Conversion kg/h to Nm3/h... and real gas compressibility effect

[rotw]

Hello All,

Sorry for this boring question...since this is a subject that has been quite exhausted ; problem is, I found myself puzzled by this:

Conversion from kg/h to Nm3/h should be quite straightforward exercise AS LONG AS we all agree on what is "Normal" conditions (vary from contract to contract);

So lets assume here that "Normal" refers to conditions as follows:

Tref=273.15 K
Pref=1.01325 Bar-a (by the way, I found many books missing the "-a" standing for absolute)

Problem is that compressibility is NOT EXACTLY equal to 1.000 at reference conditions given real gas mixture, even if reference conditions are that "low"...

In other words, a formula such as :

[1] ...... Flow [Nm3/h] = MassFlow [kg/h] * 22.4136 [m3/kmol] / Mol. Weight [kg/kmol]

which is commonly recalled in litterature for the subject reference conditions would deviate from a calculation that takes into account real gas compressibiity effect (i.e. via Equation of State) in order to determine Density @ (Tref, Pref) - see [2] below:

[2] ...... Flow [Nm3/h] = MassFlow [kg/h] / Density_eos @ (Tref, Pref) [kg/m3]

Unless the formula [1] is standardized and is agreed upon as such, I suppose that as long as normal reference conditions are clear to everyone, there is no reason why we would not consider real gas compressibility (not ideal gas). Unless am wrong?

 

[zdas04]

The reason that many references do not specify absolute or gauge for the unit "bar" is that until about 1990 bar(g) was not a defined term and pressure gauges were not ever calibrated in bar. Sometime during that decade someone decided that kPa was just too awkward and we started seeing pressure gauges calibrated in bar. About the same time someone decided that since psi worked so well, they would begin to use kgf/cm². I don't know the value of these pressure units, but I do know the confusion that they've caused. No one seems to know what zero is in the "bar" scale (is it zero bar(g) so that a complete vacuum is -0.8 bar(a) in the mountains, or is a total vacuum 0 bar(a) and local atmospheric pressure in the mountains is 0.8 bar(a) and the gauge at rest does not point to zero?)

This is all very frustrating, but in answer to your question, the proper representation for standard conditions includes a "deviation from ideal behavior" or "compressibility" term.

The next thing is "what do you call this imaginary unit?" since "standard conditions" are unlikely to physically exist in a commercial gas stream. I use the term SCm, to indicate that it is not a physical unit, and I truly hate the fabricated difference between SM3 and NM3 or using some form of "m cubed" since they sort of indicate a physical thing--when I'm working in volumes at actual conditions I use m3 or m^3 or m³, when I'm working in standard conditions I use SCm or kSCm or MSCm

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[rotw]

Hi Zdas,

Thanks for your precise answer.

I will calculate Nm3/h taking into account compressibility, that is to say use an equation of state to determine the density at reference conditions and from there derive normal flow.

For sure this does not look nice, because even estimation of compressibility effects deviate from equation of state to the next.
Well it is a bit of overthinking the issue since this all boils down to small discrepancies but just for consistency of the reasoning.

Actually I happen to think it would be even better to ignore compressibility so that normal or standard flow conserve their "fabricated" nature. But in that case, there should be some code/standard that normalizes the formula one time for all so there is no ambiguity about it.

I found it very confusing that spreadsheets or reports end up with unexplainable discrepancies when the basis (gas composition, etc.) is exactly identical. In such situation, for example in the context of equipment design/sizing etc. I suppose it would be better to input information as massflow instead of Nm3/h because the former is not dependent upon an equation of state or alike.

 

[gwalkerb]

Standard or Normal gas conditions are probably one of the most confusing things in the gas industry. Standard cubic feet or Normal cubic meters are really just a mass unit in disguise. And they also are not very useful without knowing the specific composition of your gas - at least regular mass flow is independent of gas analysis. Basically, they just require too much additional data to be useful on their own (gas composition, compressibility, definition of standard or normal conditions), whereas at least a mass flow is unambiguous, even if you do need some of that other information to do useful things with your mass flow. Don't get me started on measuring gas flow in BTUs/hr - if you find measuring flows in standard volume nonsensical, measuring it terms of energy capacity is even worse.

Whenever I'm doing conversions, I always include the z compressibility factor in my formulas. Leaving out z means that you are over-estimating actual volume & mass when converting from standard to actual, and under-estimating when converting in the opposite direction. I disagree that leaving out compressibility ever makes sense, unless you're working at pressures where your z value is sufficiently close to 1 (depending on your gas, this is going to be something less than 100 psig at most). Working in mass flow whenever possible is definitely a good choice, but eventually you do need to convert to actual volume, for example when sizing pipe or similar considerations. We use the Redlich-Kwong equation of state to determine z values when we don't have them from another source (process simulator or compressor software), but it does add a bunch of extra calculation steps. And of course it's still not completely accurate, but every engineering calculation is based on empirical results to some degree.

 

[rotw]

Sure.
Thanks all for pointing me in the right direction.