Perhaps because of this mistake is the neglect of the enthalpy or entropy

There are two possible source of error. 

1. The properties (entropy and enthalpy) that you are using to estimate the exergy are based on different equations of state and reference conditions. You can solve this issue by calculating exergy loss of stream and compare the results. Comparing the results at single point would not give you your any benefit. 

2. There may be difference in reference condition between yours and their method. Check the reference conditions. You have a mixture and usually the mixture properties are hard to estimate. 

Regards 

Imran

 I've calculated chemical exergy of the stream above and it is OK. There is tiny difference caused different standard exergies used probably. Bigger difference occurs in physical exergy, and as two values are needed to calculate it: enthalpy and entropy of the stream, I checked the quality of them. 

I've done the calculations using two different simulators and got exactly the same values of enthalpy change between actual and reference conditions but the entropy change differs approximately 15% between simulators. And this difference causes huge error in physical exergy, as we have to multiply the value of s-s0 by refrence temperarure 298!

Can anyone calculate the entropy of the stream mentioned above in actual and reference conditions and show the entropy change? I would like to know if the difference of 15% in entropy change between simulators is typical error or is it a bug?

Regards

Marcin

Do multiply the relation throughout with the mass flow rate so calculated. Also, the result you have calculated seems to the results of specific exergy.

Please, check the values of the enthalpy and entropy at ambient conditions used in the analysis.

Flow rate is 1 kmol/s so it doesn't affect the value of exergy as specific exergy is multiplied by 1. I think, as You suggested, that entropy values caused the deviations.

Dear

I suggested the multiplication with the mass flow rate on the grounds of various simulator results as mentioned by you, indicating mass flow rate in kg/s.

Nevertheless, entropy must be checked for getting a perfect solution.

I compared the results of specific entropy and enthalpy in 3 different simulators (commercial and freeware). For sake of fairness I would call them Sim1 - Sim3. And here are the results:

Specific enthalpy difference between actual (150degC, 101325Pa) and reference (25degC, 101325Pa) conditions [J/kmol]:

sim1: 12,37026 e6

sim2: 12,35600 e6

sim3: 12,88493 e6

The results are very similar. Seems OK.

Specific entropy difference between actual (150degC, 101325Pa) and reference (25degC, 101325Pa) conditions [J/kmol.K]:

sim1: 42825,35

sim2: 35799,00

sim3: 39562,62

And here the results are seriously scattered! As far as I understand, when calculating entropy and enthalopy change, it doesn't matter how the intergration conditons are selected when integrating heat capacity because they are cancelled when substracting h-h0 or s-s0. 

After all, the physical exergy calculated using simulators 1-3 (kW):

ph_ex_sim1: -398,1142099

ph_ex_sim2: 1682,52815

ph_ex_sim3: 1089,334715

I must say that result number 3 is comparable with literature sources. 

But huge differences in calculation of specific entropy change surprised me. I wonder if I make a mistake somewhere? Could it be somehow connected with different methods of calculation of the entropy? Unfortunately manuals are very poor and state: specific entropy without any clues about equations used...

Regards,

Marcin

I think the results need to be checked for a simple set of operating parameters using a calculator, one or two sets. There may be some error while formulating the program.

If standard formulation are used, then there should now be much different, also see if the ambient temperature is same OR different for the two cases.

Further, see if the irreversibility (losses) were considered properly.

Hope you will get the solution soon.

Please, check the values of the enthalpy and entropy at ambient conditions used in the analysis.There may be difference in reference condition between yours and their method. Units of enthaply and entropy are gvien per kmole .and hence you have to multipy moleculer wight of the substane in order to get total values because unit at RHS is W that is J/s.. 

Unfortunately we don't have such software. I've used three different process simulators and found that the results of entropy (namely entropy change betweent actual and reference conditions of the gas stream mentioned in the first post) differs significantly between them. Unfortunately I have no possibility to check the calculations in other software because simply I don't have it. If someone is interested, you can post the results of enthalpy and entropy of the stream from the first post so we can compare the results.

Regards,

Marcin Stec

Thank You. I would really appreciate your help.

I attached Excel file with composition and conditions of the stream. Values which should be passed to the simulator are marked with green color. The simulator will calculate entropy and enthalpy of the stream, just copy the values into fields with bold fonts. The sheet will automatically calculate physical exergy of the stream (red field in the bottom).

In the simulator I used the heat exchanger operating at 25degC to generate the reference stream. Stream 1 is inlet, and stream 2 is the outlet of the heat exchanger. I suppose one can simulate the stream 1 and 2 separately just changing the temperature of the stream.

For me, the entropy is suspicious in the attached file.

Regards,

Marcin

Hi Marcin.

Are you using the dead state temperature in the absolute scale? You need T_0 in Kelvin or Rankine degrees.

Regards.

Hi Jesus E Mora Ramírez.

Thanks for your interest. In fact it turned out, that the error was caused by the simulator I used. The entropy value was incorrect. When calculating the entropy manually, everything was OK.

The issue has been solved already.

Hi Marcin Stec .

I'm glad to hear that. Are you steel working on that topic?

Regards.

No, Jesus E Mora Ramírez , I've finished the topic.

Marcin

This paper explains as succinct and useful as possible:

It includes physical and chemical exergy formula as well.

Article Energy‐exergy performance assessment with optimization guida...

Please refer to this paper. It explains as sufficient as possible to clarify.

Article Energy‐exergy performance assessment with optimization guida...