-not

here is a table within that article