Propane gas specific heat - variation with temp 2

[mfqd13]

Hi,

I'm doing some precise calculations for a heat transfer problem with a propane gas stream. I need the specific heat of propane gas for different temperatures, say for example form 10ºC to 50ºc.
I found a great website, but the values of enthalpy for example are quite different from what i was expecting.
Link

Can anyone help me?

 

[ione]

What did you expect?

 

[MortenA]

I think you can trust NIST

 

[srfish]

One source is the ASHRAE Handbook Fundamentals I-P Edition. Thermophysical properties of propane are in the refrigeration section. For example the specific heat of vapor at 50 F (10 C) is 0.4515 Btu/lb-F.

 

[mfqd13]

Hi,

I'm not questening NIST values, but i'm trying to undersand them.
I supose that propane gas has an enthalpy value of 46300 kJ/kg, but in NIST the value for example 15ºC is 591,2 kJ/kg.

I'm confuse...

 

[TD2K]

Enthalphy values can vary between different sites depending on the conditions they use to define 0 enthalphy.

Look at the change in enthalphy between two sets of conditions using each site and see how the difference varies. I hope I've understood your question correctly.

 

[MortenA]

@macrodias,

You have to know your reference conditions (temperature) for your enthalphy of formation. Some uses 0ºC, i think that HYSYS uses 25ºC (at least not the "standard condition". So know your refere temperature (i didnt find the but it says "standard conditions" this could be 0ºC (or 60ºF) i dont know. Your enthalphy will depend on this at any given temperature - but the dH is the same.

 

[MortenA]

Guess i just repeated what TD2K said - sorry about that

 

[mfqd13]

ok, but i still quite confuse. The value that i gave of 46300 kJ/kg was the LHV.

1st question: may i say that the LHV is the enthalpy value? (i think not, but...)
2nd question: the only reference values that i foud were:

Internal energy U = 0 at 273.16 K for saturated liquid.
Entropy S = 0 at 273.16 K for saturated liquid.

May i assume that the reference temperature is 0ºC?

Thanks

 

[ione]

I think you're mixing two different things: heat of combustion and enthalpy of formation.

http://en.wikipedia.org/wiki/Heat_of_combustion#Lower_heating_value

http://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation

 

[Montemayor]

Marcosdias:

You are obviously all mixed up in your basic Thermodynamics (as well as fuel combustion). It is far wiser to stop trying to understand an accurate thermo database - such as NIST - and go back to basics, studying what Enthalpy, Entropy, and Heats of Combustion are and how they are calculated. You will find that the Thermodynamic states of Enthalpy and Entropy both rely on a relative reference temperature and pressure in order to be calculated. As defined, they are not absolute values, they are relative values and you must specify the base conditions under which they are calculated. Simply go to:

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

Now select “Propane” in the drop-down window as well as all the units that you prefer to use.
Select the Isobaric process and continue on. You will be asked for the range of temperatures (10 – 50 [sup]o[/sup]C) and the base pressure. I selected 10 bar(A).
Press for Data and subsequently, on the next page, press “View data in HTML table”.
You will get a result that can be easily copied and pasted into an Excel spreadsheet, listing all the pertinent Thermo data you ever need – and the physical state the fluid finds itself in. You will see that NIST gives the specific heats (at both constant pressure and constant volume) for each specific temperature at that pressure (10 bara).

Most importantly here, look at the bottom of the table and you will see the immediate important information relative to the printed results, the Reference States:

Reference States
For Internal energy, U = 0 at 273.16 K for saturated liquid.
For Entropy, S = 0 at 273.16 K for saturated liquid.

The Lower Heating Value (LHV) of propane has absolutely nothing to do with Thermophysical data. It has everything to do with the chemical properties of the fuel and its combustion.

 

[MortenA]

Agree Art, But not ALL "publishers" uses the reference state at 0ºC. I put "publishers" in brackets because e.g. HYSYS uses a reference state at 25ºC. So if you compare "enthalphy" across publishers you MAY have different values. Then it appears that the poster wants to know about LHV, but i didnt see that first (and i agree with your last comment wrt this.)

 

[Montemayor]

Mort:
You are absolutely correct. I'm sorry if I didn't stress the fact that enthalpy and entropy can be different values under different databases - and still be correct. It is the difference between these State Values that is the same. I thought the OP would take note of the statement I made: "..they are not absolute values, they are relative values and you must specify the base conditions under which they are calculated" and assume the obvious fact that different humans have different tastes for base conditions.

This type of common, horse sense engineering information is hardly ever emphasized and taught in universities and many of us graduate and proceed thinking that these State values should be the same in all countries and in all databases - when the opposite is true. Thank you for emphasizing the actual truth.

 

[apetri]

Marcosdias,
I can change the reference temperature when calculating enthalpy and other properties,
I utilize Prode Properties which gives this option,
you may take a look at that software.

 

[mfqd13]

Thank you all!