Please note that the exergy destruction of the flash separator is calculated by taking into account both physical and chemical exergy. the inlet mixture consists of methane, ethane, and propane. The major constituent is methane. whereas no pressure drop is involved and the process is below ambient i.e., -105 C.
Exergy is a property associated with the state of the system and the environment. A system that is in equilibrium with its surroundings has zero exergy and is said to be at the dead state.What are your dead state conditions (Tdeadstate?). You should check your balance equations.You should be aware that exergy destruction can never be negative because it violence second law of thermodynamics.
Anil Kumar Sakhiya Yes. That's what concerns me. Exergy destruction must not be negative. In my calculations, To = 298.15 K.
Destruction of exergy from a system may not be negative. You should check your exergy balance equation with appropriate data assumptions and calculations.
Dear, Ahmad Naquash
Researcher Anil Kumar Sakhiyas' answer is very accurate. I am 100% in agreement with his opinion. The opinions of researchers Md. Hasan Tarek Mondal are very good answers.
Irreversibilities (exergy destruction) are the exergy loss associated with entropy generation (Sgen) because of the energy transport between the system and its surroundings. In any real process, entropy generation is always positive, and it can be caused by various factors (such as pressure drop, and temperature difference, etc.). Please make sure you have correct energy and mass and exergy balances. Please also check the equation related to getting the energy loss that is used for exergy balance. Moreover, after calculating the exergy of each stream, you should find that the summation of input exergies must be always greater than the summation of the exit exergies. Anyway, exergy destruction in the calculations can never be obtained negative according to the second law of thermodynamics.
Dear Ahmad Naquash,
Greetings, Negative exergy may means that you need to apply energy to the equipment yet to it be prepare for a considered performance!
Regards, Saeed
The exergy balance around the equipment is correct as I checked. The sum of product physical exergy value is less than the value of physical exergy in feed. The problem is with chemical exergy. Chemical exergy values of the product are more than that of feed values.
It is never possible that the exergy destruction of equipment becomes negative because it violates the second law of thermodynamics. But you may mean whether it is possible that the exergy of a stream becomes negative.
It is possible that the exergy of a stream becomes negative just for the cases that the pressure of the stream is less than environmental pressure.
for more information refer to page 127 of the book "https://www.wiley.com/en-us/Thermal+Design+and+Optimization-p-9780471584674".
If the thermal part of the matter exergy would be small enough, the work done on the stream by the environment, causes the total exergy of the stream to become negative.
Mohammad Olfati Iman Golpour I know the exergy of a stream can be positive or negative depending upon the conditions. In my case, the exergy destruction of equipment is negative. I have twice checked the exergy balance and it is perfectly ok. I adopted the exergy values from Aspen Hysys stream properties.
Ahmad Naquash It is impossible. Based on Gouy Stodola Theorem (Ed=T0*Sgen), negative exergy destruction means negative entropy generation that violates the second law of thermodynamics.
The value of a certain exergy analysis can become negative when one has applied an oversimplified system boundary of the thermodynamic system of interest.
In general, when one applies the correct system boundary in a thermodynamic heat- work cycle including both heat sources and one takes one complete cycle in consideration, the value of the exergy calculation can't become negative. At least not in a system which is within the scales of classical mechanics.
In addition I would like to add:
The second point I would like to make is about a topic that is closely related to exergy: Clausius entropy. Clausius entropy is also related to the second law of thermodynamics. I would like to inform those who are interested in entropy that some interesting alternative views on Clausius entropy have been posted in another discussion group over the past two weeks.
Discussion group "Tackling a Century Mystery: Entropy" with corresponding theme:
"Why are we still unable to explain the difficulties caused by a physical concept even after more than 150 years of hard work?"
Source: https://www.researchgate.net/post/Tackling_a_Century_Mystery_Entropy
Some quotes of alternative views of Clausius entropy from this discussion group:
- There is no randomness anywhere in the Universe. If randomness could exist, it would end the existence of the Universe. The Universe must be fundamentally unified or it would not exist. Thermodynamic entropy has been misinterpreted in several ways due to the lack of understanding of what it was that Clausius discovered. Being a measurement of randomness is impossible.
- There is no non-equilibrium thermodynamic system anywhere in the Universe. If non-equilibrium could exist, it would end the existence of the Universe. The Universe must be fundamentally in thermodynamic equilibrium or it would not exist.
- Science has been able to explain many forms of spontaneous processes in the past 2,000 years. For example in biology: spontaneous generation. In the mid-19th century Louis Pasteur and John Tyndall found the underlying phenomena and also this spontaneous process could be eliminated as spontaneous; it wasn’t. Quote “Rejection of spontaneous generation is no longer controversial among biologists. By the middle of the 19th century, experiments by Louis Pasteur and others refuted the traditional theory of spontaneous generation and supported biogenesis.”
- There is no other concept in which the role of “cause” and “effect” have been interchanged. This quote is related to Clausius entropy
- . . . . I further consider Clausius entropy as a form of cognitive bias and cognitive dissonance. In my opinion, statements like "entropy always increase in universal systems and therefore ....." are similar to: "Green is the best color because it is the greenest of all colors"(source: https://en.wikipedia.org/wiki/Begging_the_question)
- It is the story of the “Greatest Blunder Ever in the History of Science
Casper Helder Thank you for answering this question. I believe system boundary has a vital role in exergy analysis. In my case, I have a cascaded system. I have to reconsider the system boundary to perform analysis.
In addition, no doubt exergy is a very powerful tool and, in my opinion, there is a large room available to apply this tool effectively. Thank you for sharing the link of the discussion group.
Ahmad Naquash Hello. Could you solve this problem? I have an adiabatic flash in the model. I applied the exergy equation to the flash using the inlet and outlet streams. The exergy destruction is negative which is not possible. I calculate the chemical exergy and take the physical exergy from ASPEN to find the total exergy of a stream.
Faruk Özdemir Can you please provide more details about the simulation? I mean stream components, pressure drop?
The inlet stream is wet syngas at 1.2 bars. The flash separator is adiabatic and at 1.176 bars, so the dry syngas and the condensed water exit at the pressure of the flash separator. In this way, I wanted to model the pressure drop in the unit.
Exergy destruction can not be negative. That issue is independent of whether you consider chemical exergy or some other form.
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