30 replies to this topic

[shan]

Hi dack77494,

It seems to me that just you and I are left to continue this topic. Anyway, I still think that the issue is something fundamental and worth of clarification, although it just starts with a Hysys PV Work Term Contribution input value.

Overall, I just wanted to make the following two points clear.

1. The gas expansion (depressuring process) is between isentropic (S=constant, 100% Hysys PV Term Work Contribution) and isenthalpic (H=constant, 0% Hysys PV Term Work Contribution). This is why Point D on my CH4 Miller Chart is impossible because it is not allowed by the Second Law of Thermo (Entropy will not decrease in all the processes) and Point E is impossible because it is not allowed by the First Law of Thermo (Energy will not be created or destroyed).

2. Work by depressured gas was done in the blowdown process although you did not “see” it or collect it.

Now let us concentrate to our example.

A. As indicated in the ideal gas law PV=nRT, P1V1/T1 is equal to P2V2/T2 under the condition same number of moles in the system (n1 = n2). Therefore, both gas left in the vessel and gas released (in your balloon) are in our finial state consideration and they are at same temperature and pressure.

B. T1 = T2 (Point F on the attached chart) is an impossible because is on the right of Point C. I agreed the assumption for simplifying your example.

C. Hysys PV Work Term Contraction is not a measurement of work performed vs. work content. It is a measurement of degree of isentropic approaching.

D. The final state of pressure (100 psia or whatever) is an arbitrary specification. The final state of temperature is NOT independent on the valve/turbine. If the letdown is through turbine (100% isenthalpic), it is equivalent to process that you have a large cold balloon (low temperature, work performed). If the letdown is through valve, it is equivalent to process that you have a same warm balloon (high temperature, no work performed or work out = heat in). Although there is no turbine and balloon, the real life depressuring is close to isentropic (87%-98% as Hysys recommended). If you still challenge work performed, please review and think over my Point 2 above. The only situation is close to an isenthalpic process that all or most friction heat generated through the valve is transferred back to stream and heat the stream up.

At this moment, I like a cup of StarBucks more than the cyberspace success.

Regards,
Shan

Attached Files

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[djack77494]

Starbucks gain is my loss. However, at this point I do think I understand what is happening. The temperature of whatever remains in the vessel following depressurization is a function of the final pressure, but is independent of the efficiency of the expansion process. The temperature of the expelled gases is very much a function of the expansion process. I accept that this process is normally fairly close to isentropic, though I don't fully understand why (not important however). Also, I totally understand and accept that an actual process would lie somewhere between isentropic and isenthalpic. Thank you shan and others for your help.
Doug

[rxnarang]

I had a tough time finding this particular thread, but it was important that I attach the following to this thread.


I have come across the following advice:

Expansion inside the vessel may be isenthalpic, isentropic or polytropic with an isentropic efficiency. The exact path is uncertain.

a) A single vessel or large vessel depressurisation ( > 6-8ft) is likely to be isentropic.

b ) A long period of depresssuisation ( > 1 hr) is likely to be isentropic.

c) Depressurisation of several vessels or smaller vessel is likely to be polytropic with isentropic efficiency of 40-50%.

d) A short period of depressurisation ( <15-30mins) is likely to be polytropic with isentropic efficiency of 40-50%

Note that this is talking only about the gas in the vessel, and NOT the consequences to metal temperature.
Regards

[JoeWong]

Rajiv,

Thanks and appreciate.

[daryon]

What an epic thread!

I thought I would just add some references:

It pains me to say it as a grudate from University College London, but Imperial College (with Shell I think) did some study into blowdown of pressure vessels and developed their own computer program called Blowdown. They published two papers which are worth reading:

Blowdown of Pressure Vessels I. Computer Model. Haque M.A., Richardson S.M., Saville G.

Blowdown of Pressure Vessels II. Experimental Validation of Computer Model and Case Studies.
Haque M.A., Richardson S.M., Saville G. Chamberlin and L. Shirvill.

[JoeWong]

Shell ? You mean SGS has the ownership of BLOWDOWN ?

Both are great papers in Blowdown studies...