# CO2 vapour pressure passed critical temperature

1. Aug 6, 2009

### redargon

I've been looking at phase diagrams and vapour pressure tables for CO2 and i notice that the data stops at the critical point. what would happen to the pressure of CO2 if I heated it to above the critical temperature and kept the volume constant? Would the pressure increase following the pressure curve or would something else happen?

Thanks for any help.

2. Aug 6, 2009

Hello redargon;the pressurewould increase and follow the pressure curve.If you cooled the gas whilst keeping the volume constant it would reach a flat portion where there is liquid and vapour in equilibrium.It is a pity that your data stops at the critical point because you cannot see the overall picture.Try searching for "isotherms of a real gas"

3. Aug 6, 2009

### redargon

Thanks Dadface. At the critical point though, CO2 is quite far from an ideal gas and as I understand it the CO2 becomes a supercritical fluid. I haven't found any phase diagrams that show data beyond the critical point, because I assume that it varies according to other parameters, but I'm not sure what those are.

What I basically have is a small cylinder with 25 grams of CO2 in it. At room temperature, the pressure in the cylinder is around 60bar. what I want to know is what the pressure in the cylinder will be at 50°C or 100°C.

4. Aug 6, 2009

I dont have any access to the data except that the critical temperature of carbon dioxide is about 31 degrees.If you compress the gas by keeping the temperature constant at room temperature it will eventually move into a flat portion where liquid and vapour are in equilibrium.The pressure remains constant as the volume reduces and the vapour is squashed into a liquid.Thereafter the pressure rises rapidly.Since you want to go up to and beyond 50 degrees your gas will not liquify and so,I think, you can use the ideal gas equation to get approximately correct answers.Take care though because as you said the gas deviates from ideal behaviour and with the pressures and temperatures you are dealing with I dont know how much this deviation is.Really you need more data.I tried a quick search but came up with nothing but I'm sure the data is out there somewhere.Perhaps you could try googling NIST.

5. Aug 6, 2009

### redargon

I think I'd be able to use the ideal gas law with compressibility factors taken into account, but then I'd have to work out the compressibility factor of CO2 at the required temperatures. I'll keep searching and see what I can find. Thanks again.

6. Aug 6, 2009

### nasu

The phase diagram does not tell you how the pressure of the gas changes with temperature. Even if you are below the critical point.
The line there shows the pressure and temperatures of the phase transition (from liquid to gas). If the temperature is above the critical point you simply have no phase transition.

In any case the relation between the pressure and temperature for a given phase (not at the transition) is given by the appropriate equation of state and not by the phase diagram. For gas may be ideal gas law (for low pressure and high temp) or one of the real gas laws for high density case.

7. Aug 6, 2009

### Q_Goest

Hi redargon,
All fluids are far from being ideal around their critical point. CO2 is a fairly normal fluid from that perspective.

As Dadface mentioned, you can check the NIST database online http://webbook.nist.gov/chemistry/fluid/" [Broken]

If you do much thermo work, you should seriously consider getting a good database such as the NIST REFPROP database found http://www.nist.gov/data/nist23.htm" [Broken]

The REFPROP database is excellent. It works with Excel and other software, so you can make up programs very easily to do all sorts of things with it. I've used it before but have a very similar proprietary database I use for this kind of analysis.

If you can be more specific about your initial variables, I can tell you what pressure you'd see, but right now you've not provided enough information to determine the initial state. If you can provide mass and volume, that would work great. Or just try the online NIST database. I've not had to use it but I have to believe it works well.

Last edited by a moderator: May 4, 2017
8. Aug 15, 2009

### TrentFGuidry

Above the critical temperature, there is no vapor liquid equilibrium. The liquid and vapor phases more or less merge at that point and it is a super critical fluid.

If you increase the temperature while holding the volume constant, the pressure would still go up even above the critical temperature, however at that point, there is no more vapor pressure curve for it to follow.

The ideal gas law is pretty bad near the critical point. You would be better off using something like the Peng Robinson or Soave Redlich Kwong equation to determine the pressure under those conditions.