N VALUE QUESTION

[dkm0038]

Can someone please help me or refer me to a good resource. I would like to know more about the 'n value' used for gas mixtures to calculate physical properties for a mixture of gases.

I would specifically like to know how to get it and the equations it is used in. I have been searching for a while with no luck on for a good reference

thank you

 

[Latexman]

You'll have to be much more specific than that. I have no idea what you are talking about.

Good luck,
Latexman

 

[dkm0038]

there is a term called 'n value' it is used in industry for gas mixtures as a means of calculating the specific heat, and maybe other things.

there is an equation that gives the specific heat and is a function of 'n value', molecular weight, and maybe a couple other things for a mixture of gases.

i would like to know if anyone is familar with this and if they can explain how to derive it or refer me to something else

 

[25362]

If the so-called N-value is the ratio of specifi heat capacities, aka k or [γ], go to thread391-244590.

 

[owg]

The Natural Gas Processors Suppliers Association (NGPSA) Engineering Data Book, 8th edition, 1966 (that's a date) page 157 uses N=Cp/Cv for ideal gases. There is also a Cp/Cv to the power n where the k is not a constant and n is a variable. I never found this much help.

HAZOP at

http://www.curryhydrocarbons.ca

 

[srfish]

N is a variable used in the calculation of the specific heat of an ideal gas. There is an article in the August issue of Hydrocarbon Processing titled "Predict Heat Capacity More Accurately". N is the exponent of temperature. The only way I know how to express the equation is to use Fortran. The equation is: CP = A + B*EXP(-C/(T**N)). The values for A,B,C and N are given in the article for 221 hydrocarbons.

 

[dkm0038]

I have found some stuff concerning my original question...

The equation seems to be of the form Cp = [n R]/[M (n-1)]
where
Cp = the specific heat
R = is the universal gas constant
M = molecular weight
n = the 'N value' from above (possible ratio of specific heats/or something concerning the degrees of freedom of the gas)

I still would like to know how it is derived (from the ideal gas law??) and what specifically is 'n'.

Is anyone familar with this and can help

 

[willem79]

n could also just be the moles...
Cp,m = (dC / dn)p
or
Cv,m = (dC / dn)v
see:

http://en.wikipedia.org/wiki/Specific_heat_capacity