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Nitrogen Vs. Helium


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#1 ahmadijaz65

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Posted 29 March 2017 - 12:44 AM

Hi

 

I am performing an experiment using both Nitrogen and Helium. In the first case when i use Nitrogen in a tank at 25 °C and 250 bar. Nitrogen undergoes expansion from 250 bar to 40 bar and attain the final temperature of -75.77 °C while in the second run, i have used the Helium instead of Nitrogen. Under the similar conditions, helium attains the temperature of -87.70 °C.

 

Why there is that much temperature difference in the final value of nitrogen and helium when the experiment is performed under the similar operating conditions.

 

Is it the heat capacity values which is responsible for this temperature difference or there is a some other valid reason?



#2 MrShorty

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Posted 29 March 2017 - 09:57 AM   Best Answer

I suspect that we need more information on the expansion.

 

Noting that the Joule-Thomson coefficients are opposite sign, if this were a JT (isenthalpic) type expansion, then one of those gases would get warmer while the other got colder. The sign on the JT coefficient would explain the observation.

 

I recognize that nothing in your post suggests an isenthalpic expansion, but you give no indication how the expansion takes place.

 

Running some calculations in REFPROP (similar to this webpage: http://webbook.nist....hemistry/fluid/), It seems that the enthalpy change (DH) for your beginning and final conditions are about the same (2.5 kJ/mol). Heat capacity probably has something to do with the explanation, but it is difficult to pin down the explanation without a good description of the experiment.



#3 Saml

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Posted 31 March 2017 - 10:18 PM

Hello, I've made this sketch to illustrate what is going on inside the tank.

 

Imagine you have some gas at the bottom of it that expands to occupy all the volume while the rest of the gas is expelled from the tank.

 

The expanding gas is almost at the same pressure than the rest of the tank. It is also at the same temperature. This is a quasi eqilibrium situation. So you can calculate the expansion as being reversible and  adiabatic.

 

If you do the numbers assuming ideal conditions that is, Cv is 3/2R for monoatimic gas, 5/2R for diatomic gas,  Cp = CV + R and that the compressibility is 1 then the calculated temperatures are:

 

-96.5 °C for N2 and -129.9 °C for H2.

 

So yes, the lower temperaure of Helium has to do with the fact that it is monoatomic, and it has a lower heat capacity than a diatomic gas.

 

Attached File  sketch.jpg   66.04KB   0 downloads

 

If you do the rigorous calculation by simulating an adiabatic reversible expansion using a method like Refprop that account for non ideal behaviour you will have other temperatures, but Helium will be a lower temperature than Nitrogen.

 

There are other facts that determine what the final temperature would be:

-  Heat transfer into the tank. Can be minimized with good insulation.

-  Turbulence inside the bottle that dissipates as heat.

-  Heat capacity of the tank itself. I would not ignore it, since the metal mass can be significant compared to the gas mass.

 

 

Regards

 






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