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Evaluating The Duty Of An Exchanger With Phase Change


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#1 RQA21

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Posted 12 June 2020 - 03:40 PM

Hi,

I am trying to determine the coefficient of performance for a refrigeration loop. It’s for the propane precooled MR process. I have data available for my streams (temperatures, pressures and compositions). The calculation approach is straightforward: determine the combined duty of all heat exchangers then divide by the work supplies by the compressor. For example, for the propane loop, i have to calculate the duties of all exchangers that use propane as the cooling medium and then divide by the work done by the propane compressor. I did my duty calculations taking the process side and not the utility (propane) side as I don’t have the flow rate readings for propane.
I have calculated the duty of the exchangers using the mcpdT , however, there are exchangers where phase change occurs ( 100% gas enters and a portion of it gets liquefied) and I cannot use the approach above. Instead, I had hysys generate the mass enthalpy (in kJ/kg) of the process side streams (based on an averaged pressure and composition for each stream) at different temperatures. Then, I copied the values to my spreadsheet. I then had the spreadsheet match the actual temperature data with the corresponding mass enthalpy and did the calculation (duty=m(H(T2)-H(T1)).
Will this approach allow me to calculate the duties correctly and are there other approaches to evaluate the duty where partial liquefaction of the gas occurs?

Thanks

#2 Bobby Strain

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Posted 12 June 2020 - 05:18 PM

Why not use HYSYS?

 

Bobby



#3 RQA21

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Posted 12 June 2020 - 05:42 PM

I am having issues connecting hysys to my spreadsheet. With this limitation, is my approach correct? If I were to connect hysys to the spreadsheet, how would you go about the calculation of heat exchanger duties? Would the mass flow multiplied by the mass enthalpy difference be the correct approach ( i.e. enthalpy at outlet - enthalpy at inlet based on the actual temperatures)?
Can I use the enthalpy approach to calculate the power supplied by the compressors as well?

#4 Bobby Strain

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Posted 12 June 2020 - 10:37 PM

Simply get the duties with HYSYS. It's a simple task with Excel VBA. If you don't use VBA with HYSYS, this is a good opportunity to learn how. Aspen Tech has lots of samples on their support site. And, this exercise sounds like an academic undertaking.

 

Bobby



#5 RQA21

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Posted 13 June 2020 - 02:17 PM

Thank you for your reply Bobby.

Can I set it up so hysys can generate the enthalpy values of the inlet and outlet streams to each exchanger based on the process conditions (actual temperature, pressure and composition) and once I get those values I can determine the duty by multiplying the mass enthalpy difference (in kJ/kg) by the actual flowrate?

Can this method be used with compressors too?

I want to make sure that my approach is correct before I look into using excel VBA.

#6 Bobby Strain

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Posted 13 June 2020 - 08:33 PM

You can get duties directly from HYSYS.



#7 Alpe

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Posted 14 June 2020 - 10:38 AM

RQA21, as suggested by Bobby you can obtain enthalpies, duties etc. as result of simulation,

 

I work with spreadsheets (Excel and LibreOffice) and I utilize predefined pages for the different operation units (compressor, exchangers, valve etc. see the attached example),

 

I prefer this approach but the results are the same.

 

 

 

 

 

 

 

 

Attached Files



#8 RQA21

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Posted 14 June 2020 - 05:23 PM

RQA21, as suggested by Bobby you can obtain enthalpies, duties etc. as result of simulation,

I work with spreadsheets (Excel and LibreOffice) and I utilize predefined pages for the different operation units (compressor, exchangers, valve etc. see the attached example),

I prefer this approach but the results are the same.


Thank you for your response.

If I may ask, if I want to build a similar spreadsheet to yours, what data would be sufficient?
I can get the mass enthalpy based on the process conditions. I plan to use a stream on hysys with pressure and composition input ( and assume that those don’t change for each stream). From this stream, I will have Hysys generate mass enthalpy values at different temperatures. I will then use a function on excel to match the actual temperature with the mass enthalpy from Hysys for the inlet and outlet streams of the exchanger and use the equation duty= mass flow x mass enthalpy difference.

What do you think of my approach above as an alternative to using excel VBA? I decided to use this method as I couldn’t find a way to calculate the duty with partial condensation of the feed stream.

Thanks in advance :)

#9 Bobby Strain

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Posted 14 June 2020 - 07:09 PM

You don't need to do all that. Build your model in HYSYS and it will calculate everything for you. Then all you need to do with VBA is fetch what you want.



#10 Alpe

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Posted 15 June 2020 - 02:55 AM

as said I work mainly with Excel, LibreOffice and similar tools,

to generate the Excel page for Heat Exchanger zone analysis see this thread

 

https://www.cheresou...de/#entry115954

 

I have a different tool (Prode Properties) but you can obtain equivalent results with your simulator.



#11 Bobby Strain

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Posted 15 June 2020 - 04:06 PM

RQA21,

Do you know how to use VBA in Excel?


Edited by Bobby Strain, 17 June 2020 - 09:06 AM.





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