Vapour Pressure above critical temperature?
Vapour Pressure above critical temperature?
(OP)
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?
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?





RE: Vapour Pressure above critical temperature?
Good luck,
Latexman
RE: Vapour Pressure above critical temperature?
There are no references for the wikipedia values anyway so I guess I should have figured that out.
RE: Vapour Pressure above critical temperature?
h
"We don't believe things because they are true, things are true because we believe them."
RE: Vapour Pressure above critical temperature?
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.
RE: Vapour Pressure above critical temperature?
Good luck,
Latexman
RE: Vapour Pressure above critical temperature?
For estimating bubble and dew points of light hydrocarbon mixtures, why not use published K factors?