7 replies to this topic

[ARAZA]

Dear All,

I've got a ethylene feed coming in at 15 deg C at 12 kg/cm2 (g)and I have to drop the pressure to 1.5 KG/cm2. I need the auto refrigeration temperature. I used Chemcad to calculate the temp. which is giving me -76 deg C which I think is very cold. The P-H diagram gives me much higher value.

Can somebody have this value readily available or throw some light over the temperature whether -76 deg C sounds right??

Thx

ARAZA

[Art Montemayor]

Araza:

I wish you would give us ALL your basic data. You don’t tell us what your “much higher value” is and how you derived it. I am willing to tell you my values and how I came up with them (complete with references) – please refer to the attached workbook.

I come up with very different values as well, but my graphical sources are old. The graphical sources are close to each other – but the NIST database gives a value that is approximately 33 ^oF colder than that of the graphical data.

As in all these cases of free expansion, I have assumed an isenthalpic process to occur across the expansion valve. I am assuming that you did likewise.

Since I don’t have your results, basis, or reference I can’t make a further comment except to say that I would probably go with the NIST results (if I didn't have a more credible and proven database).

Attached Files

•  Ethylene Expansion.xls   46.5KB   107 downloads

[fallah]

Isenthalpic or Isothermal?

I think it would be isenthalpic expantion.

[Zauberberg]

Art's analysis is correct, and the exact figure (if we are talking about 1.5 kg/cm2 ABS pressure downstream of the valve) is 1.3 degC as final temperature. Ethylene undergoes an isenthalpic expansion across the J-T valve. The same results are obtained by using enthalpy tables and Hysys.

It is not logical to have such a huge temperature drop due to isenthalpic expansion. The results you've got are closer to isentropic expansion so check if, for some weird reason, ChemCAD has not included this type of expansion in the internal calculation sequence.

[Art Montemayor]

Fallah:

Good eye! I meant AND wrote "isenthalpic" - but on my word processor, where I prepare all my responses. Unfortunately, Bill Gates probably doesn't like (or understand) the term "isenthalpic" and the word processor changed it to "isothermal" - a term I don't like because of its impracticality and un-applicability as a Thermo process.

I have edited the post and corrected the term. Thank you for pointing out what I have noted in the past. Regardless of what the computer does, WE are responsible for what we post. I will have to revise all my word processing before posting in the future.

[Aaron Herrick]

I used Chemcad to calculate the temp. which is giving me -76 deg C which I think is very cold. The P-H diagram gives me much higher value.
ARAZA

I'm not sure what you did to get that value, but plugging your numbers into a feed stream, and then using a Valve to drop to 1.5 kg/cm2 CHEMCAD gives a value of 2.6C, which is much more consistent with Art's chart (CHEMCAD is using DIPPR's values, not NIST's). Give 1-800 CHEMCAD a call and they'll be happy to help.

Best
Aaron Herrick

[chemsac2]

Isenthalpic expansion:

Inlet conditions (P / T / H) = 13 kg/cm2g / 15 deg C / 51.197 kJ/mol

Outlet conditions (P / T / H) = 1.5 kg/cm2g / 2.6 deg C / 51.161 kJ/mol

Isentropic expansion:

Inlet conditions (P / T / S) = 13 kg/cm2g / 15 deg C / 195.265 J/mol-K

Outlet conditions (P / T / S) = 1.5 kg/cm2g / -70 deg C / 195.271 J/mol-K

From this, it seems you are carrying out isentropic expansion in your CHEMCAD (e.g. in an expander). Expansion across a valve is iesnthalpic expansion.

Regards,

Sachin

Edited by chemsac2, 01 August 2010 - 06:43 AM.

[Zauberberg]

There must have been some error in your simulation setup, as I have asked one of my former colleagues to verify the results in ChemCAD and he got the figure 0.89 degC as the temperature downstream of the J-T valve. Peng-Robinson EOS was used. The flowsheet including process data is attached below.

Attached Files

•  ChemCAD Case.jpg   299.68KB   19 downloads