I've seen basically two ways to calculate the expression "the maximum work that can be extracted from a system" regarding the thermomechanical evolution of the system towards equilibrium with the environment.
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One is what Breizh linked. In this case, the expansion work that a system does against the environment is considered as "extracted work" and is part of the calculated exergy. That portion of the exergy is the term
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Here, the thermodynamic properties to use for exergy calculation are internal energy, entropy and volume differences.
Another way is to consider an open system where a stream is continuously flowing. The system volume is fixed. The entrance and exit work are not considered as part of the "maximum work that can be extracted from the system". A better explanation is in the following link:
https://www.tbm.tude...y_of_energy.pdf
For this case, the properties to use to calculate exergy are enthalpy and entropy.
This last method it what Aspen is calculating. By the way, I've verified the number in Aspen Plus assuming that you are expanding propane at it's bubble point.
Numbers are similar to those obtained from DWSIM
Now, regarding your original question: it is not possible to produce an irreversible transformation (the pressure reduction) and be able to obtain more work than before. This would make a perpetual motion machine of the second class.
Let's say that
- A is your inlet stream condition,
- B is outlet, and
- C is at equilibrium with environment.
You evolve reversibly from A to C generating 122,3 J per Kg. You can evolve the opposite way buy using the same 122,3 J per Kg (because it is a reversible transformation)
You can also evolve from B to C generating 143,4 J
So that perpetual motion machine would work using the following cycle:
- From B, evolve to C reversibly. Generate 143,4 J/kg exchanging heat between your system and environment only.
- Evolve reversibly from C to A using 122,3 J/kg and exchanging heat between your system and environment only.
- Cause an irreversible pressure drop from A to B without exchanging heat.
We've got 21,1 J per Kg, exchanging heat against the environment only, with no other hotter/colder source (this is a perpetual motion machine of the second class)
Edited by Saml, 26 November 2016 - 10:04 PM.