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

hysys entropy entropydiscrepancy simulation properties thermodynamics chemicalengineering aspenhysys aspen

12 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 -120Â°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.

### #7 PingPong

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Posted 22 October 2020 - 10:52 AM

The value of entropy or enthalpy is always relative to a certain reference (datum, basis).

Different sources, or software, use difference reference levels so they give different values.

Therefor you can never use a combination of entropy or enthalpy from multiple sources in your calculations, unless you know for sure that they happen to use the same reference.

Absolute values of entropy or enthalpy are not important. What matters are differences, and differences from a certain source are the same no matter what reference that source uses.

So it does not matter what reference Hysys uses for entropy and enthalpy as long as you only use values from Hysys in your calculations.

And it does not matter what reference NIST uses for entropy and enthalpy as long as you only use values from NIST in your calculations.

### #8 PingPong

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Posted 23 October 2020 - 03:55 AM

It is not clear what you are talking about.

You mention some numbers, but you do not indicate which component, which phase, which T, which P.

If two different sources (e.g. Hysys and NIST) report different values for the same component at the same T and P , then those two sources use a different base. It's as simple as that.

### #9 breizh

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Posted 23 October 2020 - 07:20 AM

Hi,

Attached a link (Korean Data base) with the properties related to N Butane .

https://www.cheric.o...rop.php?cmpid=4

Hope this is helping you .

Breizh

### #10 PaoloPemi

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Posted 23 October 2020 - 09:14 AM

you need to define a base (reference) value for enthalpy / entropy to start integration,

yes, the values for enthalpy of formation of N-Butane available in many software applications (I attach a screenshot from Prode) are those mentioned by Breizh,

note that if you add reactions you may consider Gibbs energy of formation too (see attached screenshot),

your point about the values for the different constants stored in Aspen should be in my opinion discussed with their technical support.

### #11 PaoloPemi

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Posted 23 October 2020 - 11:33 AM

that is probably the best option as we do not know the details,

I like the possibility to specify the base / reference / initial condition (see my previous posts) but that is really not necessary.

Also the definition of standard condition (1 atm of pressure and 298.15 K) without a definition of a hypothetical state, may introduce some ambiguity due to phase change solid->liquid->gas etc.

### #12 PaoloPemi

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Posted 24 October 2020 - 01:04 AM

please do not simplify, my considerations are based on definitions available in IUPAC or equivalent standards,
for a short introduction, see  4.1 The concept of the standard state in

http://www.degruyter...ticle-p1239.pdf

### #13 PaoloPemi

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Posted 24 October 2020 - 11:06 AM

sure,

if you need to justify a value calculated by a commercial application better to talk with developers,

after all, they have been paid for that...