What are the equations of state for air at high pressures and temperatures?

[kandelabr]

Hello,

I am looking for a real gas equation of state for air, if possible a MATLAB script. I am working on pressures up to 500 bar and to 500°C.

The thing is that I not only need a p-v-T relation, I also need to calculate work done by compressed air and heat that is involved in warming/cooling at these pressures and temperatures.

I know the TEST apps, but I need the equations (or tables) so I can write my own MATLAB script.

I also found the document that is attached to this post (I downloaded it for free two days ago, but now I can't find the link o.0 ), but I don't understand a thing. Why Helmholz free energy? And about the equation for Cp/R on page 6, why not simply Cp? Is R constant or not? Why?

Thanks,
kandelabr

 

[kandelabr]

The attached file is here, don't know why it wasn't in the first post. (?)

 

[stewartcs]

Hello,

I am looking for a real gas equation of state for air, if possible a MATLAB script. I am working on pressures up to 500 bar and to 500°C.

The thing is that I not only need a p-v-T relation, I also need to calculate work done by compressed air and heat that is involved in warming/cooling at these pressures and temperatures.

I know the TEST apps, but I need the equations (or tables) so I can write my own MATLAB script.

I also found the document that is attached to this post (I downloaded it for free two days ago, but now I can't find the link o.0 ), but I don't understand a thing. Why Helmholz free energy? And about the equation for Cp/R on page 6, why not simply Cp? Is R constant or not? Why?

Thanks,
kandelabr

There are quite a few equations of state that can be used to describe fluids in the gas phase. A very simple, although not to accurate one is Van der Waals. A more accurate one is Redlich-Kwong. The latter gives good results.

http://en.wikipedia.org/wiki/Redlich–Kwong_equation_of_state

CS

 

[kandelabr]

I found some useful data in Perry's Chemical Engineer's Handbook. Now that I have compared that data to Redlich-Kwong equation, the equation seems to miss for 30 bar or even more.

I decided program Matlab to use tables and interpolate, I guess this will still be more accurate than equations of state - it's curious these equations only deal with near-critical point, but I have heard no word of high pressures and high temperatures.

There's only one thing I wonder - if I interpolate linearly of quadratically - what should be more accurate? Will the complication pay off?

Thanks for the answers.

Just in case you might be interested, I drew a surface with pressure (1-500 bar) on x axis, temperature (240-500 K) on y and specific volumes on z axis and tried to fit a surface. I didn't work well because specific volumes are so much much different at 1 and 500 bar (so much much is a very scientific term in this case).
When I did the same, but with compressibility factor instead of volume, a polynomial surface function did very well. I guess this might yield a kind of equation-of-state for this very case. (?)