Standard flowrate of individual component in gas mixture

[sheiko]

Dears
I would like to have a confirmation here:

Suppose I have 1000 Nm3/h of gas containing 75% vol of hydrogen (or around 2% mass).
If I want the flowrate of pure hydrogen in Nm3/h, shall I multiply by the hydrogen mass fraction or volume fraction?
TO me it is clear that Nm3 (standard volume) is a unit of mass and not of volume (standard volume = mass * constant) . So I believe I should multiply by mass fraction.
Could you please share your opinion ?


@Lionel_Sheiko / geniechimiquefacile.com

 

[zdas04]

You can either use mass fraction or calculate a density from the partial pressure at your "normal" pressure and calculate a standard volume directly. Either way gets you the same answer.

[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

 

[Latexman]

Volume fraction.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[sheiko]

Thanks
So Zdas says mass fraction and latexman says volume fraction.
I need more feedback please

@Latex
Shall I use volume fraction at standard conditions so?

 

[Latexman]

You know you have 1000 Nm3/h of gas containing 75% vol of hydrogen, and you want flow rate of hydrogen in Nm3/h. No unit change, same standard T and P condition. This is 9th grade physical science question. 750 Nm3/h.

Why? Why not?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[sheiko]

Latexman
Sorry but I believe 75% is not the volume fraction at standard condition but at actual condition (lab measurement).
Maybe it looks a 9th grade question but sometimes things look obvious but are not. I remember from school that gas composition change with T and P...so what if 75% vol composition is at actual conditions rather than standard conditions?

PS: sorry for my bad english which is not my mother language.

 

[Latexman]

Why does the gas composition change with T and P? It would have been good to know this back in post #1.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[MortenA]

In reality standard flow is another unit for molar flow. E.g. a Nm³ of _any_ gas is 0.0446 kmol of gas - when the reference condition is 1.01325 bar and 0ºC (not the UIPAC definition).

So if you wish to know your H2 share of the 1000 Nm3 in Nm3 you will have to multiply the 1000 with the _molar_ concentration of H2. However, as Latexman says, it will work out to be the same as the volume fraction IF your P/T is not to far of from atmospheric and the rest is not water (i think that could throw if off but m not sure if it would matter. I never worked with syn gas).

Best regards, Morten

 

[rotw]

To OP: Regarding your statement on gas composition being function of P and T.
Mass fractions have not to be necessarily function of P and T; for instance if your P, T conditions correspond to the homogeneous stable (vapor) phase, the total system composition (or mass fractions) is equivalent to the composition of the homogeneous vapor phase, irrespective of how P and/or T are varying.

If you know your mass fraction (not to confuse with molar), then you apply this straighforward to obtain your massflow of H2 as individual stream, assuming you know (or know how to calculate) your total massflow.

Knowing H2 massflow you calculate normal flow Nm3/h using density of H2 at the partial pressure of H2 considering the gas at normal (reference) pressure, as pointed out above.