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# Hysys Reference For Entropy? Why Hysys Entropy Numbers Are Different F

hysys entropy entropydiscrepancy simulation properties thermodynamics chemicalengineering aspenhysys aspen

5 replies to this topic
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### #1 stratovona266

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Posted 31 May 2017 - 01:25 PM

I'm having a bit of a problem with HYSYS. For example, based on the literature I'd expect water entropy at 0.1MPa and 800C to be around 235.7 J/(mol.K), but HYSYS gives me 220.8 J/(mol.K). Does anyone know why HYSYS entropy numbers are like this?

It seems that for all temperatures the entropy that HYSYS gives me are a certain amount higher than what I expect them to be. For example, for water it's around 15 J/mol.K.

I'm also wondering why in HYSYS entropy goes to 0 at -120C instead of -273C which is 0K. At around 0K HYSYS actually gives me astronomically large numbers.

### #2 Saml

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Posted 31 May 2017 - 10:39 PM

What are you trying to do?

For hight temperature steam you should use a thermo model specific for water like Steam Tables or IAPFW. If you are using a generic equation of state, then the difference you are seing is reasonable.

I have not checked this, but I would not expect Hysys to implement models that are consistent with third law of thermodynamics down to 0K when their expected range of use is way above that. What are you trying to simulate with water at -120°C?

### #3 PaoloPemi

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Posted 01 June 2017 - 03:32 AM

You can find many threads at cheresources.com discussing this topic, the obvious advantage (of setting the initial values for H, S integration) being the possibility to compare directly the values calculated with different models and software.
I have PRODE Properties which allows to define the initial values for H, S (then giving the same results with mixed modes) other software may include this option, but from a thermodynamic point of view I would expect that results are consistent in every case

### #4 stratovona266

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Posted 01 June 2017 - 02:36 PM

Saml I'm trying to calculate deltaS (entropy) of the electrolysis reaction at high temperatures. I'm using NIST data for water, hydrogen and oxygen, and the results are consistent with the literature values for delta S. In hysys for this reaction I can use the enthalpies that hysys has to correctly calculate delta H, but for delta S it doesn't work. The S numbers that I get for water, hydrogen and oxygen are different from literature values. So far I've fixed this by adding a dummy factor to the S values that hysys gives in hysys spreadsheets. I did this because I noticed that the S numbers that hysys gives are always (at different temperatures) a fixed amount higher than what they should be ( a different fix number for each substance, e.g., around 14.8 J/mol.K for water). I'm not happy though, it really bugs me that I've had to do this.

### #5 PaoloPemi

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Posted 02 June 2017 - 04:24 AM

May be you are comparing different things,
With NIST (REFPROP or similar software ) you calculate the difference delta S and delta H from point p1 to point p2, no reactions,
You should be able to calculate these values (delta S and delta H) with different thermo models for example a EOS such as Peng Robinson (there are many variants, with some extended versions, I know Prode PRX, for water errors in dH , dS are below 3% (except in critical area) , I consider errors in the range 1-3% acceptable for my work, but multiparameter models such as IAPWS95 or REFPROP are accurate within 0.2-0.6% (except in some areas),
But if you add reactions I do not understand how you can compare values...

### #6 maggle

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Posted 22 June 2017 - 03:03 AM

maybe you can use (www.eThermo.us), The thermodynamic properties of water can be calculated with the temperature,You can make a comparison.

Properties data fits very well with NIST,,have a try.