Vapour Pressure above critical temperature?

[johns147]

Hi all, can anyone explain the following? Background is I am looking into some pressurisation that occured following a small volume of liquid nitrogen leaking into a vessel.

As I understand it, the vapour pressure of a substance at a given temperature can be measured by introducing liquid into a barometer and measuring the displacement of the mercury column. In order to ensure that it is the vapour pressure at equilibrium, enough liquid must be added so that some remains (i.e. it does not all evaporate and a two phase equilibrium is established).

Also, above the critical temperature for the substance, liquid cannot be formed, no matter what the pressure is increased to.

However I have seen examples of data being given for vapour pressure values at temperatures way above the critical temperature (and above the critical pressure).

e.g. see the value for vapor pressure of nitrogen 63.2 MPa at 20°C in this article:

http://en.wikipedia.org/wiki/Vapor_pressure

Does anybody know what vapor pressure value given actually represents in this case?

 

[Latexman]

No, a vapor pressure above the critical temperature is meaningless.

Good luck,
Latexman

 

[johns147]

Thanks. Presumably this figure is some other value applied in error. I wonder what it is.

There are no references for the wikipedia values anyway so I guess I should have figured that out.

 

[sheiko]

For info.

http://encyclopedia.airliquide.com/...aporPressureGraph/nitrogen_vapor_pressure.gif

"We don't believe things because they are true, things are true because we believe them."

 

[dcasto]

we assign an apparent vapor pressure to ethane of 800 psia at 100 F, yet there is no such thing as a liquid ethane at 100F. We also assign a VP of 5000 psia to methane. These values are used to approximate the VP of a mixture that can contain small amounts.

We run into issues when a product specification allows a VP of 400 psia, but when you use the VP approximation of sum of (lv% * VP) for C1 to C8 and you get 430 psia, outside of specification. If you then run a PengRobinson flash on the mixture, you'll get a 380- psia VP, much lower because of mixing.

 

[Latexman]

dcasto - that sounds somewhat analagous to Henry's constant to me.

Good luck,
Latexman

 

[25362]

For estimating bubble and dew points of light hydrocarbon mixtures, why not use published K factors?