Isentropic Efficiency in Depressuring 6

[JoeWong88]

Dear all,

Low temperature embrittlement due to system blowdown shall always be considered during material selection study. During Basic Engineering phase, normally depressuring unit in HYSYS and / or PRO-II will be used to carry out blowdown and it low temperature effect. One of the parameter seriously affect the results especially temperature is ISENTROPIC EFFICIENCY.

The higher the isentropic efficiency, the lower the final temperature. In some case , when we use Isen. Eff = 80%, its final temperature is lower than the pre-selected material (LTCS) low temperature limit (-46 degC) and a much expensive material (SS) is required. If we apply lower isen. eff=50%, its final temperature is still higher than the CS LT limit.

I have gone through several project. Some project use isen. eff= 50%, some use 80%, etc. Some use 100% for gas system and 50% for vapor-liquid system, etc...

I really would like to take this opportunity to gather some informations, thoughts and advices from all of you.


Looks forward your advice.

JoeWong

 

[EmmanuelTop]

During depressuring, like in any other gas expansion case, the work against external (downstream) environment is performed - it is just not used for anything. Remember R. Meyer's experiment for determining Cp & Cv? I found it to be very similar to depressuring, but maybe I missed the point. Observing things in that way leads me to the conclusion that efficiency in our case is nothing more than:

EFF = 1 - (friction and other losses divided by energy difference between initial and final state of the fluid)

Dcasto, one big star for you. That is exactly what we have been looking for - practical experience and inductive way of thinking. Thanks for sharing!

Regards,

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[JoeWong88]

ALL,

Thanks for your inputs...

Work
My understanding of work is...

Let see the following image.
When system change from state 1 (P1) to state 2 (P2), work (thermodynamic) is done when boundary is move from boundary 1 (V1) to boundary 2 (V2). See work definition in wikipedia . (area under curve)

When P1 approaching ATMOSPHERIC, then work (thermodynamic) will be integrated from P1 to ATM. Above explained the work for expansion process (e.g. valve throttling, depressuring, etc).

Deviation
However, moving from P1 to P2 or ATM may take different path such as isothermal, adiabatic or between isothermal and adiabatic. The process may be reversible and irreversible.

Isentropic process is adiabatic and reversible process.
Isentropic work will be work done by the system by expansion without any heat input/withdraw and reversible

However, in real life there will be heat input from surrounding (e.g. metal and ambient heat input in depressuring process) , so the depressuring process is not adiabatic.

According to Prof. Haroun Mahgerefteh and Shan M. A. Wong in their paper "A numerical blowdown simulation incorporating cubic equations of state", Computers & Chemical Engineering, Volume 23, Issue 9, 1 November 1999, Pages 1309-1317, "Although the rapid expansion of a gas in a vessel may initially follow an isentropic path, heat transfer from the vessel walls ensures that the gas temperature will never reach the isentropic value."

Private communication with Prof. S.M. Richardson that " The fluid expansion in the vessel is (nearly) reversible...As blowdown is not adiabatic (involve heat transfer), it is pseudo-isentropic and this has led to use of isentropic efficiency.” Isentropic efficiency would be used to quantify how far the actual work deviate from Isentropic work.

Thanks to sshep (a star for you). sshep has triggered to rethink why HYSYS used "PV work contribution term" and the statement "PV work contribution term is used to approximate isentropic efficiency...". This "Isentropic efficiency" is not identical to the isentropic efficiency that we used in turbine.


Additional literature :
i) Richardson, SM, Saville, G, Blowdown of LPG pipelines, PROCESS SAF ENVIRON, 1996, Vol: 74, Pages: 235 - 244, ISSN: 0957-5820

ii) HAQUE, MA, RICHARDSON, SM, SAVILLE, G, BLOWDOWN OF PRESSURE-VESSELS .1. COMPUTER-MODEL, PROCESS SAF ENVIRON, 1992, Vol: 70, Pages: 3 - 9, ISSN: 0957-5820

iii) HAQUE, MA, RICHARDSON, SM, SAVILLE, G, et al , BLOWDOWN OF PRESSURE-VESSELS .2. EXPERIMENTAL VALIDATION OF COMPUTER-MODEL AND CASE-STUDIES, PROCESS SAF ENVIRON, 1992, Vol: 70, Pages: 10 - 17, ISSN: 0957-5820



JoeWong

 

[sshep]

Emmanual,

I respectfully must disagree that PV work is like any other (i.e. shaft) work. A lot of historical effort went in to defining the concept of enthalpy and calculating tables of it for real fluids.

Sure we could use internal energy and include PV work in every pressure drop calculation, but no practicing chemical engineer does this. Rather we take the up and downstream enthalpy to be equal in an adiabatic throttling process. The enthalpy concept makes the downstream temperature easy to calculate, even when heat transfer is included. Other energy outlets like velocity, noise, etc can usually be neglected.

This sure became a long thread for a simple concept, but that is my opinion anyway.

best wishes,
sshep

 

[sailoday28]

I'm still waiting for the definition (not opinions) for effeciency during the blowdown process.
EXERGY is a way to releate to efficieny of a process.
I'm not expert in exergy, but a google search will give many examples. See for example

http://berrygroup.uchicago.edu/papers/340.Mironova'94 JAP00629.pdf

Regards

 

[dcasto]

Facts is facts, there is no work across a valve. The diagram is showing a piston moving and removeing work! What it shows is an expander, not a "blowdown".

