Impact on discharge temperature by recycling N2 (instead of Propylene) in a refrigeration system

The boiling point is much lower for N2 than propylene so i dont think you can get the pressure high enough to see a liquefaction?

And just expansion wont give that much "cooling". If you consider isenthalpic expansion (throttle valve) then if you start with N2 at 25ºC (77F) and 100 bar (1450psi) and drop the pressure to 1 bar then the temperature of the N2 would only drop to 5ºC (41ºF)

And at e.g. 10 bar the boiling point of N2 is -170ºC!

(all data is taken from REFPROP/NIST)

Best regards, Morten

BTW if you have an expander instead of the throttle valve you will get much lower temperature

Morten is correct, nitrogen is not suitable to replace propylene in a refrigeration plant (i.e. Reversed Carnot Cycle),
in addition to Morten's comments, compressing gaseous Nitrogen or Propylene, with the same compression ratio, discharge temperature will be much higher for N2 due to the different cp/cv values...

Discharge temperature respective to which equipment???
If you are referring to compressor, this would be my bet:
Propylene has higher MW than N2. If the compression is done with a centrifugal, then in order to keep compression ratio identical you would need to have the machine spin at higher rpm with N2, and you need to make sure you have room for that speed increase, alternatively would could reduce the capacity at identical speed but allowance for doing so is usually limited, or combination of both. Because of the lower MW, all things being kept equal while operating within machine envelop - you would have to expect a lower temperature increase due to compression when running with N2 than when it is with Propylene. Goes without saying that this comment is from a compressor view point. Whether this makes any sense (physically) with respect to your entire system or not, is another story.