Supercritical carbon dioxide

[AWCG]

https://en.m.wikipedia.org/wiki/Supercritical_carbon_dioxide

This question comes from following too many Wikipedia links.

Are we forming environmentally friendly dry cleaning fluid in our PCPs?

I think most of the atmospheric gasses are above their critical point (can’t be liquified by pressure alone when above x°) at room temp. But what’s going on with the CO2? Is it cleaning my barrel for me?

 

[T3PRanch]

You are only running the gas phase of CO2 so no cleaning action!

Thurmond

 

[AWCG]

Darn. 

 

[prfssrlee]

Huh, I never even thought of that before. In the cylinder at room temperature if the pressure is higher than about 1000 psi, the CO2 will be supercritical (sCO2). Below room temp, it will just be a liquid at those pressures and higher. But the dry cleaning action from sCO2 is due to the rapid gasification of sCO2 when the high pressure is released. So clothes are first washed in a cleaning fluid then the chamber is pressurized and sCO2 is introduced. It permeates all the pores of the clothing. Then when the pressure is released, the sCO2 becomes a gas and takes the cleaning fluids and dirt/oils with it into the vapor phase, instantly "dry cleaning" your clothes. Then the CO2 gas and cleaning fluid are recaptured and reused.

But I think you are right - the tiny amount of CO2 in air would be fluidized (turned to sCO2) in the cylinder when the pressure & temperature are high enough, and it would be small drop of liquid CO2 when the temperature isn't quite high enough but the pressure is. Air is only about 0.03% CO2 by volume, so it doesn't amount to much. Still, that's neat. Sorry, its the teacher in me coming out. I'm pretty much always just a student around here, so it was surprising to run across a bit of physics!