Supercompressibility

[maddocks]

I know that the molar volume of a gas is 22.4 mol/m3 at reference conditions. I then calculated a shifted molar volume at my regulatory conditions of 15 Deg C and 101.325 kPa to be 23.645 mol/m3. So far, so good. Now my regulator is insisting that I account for supercompressibility in my calc. Molar volume is equal to:

RTz/P - this means that as z decreases, the molar density appears to decrease but this seems backwards to me. I would have thought that a compressibility less than 1 would result in a higher density.

So, my question is, once I get the supercompressibility out of my DCS, how do I apply it to my molar density calc?

 

[pmover]

i'm relying on memory, but i believe

supercompressibility = 1/Z^2

hope this helps.
-pmover

 

[maddocks]

I read this in AGA 3 but it's a bit hinky... (if that's a real word).

So, suppose (as an example) I calculate a z of 0.991 at 15 Deg C and 101.325 kPa, the resulting supercompressibility should be 1.01825. This means that I would then use this as a flow multiplier. This seems sort of correct to me, but I'm curious as to why the z is squared and inverted. If the z is a direct compressibility function, why is it adjusted in this manner?

I still revert back to my PV=znRT basics, and the molar density is a direct relation to z.

 

[ione]

It's just a matter of definitions.

I have added some papers hoping they could help

 

[dcasto]

I believe what the OP is asking is about stating a volume at a new pressure base and temperature base. So he needs z2/z1 correction. the result of z2/z1 is less than 1 part in 10,000 if you have changed the pressure and temperature only 1 or 2 kPa and 3 or 4 Degrees C. the limit of measurement is 1/10,000.

 

[maddocks]

Thanks dcasto - I was hoping you would wade in to this one. Just wrestling with regulatory boards when installing a mass flow meter on acid gas compressor discharge. They keep on insisting on an AGA calc and I am telling them that I want to ignore AGA and use molar density corrected with compressibility at 15 Deg C and 101.325 kPa. They've asked me if I am using supercompressibility but I don't think it applies in my case. Supercompressibility is the correction to a volume flow meter to account base compressibility - yes?

 

[dcasto]

supercompressibility in the AGA flow measurement is to account for the density of the gas under pressure, thats why it is a square root function in the equation.

you are looking for a similar adjustment to correct the density from 1 standard condition to another, the way I read it.

here is the AGA program that will correct all the volumes, densities, energy from 1 pressure base to another. It uses the AGA8 zfactor method of summation factors.