5 replies to this topic

[grad]

Pardon me if this has been discussed earlier. I am new to the forum.

Thermodynamics books, Perry's Handbook and my education all taught me that absolute enthalpy is an abstract concept, and what matters is the delta H. However, all these books tended to suggest that scientifically, enthalpy is zero at absolute zero temperature. For practical purposes, liquid water at 32 degrees F was set to have zero enthalpy. Smith and Van Ness's Thermodynamics book used that.

Recently, I started using simulation softwares such as HYSIS (AspenTech) and Korf who use databases such as NBS Steam and ASME Steam. They often show enthalpies as negative. For example, enthalpy of saturated steam at 365F and 148 psig is shown as negative 1.018E5 BTU/lb mol, which becomes negative 5,649.3 Btu/lb. Perry's and Smith and Van Ness's Thermo book show enthalpy at these conditions of positive 1195.7Btu/lb. Interestingly, entropy values are all positive, and exactly the same as in Perry’s or Smith and Van Ness.

Can somebody explain why the simulation databases and NBS and ASME Steam databases use such large negative numbers for enthalpy? Thanks.

[joerd]

You give the answer to your own post in the second paragraph. The reference state where you define enthalpy equal to 0 is arbitrary. So, different people, software, books, define different reference states, and the enthalpy values they report are going to be different. For example, in the Hysys Simulation Basis reference you'll find:

Ideal Gas Enthalpy basis used by HYSYS is equal to the ideal gas Enthalpy of Formation at 25°C

So, since the enthalpy of formation of water is (very) negative, the reported values are very negative.

[grad]

Thanks Joerd. That really answers my question.

[engware]

Hi there:

Here is a plot on physical properties for a few species to help clarify the reference temperature value and enthalpy values.



Also, I would suggest to check out the following web site for physical properties:
http://www.nist.gov

Thanks,

Gordan

[Syed Faizan Ali]

Hi,

So of we are to consider and calculate the enthalpy for the steam, which value is supposed to be considered? Or how are we supposed to calculated it in order to account for the energy withheld by the steam in Aspen HYSYS?

[PaoloPemi]

perhaps I am missing something but what about to let the user define the basis for enthalpy calc's ?
I say this since both my software tools ( Prode Properties and Nist Refprop ) allow this, it's very easy,
for example for enthalpy Prode allows to set as base
-a value (of enthalpy, specified by user) and a temperature (specified by user)
-enthalpy of formation in database
guess also simulators do that (but i may be wrong).

Edited by PaoloPemi, 29 May 2012 - 11:41 AM.