Cp equation

[HenkF]

Hi all, I have data available regarding H, S and Cp values (g,l,s, and water soluble), and like to integrate part hereof with COCO/TEA.
The Cp equation of these data is formatted like : A + B*T/10^3 + C*10^5/T^2 + D*T^2/10^6.
I can think of using one of the available equation formats, A B*T + C*T^2 + D*T^3 + E/T^2 fits pretty well, all coefficients can be entered without major adaptations (apart from a J=>kJ thing)
Is this the right way to go?

regards, Henk

[jasper]

Yes, this sounds proper. Simply leave D zero.

Note that you can enter the coefficients in the PCD manager as well as directly in TEA (Configure -> compounds -> [select] -> Edit -> Temperature Correlation Data)

[jasper]

Actually, most models require the ideal gas heat capacity.



The difference between the IG enthalpy and the actual gas enthalpy is given by the excess enthalpy which follows from the selected equation of state. In case you are using the ideal gas law the difference is zero, but this is not the case for any other EOS. So you can put your coefficients in, then pull gas enthalpy.Dtemperature out (e.g. plot vs temperature perhaps at several pressures, copy data out, copy to Excel), see how far you are off from the actual gas temperature derivative of enthalpy. This will tell you what ideal gas heat capacity (Cp = d h / d T at constant P) should be as a function of temperature and you can refit the coefficients using any of the formulas (which ever fits best).

Then for the liquid phase, if you are using an equation of state there are no more degrees of freedom as the EOS parameters are all determined from critical properties, which I presume are known. So if you have determined your IG heat capacity, your liquid enthalpy should be ok already, within the restrictions of the EOS you are using. If you are using an activity model however, there is the heat of vaporization that you will need to refit from the liquid enthalpy predicted and the liquid enthalpy you have. Refit any correlation, put it back in.

For solids, TEA only deals with compounds that cannot live in the solid phase, or can only live in the solid phase, at this point in time. The solid enthalpy model requires not IG heay capacity, but solid heat capacity. Best to fit that one immediately to the d h / d T data for the solid phase of course.

[HenkF]

Hi Jasper, that's a clear line up of the requirements. I guess I will need your assistance once I've entered some data...
esp. because I'm more confident with using activity models, which will be required by some of the models I intend to use.
Anyway, thanks for your kind assistance so far.
Henk

[HenkF]

...follow up:
I have data available from a different source. From that source I have compared data for SiO2/Sand with data available within solids.pcd, after a simple conversion step.(as indicated in an earlier message in this thread). A pretty good match!
When I however use these converted data within the PCD manager, very different values seem to be calculated. As an example: I entered Solid Heat Capacity Data, using equation 6, with A=58082, B=-0.033, C=0.028221,D=0 and E=-1425900000.

Plotting and calculating using Excel shows a good match, but once entered in the PCD manager, very different results are calculated. Where did I go wrong? pcd and excel file have been attached.

I also exported data to XML, and tried to import it with the PCD manager. Unfortunately ddn't yet succeed. Guess something in formatting is not correctly passed, although the XML file seems identical to one I used as an example (=> an export to XML and then re-import data from that file, with some minor changes. It did succeed) Anything I should be aware of? Formatting issues? Character encoding?

regards, Henk

[jasper]

How did you get the numbers in COCO? If you enter the same coefficients in TEA I would expect an exact match in your graph, so I think something is off there.

Not sure what is wrong with the numbers in the PCD manager. Something for the ChemSep developers to look at. To be continued.

[HenkF]

...I entered the numbers by hand, using the PCD manager: defined a new compound, and then filled the required blanks for solid compounds (name, CAS, MW, solids Cp, solids density, H, S and G IG formation values (with respect to solids the latter naming is then a bit confusing. Are the enthalpy, entropy and resulting Gibbs Energy of formation intended, for the stable state at normal conditions?) Maybe just another equation is used, instead of the intended one?
regards, Henk

[jasper]

The PCD manager is obviously not giving the right values. But can you send me the fsd that you used to pull the COCO data from? It looks suspicious too. I do not think the match is that close.

For solids reference state, see here (scroll down to solids), or the entropy page.

[jasper]

[HenkF]

Thanks! So this does imply that the calaculated parameter values might be OK to work with. Just curious : the formula 6 implementation thus was in error, and has been adapted. Is this implementation used by TEA as well, or does this also needs adaptation? This helps me a lot in using COFE with solids.
Once ready-and-checked I will pass a solids data set. Please don't hesitate to make a request for solids data. But in general I only have Hf,Sf, Cp and density data available.
Henk