I encounter the following behaviour when using a pipe (e.g., of 500 m length) to transport a gas (e.g., CH4 or CO2, Chemsep components) with a heat transfer coefficient of zero and Joule-Tompson cooling turned off (two material streams and an energy stream connected):
Pressure drop and temperature drops are correctly calculated for the pipe unit (temperature drop and energy stream are both zero). However, the connected output stream shows a change in temperature.
Is this a know issue? I can attach the input file if required.
Thanks and best regards,
Hans
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This is a consequence of the heat balance assumptions. The calculation with Q = 0 or U = 0 assumes an adiabatic flash where the enthalpy remains constant. With a lower pressure and the same enthalpy, the temperature changes.
You can try isothermal mode, and DWSIM will calculate the energy necessary to keep the temperature constant.
Daniel
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I'm working on the model right now. Indeed it takes much longer to calculate, but I feel more comfortable this way... currently testing all combinations of thermal settings + JT coeff.
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Modified UNIFAC (NIST) uses the ideal model by default to calculate enthalpy. At 34 C, enthalpy of vaporization is zero for CO2, because it is at supercritical conditions, so there's no difference between liquid and vapor phase enthalpies. This causes some errors in the PH flash: when the enthalpies are equal, DWSIM assumes the fluid is in vapor state. At the inlet pressure, saturation temperature is 307.7 K and the current temperature is 307.2 K, very close.
You can either change the enthalpy calculation method to Lee-Kesler or change the property package to an EOS-based one, for instance. Then it will give you the correct results.
Regards
Daniel
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I meanwhile remembered your previous suggestion on adding a very low fraction of another component, what actually does the job (resulting in Q=0 for U=0 etc.).
And yes, switching the property package is also a good idea.
Best, Hans
Last edit: Hans Eisenstein 2016-05-20
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Hi Hans,
the pipe model has been reworked significantly by Daniel (DWSIM4).
Your assumption of a constant temperature along an adiabatic pipe is wrong! Due to the pressue loss the temperature needs to decrease!
You also can uses all three available calculation models in 64-bit mode now. The editor and result diagrams were improved and some other bugs were fixed.
All should work perfectely now without depending on any special workarounds.
Gregor
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Great! Thanks a lot for the improvements. I am especially grateful that you considered the temperature gradient option for the pipe unit. This helps a lot.
Hans
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Dear Daniel and Colleagues,
I encounter the following behaviour when using a pipe (e.g., of 500 m length) to transport a gas (e.g., CH4 or CO2, Chemsep components) with a heat transfer coefficient of zero and Joule-Tompson cooling turned off (two material streams and an energy stream connected):
Pressure drop and temperature drops are correctly calculated for the pipe unit (temperature drop and energy stream are both zero). However, the connected output stream shows a change in temperature.
Is this a know issue? I can attach the input file if required.
Thanks and best regards,
Hans
Hi Hans,
This was an issue on earlier builds, which one are you running?
Thanks for the prompt reply, Daniel.
3.7.5905 binaries from sf.
Best, Hans
Hans,
This is a consequence of the heat balance assumptions. The calculation with Q = 0 or U = 0 assumes an adiabatic flash where the enthalpy remains constant. With a lower pressure and the same enthalpy, the temperature changes.
You can try isothermal mode, and DWSIM will calculate the energy necessary to keep the temperature constant.
Daniel
Rigorously speaking, I should calculate equilibrium after each section, but this is super computationally expensive...
OK, so the workaround is to use a cooler or heater to shift the stream to the desired temperature, what does only require a low computational effort.
Thanks, Hans
I'm working on the model right now. Indeed it takes much longer to calculate, but I feel more comfortable this way... currently testing all combinations of thermal settings + JT coeff.
Last edit: Hans Eisenstein 2016-05-20
Hi Hans,
Modified UNIFAC (NIST) uses the ideal model by default to calculate enthalpy. At 34 C, enthalpy of vaporization is zero for CO2, because it is at supercritical conditions, so there's no difference between liquid and vapor phase enthalpies. This causes some errors in the PH flash: when the enthalpies are equal, DWSIM assumes the fluid is in vapor state. At the inlet pressure, saturation temperature is 307.7 K and the current temperature is 307.2 K, very close.
You can either change the enthalpy calculation method to Lee-Kesler or change the property package to an EOS-based one, for instance. Then it will give you the correct results.
Regards
Daniel
Thanks Daniel.
I meanwhile remembered your previous suggestion on adding a very low fraction of another component, what actually does the job (resulting in Q=0 for U=0 etc.).
And yes, switching the property package is also a good idea.
Best, Hans
Last edit: Hans Eisenstein 2016-05-20
Hi Hans,
the pipe model has been reworked significantly by Daniel (DWSIM4).
Your assumption of a constant temperature along an adiabatic pipe is wrong! Due to the pressue loss the temperature needs to decrease!
You also can uses all three available calculation models in 64-bit mode now. The editor and result diagrams were improved and some other bugs were fixed.
All should work perfectely now without depending on any special workarounds.
Gregor
Great! Thanks a lot for the improvements. I am especially grateful that you considered the temperature gradient option for the pipe unit. This helps a lot.
Hans
You're welcome, Hans. Please let us know if you find any (new) bug.