Daltons Law Explained 2

[Metalguy]

Here's the question. A very common MIG/GMAW welding gas mixture is 75% Ar and 25% CO2. We know that at temps. below ~89 deg F a cylinder of CO2 will have both liquid and gaseous phases present if the pressure is higher than some psi (I think it's around 900 psi). Now, if we have a cylinder with only a small amount of CO2 (assume all gas phase), and we then fill it to ~2200 psi w/Ar, will most of the CO2 change phase to liquid?

 

[waross]

If that were the case, scuba divers would get pure CO2 when they were swimming downward and their tanks were inverted. The liquid CO2 would flow to the outlet and flash to gas in the primary 140 PSI regulator. The diver would get a lung-full of cold CO2 that would trigger the bodies involuntary gasp reaction. The scuba industry would have been limited to oxegen re-breather type equipment from the start.
yours

 

[Montemayor]

The basic question in this thread is a fundamental and practical one that has everything to do with (& about) Thermodynamic Phase Equilbria and very little or nothing to do with Dalton’s Law. This is a let down for me because John Dalton and his famous Law have always been favorite discussion pieces with me. John Dalton’s famous relationship is centered on gases and their vapor-state composition. What is being asked here regards the liquefied state of gases – Phase Equilibria.

This is all about basic Thermodynamics and this thread is certainly in the right Forum. I’m eagerly looking forward to what the real experts can contribute and throw in here with regards to this very fundamental and important Thermo subject. However, one should be warned: we’re going to have to talk about the Critical Properties of Pressure and Temperature as well as Bubble Points and Dew Points in order to truly address the basic question. To do that, we also have to say something about Equilibrium Constants.

In order to leave the door open for all who want to (or can) contribute valued comments, I’ll basically answer with the statement that all gases can be liquefied. However, when a mixture of gases is dealt with, the amount of condensation depends on the equilibrium constant and conditions under which the gaseous mixture finds itself. If a mixture is in equilibrium at a temperature between its bubble point and dew point, then by basic Thermo definition it must consist of two phases – liquid and gaseous. It is by making a Dew Point calculation (at a given pressure) that we can identify the temperature at which we will find the first formation of liquid. The manual reiterative calculations are a little tedious; a simulation program makes a quick resolution of the problem.

Depending on the amount of CO₂ in the binary mixture, it is possible to have some of it liquefy – although I doubt this will occur at 2,200 psig with a nominal CO₂ content less than 10%. What waross is stating is far-fetched and has little or nothing to do with the basic question. The amount of CO₂ hasn’t been identified, so one can’t state if it would drop out of the gaseous phase by liquefaction. Scuba cylinders are indeed filled with high pressure air but the amount of CO₂ in air is miniscule. However, I can easily prove that the partial condensation of the components of air is being carried out every day inside an air separation column. I did this for many years, producing both gaseous and liquid Oxygen and Nitrogen – from atmospheric air.

 

[25362]

Mixtures of (unreacting) gases behave differently in that the vapor pressure curves on a phase equilibria P-T chart form a loop, rather than a single line on which the bubble and dew lines coincide as for single species.

This means that the P-T relation is not the same for the saturated liquid and the saturated vapor of a mixture of constant composition. The critical point would be somewhere near the apex of the loop.

It is common to see the dew line (to the right of the critical point of the mixture), on a P-T diagram, bulging somewhat towards higher temperatures (on the abscissa).

This means that even when at temperatures higher than the critical point of the mixture one can still condense by compression or by decompression (retrograde phenomena).

Although, as Montemayor says, it seems unlikely that a binary mixture of argon (T_c=-122^oC, P_c=48 atm) with a minimal amount of carbon dioxide (T_c=31.1^oC, P_c=73 atm) would show a dew point at, say, 25^oC and 2200 psia (~150 atm).

 

[Metalguy]

Good posts-a bit over my head but good anyway. Keep in mind the gas mixture in the first post was 75% Ar and 25% CO2.

 

[joerd]

The "loop" that 25362 refers to is commonly called a phase envelope. You can calculate bubble point and dew point curves for varying gas compositions. This will show you that all Ar-CO2 envelopes are always below 2000 psia ("cricondenbar). This means that above 2000 psia (at 40 mol% CO2) or above 1940 psia (at 25 mol% CO2), mixtures of Ar-CO2 are always single phase. Likewise, if the temperature is high enough, the mixture will not condense, e.g. above -35 F for 25 mol% CO2.

Cheers,
Joerd

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