Mollier diagram fro CO2 5

[MortenA]

I have been looking at a Mollier diagram for CO2 in order to determine the downstream temperature when letting down pressure drom 200 barg/3000 psig to 0 barg from e.g around 20 deg C/60 deg F.

Thats isenthalpic depressuring so i follow the constant enthalphy line to the phase split envelope, then continue downward (in a PH diagram) a- but then there's a line around 8 barg that says triplpoint and below this line there are no more line - no const enthalpy, entropy, density, temperature...

Now I tried to look at my textbooks, but none of the "normal" (water and propane are quite common)PH diagrams has a triplepoint in this region - so the description dosnt cover this. How do I read it - because of course I can let down the pressure - but what will the temperature be and how much of the phase will turn into dry-ice?

A question comes to mind: Does the phases have the same density at the triplepoint?

Anyway how do I read this map - and how do I get from the triplepoint line to 0 barg? Is the problem that you can get experienment in the solid/vapour system because its to difficult to measure - or is it that further depressuring cannot be isenthalpic - or will all turn to ice at a instant (and wouldnt that require enthalpy?) or will i the follow the P/T curve in the phase diagram separating gas and solid - with a part of the co2 in solid and the other ingas?

I dont trust HYSYS/PR because it predicts liquid CO2 at atm pressure - not possible! I also tried to switch to antonie og K10 - but they also find liquid co2 at atm.

Hope somebody can explain how the Mollier diagram works below the triplepoint line.

Best regards

Morten

 

[zdas04]

REFPROP has the triple point of pure CO2 at 75.1 psia (5.1 bara) and -69.8F (-56C). At the triple point by definition the three phases have the same density, enthalpy, entropy, temperature, pressure, etc. You follow the sublimation line from the triple point to 0 barg (on a phase diagram that would be -100F (-73C) or so

Do a search on CO2 and "Montmeyer". Art had a masterful description of what happens as you depressurize CO2, but I can't remember where it was (maybe in Compression?).

David

 

[Montemayor]

Morten:

You are absolutely correct in your suspicions. Any properties database or Equation of State that predicts a liquid phase for CO2 at atmospheric pressure (on planet Earth, at least) isn’t worth the paper it is printed on.

The best visual representation of the thermodynamic property values of CO2 has to be the T-S Diagram as published and freely distributed by the Liquid Carbonic Corporation during the 1960’s. It was detailed, color coded, and 13” x 22” on heavy, quality paper. It was originally assembled under the German data of Kupriniof and used by their engineering department for designing everything from compressors to dry ice presses and all cooling applications. I know it was accurate – because it worked! I sized a large number of compressors using nothing more than this diagram and a slide rule. I have only two decent copies in my possession as mementos. Liquid Carbonic was purchased by Praxair and I don’t think the printing and distribution of the T-S diagram has been continued anymore.

The other diagram I have and obtained from the NGPSA is a Mollier type (pressure vs. enthalpy) and it also depicts all the phases. It was constructed in 1981 from “Fluid Thermodynamic Properties for Light Petroleum Systems”, by Dr. K. E. Starling; copyrighted by Gulf Publishing Co.

Another source for CO2 properties below the Triple Point is the published tabulation of saturated data of ASHRAE, as found in their Handbook of Fundamentals (1965), page 325. I have this and other CO2 data in Excel flat files and you are welcomed to it if I can send it to you. Unfortunately for us, the great and free NIST database found at

http://webbook.nist.gov/chemistry/fluid/

doesn’t include thermo properties below the Triple Point. In other words it doesn’t go into the solid-vapor equilibrium phase.

Now to the subject of isenthalpic expansion of liquid CO2 below the Triple Point and each of your queries:

1) “I can let down the pressure - but what will the temperature be and how much of the phase will turn into dry-ice?”
The temperature and the “quality” (or % vapor) can be read directly on my diagrams. If you know the saturated vapor and saturated solid enthalpy values at the expanded pressure (presumably, atmospheric) and you know your thermo and Mollier Diagram (which I’m sure you do), then calculating the resulting quality or % of solid dry ice snow formed is easy.

2) “Does the phases have the same density at the triple point?”
No. The Triple Point is defined as the condition where the solid, liquid, and gas phase are in equilibrium. So you have 3 different densities in question at the same time. It is an unstable condition in that if you establish the triple point, it will remain there until you evacuate the vapor being formed in order to drop the pressure any lower. In a dry ice press chamber, you can actually see this phenomena happening as the pressure gauge swings back and forth at approximately 60 psig (75.1 psia) while the formed vapor is evacuated and the chamber is expanded.

3) “how do I get from the triplepoint line to 0 barg?”
Easy. You simply follow – on the T-S or Mollier Diagram – a constant enthalpy line down to the 0.0 barg line. On my diagram, you can read the point values for enthalpy, entropy, pressure, temperature, and specific volume.

4) “Is the problem that you can get experienment in the solid/vapour system because its to difficult to measure - or is it that further depressuring cannot be isenthalpic - or will all turn to ice at a instant (and wouldn’t that require enthalpy?)”
There is no problem in going through the Triple Point on your way to a lower pressure, dry ice + vapor system at -109 oF. The thermodynamic path is very clear and direct. What you are having trouble visualizing is nothing more than what is happening every time you activate the valve on a CO2 fire extinguisher. The CO2 in the extinguisher is in a high pressure, saturated liquid state at ambient temperature (which conveniently is lower than the critical temperature of 88 oF for CO2. This HP CO2 liquid is expanded from approximately 75 barg down to 0.0 barg – in ONE STEP – and the result is an instant mixture of dry ice snow and saturated vapor at -109 oF.

