This is a fun topic because it involves what we would normally classify as a mundane and ordinary procedurre involving a common, mundane "gas". It turns out that it isn't so; it really involves a compressed gas liquid that has extraordinary and interesting characteristics as it either cools down or heats up.
This is one thread that has certainly been posted in the CORRECT forum: Heat Transfer & Thermodynamics engineering. My compliments to bnrg for his insight. I will attempt to address the last two interesting posts:
MintJulep:
The best way I know how to prove what I stated is for you to rig up a CO2 cylinder with a pressure regulator set at approximately 50 psig and open the down stream valve 100% while placing the palm of your hand on the regulator's body. You will soon experience the absolute visibility of the system's lower temperature as the saturated gas is evacuated from the top of the cylinder. I know of no better way to prove what is clearly shown on the CO2 T-S (or Mollier) diagram. If you deplete the saturated gas in the cylinder faster than you can re-generate it by heat transfer through the cylinder steel walls, the pressure and the temperature will come down within the cylinder (& the system). And the effect will be very visible on the outside of the cylinder in the manner of ambient moisture freezing on the cylinders external surface and forming a "frost line" that can be clearly seen.
This is not theoretical Thermodynamics. This is the same process by which I have industrially produced thousands of tons of Dry Ice (solid CO2 @ -109 oF). I have done this starting with HP CO2 (like what exists in a cylinder) as well as with LP CO2 (Liquid CO2 @ -8 oF & 250 psig). This is still the way Dry Ice is produced today.
25362:
While I'm presently not at my customary library at home but helping my daughter move into a new home in Tucson, AZ, I'm still reminded of old problems and process headaches in reviewing your interesting 6 questions. I don't need access to my T-S diagram to identify the critical zone (or nightmare, as we used to call it) for CO2. I enjoy sharing my thoughts on this subject with you since I personally have read your many knowledgeable Thermodynamic discussions on this Forum for some years now. I don't think I can reveal any new Thermo knowledge to you.
Your first 2 questions have to do with theoretical and empirical findings regarding the supercritical zone. You are, as usual, correct in your analysis according to what I have witnessed in the field. For years, we called this nebulous zone "Mush" - or something similar.
Question 3 is temporarily out of my grasp since I don't have a T-S diagram to identify the phase site and the process path. The same applies to #4.
You have re-inforced my finding with #5. This is why the "old timers" used tap water (ambient temperature) - and not anything hotter - like steam - which could "pop" the cylinder's relief device. And, yes, if you want to heat up faster you can employ an inline heater downstream and also electrically trace the gas regulator.
#6 has been answered above. I can make Dry Ice by "evaporating" the saturated liquid's vapor pressue down to a temperature where, if you look on the T-S diagram, you will land on the horizontal equilibrium line connecting the saturated SOLID state (on the far left) and the saturated vapor curve (on the far right). This is the strength and simplicity demonstrated by the T-S diagram. Unfortunately the NIST data on their website doesn't identify the CO2 solid phase. For Dry Ice thermo data I resort to my own. In fact, the process vessels that were used for this purpose back in the old Liquid Carbonic Dry Ice Plants were called "evaporators" - simply because that was what was going on: a batch quantity of liquid CO2 was introduced into the vessel and then the vessel's saturated vapor was subjected to the suction of CO2 recompressors. The result was progressively colder and colder liquid CO2 that was eventually injected into a Dry Ice Press at a temperature and pressure just above it's "Triple Point". At that point, the remaining available saturated vapors were "sucked" out by the recompressors and the result in the Press chamber was solid, CO2 "snow" - which was ultimately compressed, using hydraulic rams, into a solid Dry Ice block.
I never like this method because I found it wasteful and slow. That's why I patented an automatic machine that produces pellet-sized Dry Ice on a continuous basis. This makes more sense and is far more economical and cost-effective.
I hope I furnished useful experience to your notes and I apologize for not being in a place with access to my files and databases.
Art Montemayor
Spring, TX