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Supercompressibility

Supercompressibility

Supercompressibility

(OP)
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?

RE: Supercompressibility

i'm relying on memory, but i believe

supercompressibility = 1/Z^2

hope this helps.
-pmover

RE: Supercompressibility

(OP)
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.

RE: Supercompressibility

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.

RE: Supercompressibility

(OP)
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?

RE: Supercompressibility

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.

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