SPECIFIC VOLUME 7

[mikebb]

I need to calculate the specific volume of nitrogen (in m3 / kg) of Nitrogen at different conditions of Pressure and temperature ( eg 424.7 kPa and 183 K). Thermodynamics is not my strongest area, but I have tried to calculate values as follows:

Using gas law, From V = n R T /P :
n = amount of substance of gas.
If amount = 1kg, then n = number of moles in 1kg of nitrogen . Molecular weight of N2 = 28.0134 g/MOL, therefore in 1g, n = 1000/28.0134 = 35.69720205 moles

R = gas constant. = 8.314472m3 Pa K-1MOL-1
T = 183.7074046 K
P = 424710 Pa
Now, v = 35.69720205 x 8.314472 x 183.7074046 / 424710 = 0.128381672 m3/kg

However, I am not sure if this is correct ( for example, should I be using a value of R which is specific to Nitrogen ? )

Any guidance much appreciated.

 

[SNORGY]

This explains it all, I think...

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

Regards,

SNORGY.

 

[ione]

http://www.peacesoftware.de/einigewerte/stickstoff_e.html

Regards

 

[zdas04]

As long as you're using "n" as a number of moles, then you should use the universal gas constant. If you're working with mass units, then you need to divide the universal constant by molecular weight. Most often I have Specific Gravity relative to air so I use:

[γ] = R(air) * T /(SG*P)

With R(air) in units of m^3*Pa / (K * gm) [or kg]

David

 

[StewPad]

Why did you ask about the Z factor if you don't use it?
Z (ratio between real and ideal Gas)

"R = gas constant" for any gas!

 

[zdas04]

StewPad,
Even with the exclamation point, "R" is just the 18th letter of the alphabet. R(universal) or R(bar) is a symbol used in some contexts for the universal gas constant that applies to any gas, like you seem to be trying to say. There is also a gas-specific version which is the universal gas constant divided by the molecular weight of a gas that is pretty useful for a lot of real-world calculations.

Also, I can't see any reference to compressibility in anyone's posts above. What exactly are you talking about?

David

 

[25362]

Isn't it better to use the data from

http://webbook.nist.gov/cgi/fluid.cgi?ID=C7727379&Action=Page

 

[zdas04]

I got kind of tired of the navigation kludges in the native (free) NIST site and bought their REFPROP program for around $100. Never regretted it when I need a number. I still go back to EOS models when I want to program them into an application or want to explain to someone why their wishful thinking won't make their recip do 25 ratios/stage.

David

 

[SNORGY]

Also useful, on the topic of compressibility, is GPSA Volume II, Chapter 23, Equations 23.1 and 23.2.

In any event, at T = 183 K and P = 427 kPa, I suspect Z is almost equal to 1.0?

Regards,

SNORGY.

 

[StewPad]

mikebb knows what I’m talking about

http://www.eng-tips.com/viewthread.cfm?qid=283858&page=1

 

[zdas04]

Maybe that question was in regard to another issue? I asked a question once about plastic pipe, not every thread I've started since then has dealt with plastic pipe.

David

 

[mikebb]

Thanks everyone for your help, and Yes Stewpad, I did ask a question a couple of days ago re compressibility which I thought was unrelated, but now I'm not so sure. Will need to do some reading and thinking. Thanks again everyone.

 

[katmar]

At high temperatures and low pressures real gases tend towards behaving like an ideal gas and then you can take Z=1 and use the ideal gas law. But at low temperatures non-idealities come into play. "Low temperature" means different things for different gases. Basically, if your actual temperature is lower than the critical temperature then be aware that Z may not be 1.0

For nitrogen the critical temperature is 126.2K so you are still above this, but getting close. You can expect a 1 or 2% deviation for Z. By contrast, hydrogen has a critical temperature of 33.2K and at 183K you could take Z=1 with confidence (at the low pressure you are working at). Carbon dioxide with a critical temperature of 304.2K would have a significant deviation of Z from 1 at your temperature.

As a free alternative to the excellent software recommended by zdas04 you could download the Uconeer units conversion program from my web site - see signature below. (Disclaimer - I am the author of Uconeer.) Uconeer has a function to convert gas flows from mass basis to volume basis and obviously you need the gas density to be able to do this. The software estimates the density using the Peng Robinson EOS and reports the density and compressibility.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[msquared48]

I thought there was another form of the basic equation for the General Gas Law:

PV = (m/M)RT (m/M = n)

Maybe this is what mikebb was getting at for the element specific equation?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask