The exergy destroyed was obtained by balance of exergy at each component at the double-effect LiBr/H2O absorption refrigeration system.

Check the considerations about Chemical Exergy of the compounds.

Exergy change could be negative, but the destruction of exergy must be positive value.

I made the question because i have read in a scientific article that the exergy loss (irreversibility) in some devices is negative. And, in the analysis of my final project, the irreversibility of some components is negative as well. In the case of my project is the exergy destroyed.

If i made mistakes in my english, i give you my apologise.

Exergy destruction is the irreversibility, and never be negative, you must made a mistake in your calculations. You should notice: exergy destruction (Irreversibility) equal to exergy in minus exergy out, the result must be positive value.

Ok, I get it, I have to check again the exergy balances at each component of the system. How the irreversibility of some devices is negative, this means that the exergy balance is not the appropriated. In the case of my analysis, these outcomes have to be positive.

@ Giuma thank you to clarify this kind of concept! Many people confuse energy loss and exergy destruction.

thank you Marta

Thanks a lot for your advices, I at last could find the errors at the analysis and these are already fixed it. All my results are what I have been expecting. 

Hi everyone,

Thanks for sharing your concepts and suggesting about exergy. I already finished my final project and I got good outcomes that have useful for validating the refrigerations absorption systems. I had to take into account the components of exergy such as the physical and chemical exergy, and when I compared results, I concluded that it was very important consider these component into absorption systems. The exergy destroyed of each device of double-effect absorption system and the combined absorption system was positive, and this gave me idea that I had to fix some mistakes into the simulation.

Thanks again for your contributing for my investigation.

Negative physical exergy occurs if your environmental conditions (p0 and T0) are chosen in an inconvenient way. Mostly streams with pressures and/or temperatures under the environmental conditions show negativ physical exergy.

Considering chemical exergy masks the problem and since chemical and physical exergies are added to calculate overal exergy, negative physical exergies would reduce the chemical exergy although no chemical change has occured for instance in a valve or a pipe with friction leading to pressure drop. That would be incorrect!

Using an appropiate environmental pressure and temperature will result in positive physical exergies.

Aspen Plus allows to set pressure and temperature for any exergy calculations.

I think that from the thermodynamic point of view it is nonsense (negative ExD or negative ex. efficiency). On the other hand, it is not always possible to manipulate by  choice of the reference temperature. For those interested would be helpful to become familiar with the approaches in this case study.: 

The exergy efficiency assessment of heat recovery exchanger for air handling units, using a state property – Coenthalpy. Applied Thermal Engineering. Vol. 108 (2016), p. 388-397

mass exergy flow stream simulated in aspen hysys give negative value for some streams although the stream has value more than 25 C and 1 atm .why?