Mongo Blongo
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critical pressure
I have something which is confusing the fuck out of me. I have obtained a refrigerant which i will be using for an improvised freezing apparatus. It
is 1,1,1,2-tetrafluoroethane, AKA HFC-134a, Freon 134a, SUVA-134a Genetron-134a, Forane-134a KLEA-134a etc.
It's critical pressure is known to be 588 psi which is a shit load of pressure. Well, i have obtained it in the form of an aerosol spray called
"Freez-It" which is used for freezing electrical components. The contents is a liquid and it is very pure! The data sheet can be found at http://www.farnell.com/datasheets/8594.pdf
I thought no way could an aerosol can stand up to 588 psi!! The can is a little bit thicker than normal aerosols but still..
I thought I would check the pressure myself and I found it to be 115 psi !
I was defiantly measuring the pressure of the liquid.
So does anyone know how it could possibly be a liquid at 115 psi which is well below the known critical pressure??
P.S.- i posted this in the e&w forum as well.
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Mr. Wizard
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Critical pressure of Genetron-134a
Your confusion is mixing up Critical Pressure and Vapor pressure. Vapor pressure is the pressure exerted by the liquid's vapor (Genetron-134a)
against the sides of the container at its temperature, while at equilibrium. In your case I guess the pressure was about 115 psi. If the temperature
was raised over the Critical Temperature of the liquid it would become a gas and no increase in pressure would convert it back to a liquid. The
Critical Pressure is the pressure needed to maintain a gas liquid interface at the Critical Temperature. If you raise the liquid above its Critical
Temperature, while pressurized, is will become a gas with the density of the liquid, but it won't act like a liquid, or stay at the bottom of
the tank. There won't be a liquid gas interface. Another common liquid that has a Critical Temperature is CO2. Above 98 degrees (approx) and
about 900psi, the liquid line vanishes, making it hard to fill smaller tanks unless you cool one of them. Maybe someone else can give a better
explaination.
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Organikum
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If the critical temperature and pressure are exceeded the compound becomes "supercritical" not a gas. SC or supercritical is a own state in
which matter can exist (solid-liquid-gaseous-supercritical). Compounds in this state have often astonishing properties, combined from liquid and gas.
For example SC-CO2 can dissolve other compounds like a liquid whereby filling the the whole available space all time like a gas. In this way for
example coffeine is extracted for decoffininated coffee. CO2 is often used for it is available and quite safe to handle. Early experiments on aerogels
had been made by using SC-MeOH. The reports make one believe SC-MeOH is something one should avoid by all means. It was rated a success when the
apparatus was NOT exploding.
About Supercritical (SC) Fluids
History of Aerogels
More on Aerogels
[Edited on 4-5-2003 by Organikum]
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PHILOU Zrealone
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When you have a fluid in a closed vessel of constant volume (isochoric device); increasing the heat will induce volatilisation of the fluid to a
gaseous form until inner gas pressure reaches the equilibrium with the fluid under it.
By doing so you get pV=nRT (perfect gas law) curve p(T).
At a certain point called critical temperature you reach the critical pressure where the liquid phase is in gaseous form all over the vessel (like a
smog) and the gaseous phase condensate all over the place.
At that point viscosity of the supercritical fluid is mitigeate between liquid and gas, properties change a lot and polar solvants can dissolve non
polar ones.
When a fluid is supercritical (over its critical T and pressure) it is a perfect gas and shaking the bottle will allow no "water fall sound"
since the liquid acting like a gas will have nor real interface gas-liquid.
For most fluids Tc and pc are elevated, some gas does display SC properties at ambiant T... Helium is such a compound (Tc is a few d° Celcius over
its boiling point)...thus despite Helium is in a gas bottle under tremendous pressure it will never display a liquid liquid phase if the gas bottle is
at 20°C (it might do so if cooled under its critical T°).
PH Z (PHILOU Zrealone)
"Physic is all what never works; Chemistry is all what stinks and explodes!"-"Life that deadly disease, sexually transmitted."(W.Allen)
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