5) “will I the follow the P/T curve in the phase diagram separating gas and solid - with a part of the CO2 in solid and the other in gas?”
Yes, at the end of the expansion you have described, you will wind up at a point on the horizontal (pressure) line joining the saturated vapor and the saturated solid (dry ice). In your case, this line is the 0.0 barg pressure line. The termination point of your liquid CO2 expansion will be a point on that horizontal line, inside the “dome” curve. The ratio of this point’s location to the overall length of the line represent the “quality” – or percentage of vapor formed. I’m sure that when you reflect on this explanation you will quickly recognize that it is nothing more than what you learned in Steam Lab as a young mechanical engineering student trying to find the quality of the steam by expanding it in a steam calorimeter. It’s the same thermo process – nothing changes.

Let me know if you are interested in my thermo data and how I can send it to you. I hope I've been of help.

 

[MortenA]

I think getting the right diagram would do the trick. The one I got was someting that I more or less pulled random from the net - and not very "sharp" (the map that is ). I would really appriciate a reference to the char from Liquid Carbonic Corporation that you mentioned.

I considered checking out nist but didnt initially because i wanted to use the chart (also for a graphical presentation later on) - not just lookup that at least my memory told me was what i can do at nist - but i will revisit that thought.

Best regards

Morten

 

[MortenA]

Oh yes - and i wrote my original question from my home adress where i didnt have the chart - and its of course at 5.18 bar, -56.558ºC - but its a P-H diagram.

Besst regards

Morten

 

[MortenA]

Checked the HYSYS manual this is a (long) quote from the manual:

Aspen HYSYS does not check for solid phase formation of pure components within the flash calculations, however, incipient solid formation conditions for CO2 and hydrates can be predicted with the Utility Package.

Solids do not participate in vapour-liquid equilibrium (VLE) calculations. Their vapour pressure is taken as zero. However, since solids do have an enthalpy contribution, they have an effect on heat balance calculations. Thus, while the results of a Temperature flash are the same whether or not such components are present, an Enthalpy flash is affected by the presence of solids.

Reading this im a bit unsure: They have an "utility" - i know this one - but it only checks PT agains the phase map for pure co2 and then reports if solid co2 is present or not.

So will it correctly find the temperature and phase volumes and just report it as liquids or is it just not usefull? Maybe i should try the support line

Best regards

Morten

 

[pmover]

Art . . .

Any possibility this is the same chart? Not color coded of course . . .

http://books.google.com/books?id=h-...xsDYBg&sa=X&oi=book_result&resnum=3&ct=result

Something to ponder . . . take a good digital photo of the chart and include with posting.

-pmover

 

[Montemayor]

pmover:

Good eye!! That is a copy of the original. The original was color-coded, with constant quality, density, pressure, and enthalpy lines --- all easy to pick up and identify.

It was a great deal bigger and was not problem for picking off thermovalues as you followed the thermo cycle through its paces. I believe this text book tries to teach this same concept through use of this illustration. It worked, and it worked well enough to size industrial-sized equipment when designing multi-staged compressors, heat exchangers, dry ice coverters, evaporators and hydraulic presses. I remember designing and specifying 3-stage CO2 compressors and only missing the discharge temperature of any compression stage by no more than 3-5 degrees Fahrenheit. In the era of the slide rule, that was nailing it.

 

[MortenA]

Its a bit hard to read (im just a cry baby ) - so if you still have it in your heart (and a better copy) then i would really appreciate a link. Maybe i will just have to chekc my technical library...

Best regards

Morten

 

[MortenA]

BTW

Just got it confirmed by Aspen - HYSYS does not handle the solid phase - although it has a check of the possibility of a solid water and CO2 phase. Bummer.

Best regards

Morten

 

[Montemayor]

Morten:

The attached bitmap is all I have to send you at present. It's a horrible example of engineering data documentation, but at least it is an overview of the various Thermodynamic phase zones. Nevertheless, I am going to try to see if I can get a decent, clear, and readable .pdf version of the color-coded Liquid Carbonic T-S diagram that I have at home. If I succeed, I'll also post it here for your download.

 

[MortenA]

Art - you dont need to break your back over this diagram - i located an acceptable reference here:

http://www.union.dk/media/CO2-GB.pdf

and then its in SI units

Best regards and many thanks for the hints, tricks and help!.

Best regards

Morten

 

[Montemayor]

Morten:

Great Hunting! This is a great representation of the data you are looking for. Although the plot is very busy and not the easiest to follow and identify due to the single color used, the information is shown sharp and the resolution is very good.

As the booklet states, "the information is based on current technical knowledge" --- that of Plank and Kuprianoff, who put it all together over 70 years ago. Good data never ages. As I stated before, I always found the Plank & Kuprianoff data to give good results 50 years ago. Obviously, the Union people have found no good reason to challenge this data - even in today's instant computer databases and "improved" equations of state.

 

[MortenA]

The only trouble i had was that i need data above 110 barg - but then i have another that didnt include the data below the triple point - but here the reference enthalphy is not the same. But as far as i rember its just a superpositioning dH is always the same but "absolute H" depends on a refernce state. So i found a common reference point in the two PH diagrams and calculated the difference in reference enthalpy.

Its also a little funny that most charts i have seen are log on the pressure axis - but not this one. It make the constant temperature curves much more vertical.

But apart from the PH diagrams thers also a lot of other good stuff on CO2 including soluability data and so forth so im very pleased with my findings.

Best regards

Morten

 

[revata]

Hi,

Thanks for the helpful posts. Data on predicting dry ice formation when compressed CO2 liquid is expanded is something that I have been looking for months. Your posts are exactly what I need

revata