If you are looking for expanders, then most high speed turbo expanders can run at 80% eff. As the blowdown continues the turbo expander effiency will drop to 0% when the delta P approaches 0 psi, for a net 40%. A piston may run about the same initial eff, but the overal would be slightly higher at say 50%

 

[Latexman]

JoeWong,

Is the definition you gave from depressuring unit documentation in HYSYS or PRO-II?

What is "latent heat in metal"? Did you mean sensible heat?



Good luck,
Latexman

 

[JoeWong88]

Latexman,
I am having difficulties in getting the exact definition of "PV contribution term" in HYSYS and "isentropic efficiency" in PRO-II.

There is NO exact definition of isentropic efficiency either in HYSYS nor PRO-II manual.

HYSYS used "PV contribution term" which approximate the Isentropic efficiency...see "PV work contribution term"

PRO-II stated "The gas is depressured isentropically using either a user-defined isentropic efficiency value or the default value of 1.0....."

That's all i managed from my search. I am hoping Che Jedies in this forum can assist.

Meanwhile i will try to contact HYSYS support on this issue.


To be exact, it is sensible heat of the metal. Apologize for my mistake.


JoeWong

 

[dcasto]

There is such factor included in the WinSim depressurizer. It does allow the user to add heat to the system (ie work)which would equivalent to isentropic effiency I suppose.

 

[Latexman]

The PV Work Contribution Term sounds like it may account for what my old thermo textbook called "lost work". In this case, the loss in ability of the gas to do PV work. Besides the irreversible expansion and friction, there are at least two other changes in entropy that would not account for - heat transfer and shock waves (if you reach Mach 1). This is what has me concerned, and probably the others too. It isn't clear nor does the documentation explain how to make this functionality work for you. I guess one could figure up an equivalent total "lost work" associated with all these components and use the PV Work Contribution Term as a correction factor, but that seeems to me to be a huge letdown when you are using a sophisticated tool like HYSYS or another software. For example, it is intuitive that a reversible, adiabatic, and frictionless blowdown would have a PV Work Contribution Term of 100%, but what amount of irreversibility, friction, shock waves, and heat transfer align to 0%, and what does that mean?

Good luck,
Latexman

 

[EmmanuelTop]

"For example, it is intuitive that a reversible, adiabatic, and frictionless blowdown would have a PV Work Contribution Term of 100%, but what amount of irreversibility, friction, shock waves, and heat transfer align to 0%, and what does that mean? "

That was exactly my point in the previous post, and thank you for clarifying this issue. This is pure thermodynamic question - full stop. PV work contribution therm is relating to that amount (percent) of initial fluid energy that has not been lost through friction, shock wave etc.

0% is just a theoretical state, in my opinion - in the same way as 100% is.

One big star for you, Latexman.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[sailoday28]

I believe that thermo efficiency relations are typically for a cylic process or a process which is steady state.
The isentropic temp during blowdown is dependent upon the amount of mass in the vessel and clealy changes with time.

 

[JoeWong88]

Private communication with Aspen HYSYS support :
"The PV Work Term Contribution refers to the isentropic efficiency of the process. A reversible process should have a value of 100% and an isenthalpic process should have a value of 0%."

 

[sailoday28]

JoeWong88 (Chemical)What if the heat is transferred to the vessel in a reversible isothermal process.
dq=Tds

 

[dcasto]

There you go Joe, just what we have said.

 

[Latexman]

Could you convert the heat that is transferred, Q, to an equivalent PV work and combine that with the other PV work terms like the expansion and friction? A shock wave could be treated similarly except the formula may be a little different algebraically.

Looks like you have to convert any non-isentropic component into an equivalent PV work term and subtract that from the ideal, 100% isentropically efficient work possible. Would it be something like this:

PV Work term in % = 100 x ((PV_{100% eff.} - PV_expansion - PV_friction - PV_{heat transfer equivalent} - PV_{shock wave})/PV_{100% eff.})

Good luck,
Latexman

 

[JoeWong88]

I have received definition for "PV contribution terms" in HYSYS depressuring unit.

The PV work term (or isentropic efficiency) represents the work done on the surroundings by the expanding fluid. It is part of the dynamic energy balance [1]:

[tab]F0 Cp0 (T0 -Tref) + UA(Tbulk - Twall) + d(PV)/dt = (M Cp)holdup d(Tbulk)/dt [1]

Energy Flow Out of Vessel + Wall Effects + PV Work of Expanding Fluid = Rate of change of energy of fluid in vessel.

In HYSYS the d(PV)/dt term above is actually calculated as:

[tab]d(PV)dt ~ (VesselHoldupPressure@CurrentTimeStep - VesselPressure@LastTimeStep) / Density * fluidWorkFactor [2]

[tab]where:

[tab]fluidWorkFactor = PV Work Term Contribution/100

This correction is included so that the user can "tune" the depressuring utility to match plant or measured data.

I can see HYSYS intention but the PV contribution term and d(PV)/dt work term seem does not carry much thermodynamic meaning.

I guess we need a new term to account for energy lost due to friction, movement of liquid in the vessel, etc. Latexman suggestion may be a good start for it.

Latexman,

Do you mind to advice me the following :
i) what's shock wave in this subject ?
ii) How shock wave is formed ?
iii) Is it formed at the exit nozzle ?
iv) Any reference that i can read further ?

Thanks in advance.

JoeWong