The "Right" CO2 Mollier Diagram 1

[zdas04]

I'm doing some scoping work on a CO2 sequestration project and need a p-h chart. I started in Google and found Chemical Logic which has some really cool software. At

http://www.chemicalogic.com/co2tab/downloads.htm

I found a .pdf file for a Mollier Diagram in English units and I was starting to use it until I saw that the enthalpy scale was -200 to 80 BTU/lbm and my calcs were giving me odd answers.

Then I went to the GPSA Field Data Book and found (figure 24-23) that their x axis ran from 50-550 BTU/lbm.

Lastly I went to NIST and generated my own chart (what a pain) from their excellent data and got an enthalpy scale from around 30 to 300 BTU/lbm). I'll always believe NIST until proven wrong.

I've picked a bunch of points and converted them to kJ/kg, kJ/mole, kcal/mol, kcal/gm and none of the conversions from NIST come close to the other two.

For a specific point, if I look at where -40F crosses the saturation line (on the right side), NIST says 187.3 BTU/lbm, Chem Logic says -32 BTU/lb, and GPSA says [maybe] 355 BTU/lbm (their scale is pretty broad). The critical point was 1070 psia on all three, but it was h=155.09 BTU/lbm in NIST, -77 BTU/lbm in Chem Logic, and 320 BTU/lb in GPSA.

I've never had cause to use a CO2 Mollier diagram before, but the Water in NIST, GPSA and Chem Logic match well so I'm really confused.

Anyone have a clue what is going on?

David

 

[MisterDonut]

The enthalpies are relative to a reference temperature at which the enthalpy is zero. It looks like the enthalpy differences between the two points is similar.

 

[insult2injury]

Pick up a thermo book and review the thermodynamic properties section.

I2I

 

[zdas04]

insult,
I must say that that was about as unhelpful a post as I've ever seen in eng-tips.com. I actually have five thermodynamics texts and oddly enough none of them have a p-h diagram for CO2. What part of my post made you think I needed to look up the definition of enthalpy?

MisterDonut,
That is interesting, I've never heard of a Mollier diagram being referenced to a pseudo-zero enthalpy, but most uses of it that I know of look at a change in "h", not an an absolute "h" so it could be. None of my thermo texts provide any insight into that formulation. I just checked and the three charts are not a linear offset from each other so a linear difference between two points would yeild different answers from chart to chart.

David

 

[amorrison]

Suggest a phone call to Prof. Mansoori (University of Chicago)

http://tigger.uic.edu/~mansoori/TRL_html

His site has a T-s or p-h chart on the first page so I assume he is knowledgeable in this area.

Comment

From previous personal searches in this area the only easily available information is what is in the American Society of Heating, Refigeration and Air Conditioning Engineering, Inc (ASHRAE) Handbook - Fundamentals Volume-Refrigerant Tables and Charts chapter. These charts were produced in the 1930s by the National Bureau of Standards.

Sad to say but I doubt that more that 1% of ME's ever even look at these hyperimportant charts after they graduate.

 

[insult2injury]

zdas04: Yes, I was talking about the definition of enthalpy. Enthalpy is ALWAYS measured with respect to a reference condition. All five of your texts should cover this.

The reason that all may be slightly different is that they are each based on a different EOS (van der Waals, MBWR, etc.). By default, NIST uses a Helmholtz EOS for CO2 as published by Span and Wagner (1996). NIST also has the ability to use 3 others which would all yield slightly varied results (if you have the fully licensed software). I don't know what the other sources use.

Also, CO2 does follow the principle of corresponding states very well if you wanted to pursue that approach.

I2I