5 replies to this topic

[sheiko]

Hello

I have learnt here some times ago (from Katmar by the way) that standard volume is a unit of mass and not volume.
This is something that I completely agree with as, indeed, for a given gas composition: standard volume = mass * constant (this constant does not vary with T and P).

Now suppose I have a gas containing, let's say 70% vol of hydrogen (as measured in lab after being sampled in a process line at actual T and P), and flowing at 1000 Sm3/h (whatever the standard T and P).

If I want to calculate the standard flowrate of pure hydrogen, shall I multiply the total gas standard volume by the volume fraction of hydrogen (lab result)?

My problem is that I don't know much about lab methods of measurement and I believe that 75% vol is the hydrogen composition at actual plant condition and not standard condition, and as such, multiplicating 1000 Sm3/h per 75% would give me the wrong number.

I believe I should use mass fraction of hydrogen because standard volume is a unit of mass, but the chief process engineer of my company uses volume fraction instead. In the case of hydrogen both methods lead to very different results

Could you please help me understand why it should be volume fraction if I am wrong?

Thanks

Edited by sheiko, 16 September 2017 - 06:50 PM.

[Saml]

Probably someone with more background can give you more precise information regarding methods.

Every time I've seen lab results in percent volume of gases, those were actually a mole fraction multiplied by 100, not the partial volume of each component, that may be slightly different if the mixture departs from ideality.

 

I don't know your process, but when you are handling gases, using volume (mole) fraction is the most used way of expressing compositon.

[sheiko]

Probably someone with more background can give you more precise information regarding methods.
Every time I've seen lab results in percent volume of gases, those were actually a mole fraction multiplied by 100, not the partial volume of each component, that may be slightly different if the mixture departs from ideality.

I don't know your process, but when you are handling gases, using volume (mole) fraction is the most used way of expressing compositon.

Okay mole and volume fractions are the same for any gas because the molar volume is the same (for example: 22.4 Nm3/kmol at 1 bar and 0 degC).
But that is not exactly my question.

Edited by sheiko, 20 September 2017 - 06:10 AM.

[Bobby Strain]

But it is the right answer.

 

Bobby

[Saml]

...that standard volume is a unit of mass and not volume...
...standard volume = mass * constant (this constant does not vary with T and P)..

Just don't forget that the constant does depend on molecular weight. The standard volume of 1kg of hydrogen is morethan the standard volume of 1kg of Nitrogen.
 

Now suppose I have a gas containing, let's say 70% vol of hydrogen (as measured in lab after being sampled in a process line at actual T and P), and flowing at 1000 Sm3/h (whatever the standard T and P).

If I want to calculate the standard flowrate of pure hydrogen, shall I multiply the total gas standard volume by the volume fraction of hydrogen (lab result)?

Yes
 

My problem is that I don't know much about lab methods of measurement and I believe that 75% vol is the hydrogen composition at actual plant condition and not standard condition

Normally is it not the partial volume at process conditions. Usually, lab values expressed as "volumen fraction" are actually "mol fraction" that is the same as the ratio of the stardard volume of each component divided by the sum of the standard volume of all the components.

 

Could you please help me understand why it should be volume fraction if I am wrong

There is nothing wrong with using mass fraction. It is perfectly valid.
But unless you have a very valid reason to use mass fraction, it is better to use what almost everyone else use.

[sheiko]

Thanks Saml