"Normal" m3 (volume) for a gas? 3

[ddkm]

Given a known quantity of gas, say in kg, how do we convert this to "Normal m3"?

I'm quite sure I know this, but just wanted confirmation from you guys.

-----------------------

My take on this is as follows, rightly or wrongly:

Using Ideal Gas Law PV=nRT:

we have PV=mRT/M

where m = mass of fluid
M = MW of fluid

Thus, the "normal" m3 of the fluid would be:

V = (mRT) / (MP)

where P = pressure at atmospheric (i.e. 101325 Pa)
and T = the "standard" temperature of 25ºC or 298.15K


IS THIS RIGHT?? Any comments? Thanks all.


---engineering your life---

 

[zzznook]

yes yore right. You convert 1 kg of your gas to moles in 25C 1atm. Then you arrange V=nRT/P. This means that 1 kg of O2 has a different volume than 1 kg of H2

 

[zzznook]

one can remember n=PV/RT at 25C 1atm =101325/8.314(298)=40.9 so converting 1Nm3 to moles is always 40.9

 

[katmar]

Your math is correct, but your definitions are wrong.

Traditionally "Normal conditions" were defined as 101.325 kPa abs and 0 degrees C. Some time back IUPAC revised their pressure standard for normal conditions to be 100.000 kPa, but left the temperature at 0 degrees C or 273.15 K.

Whatever you decide to use, make sure that you clearly define which "Normal" conditions you are using. If you receive data that is not clear from others, then request them to define their normal conditions.

This subject comes up regularly here. Do a search and you will find plenty of debate on the subject.

regards
Harvey

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[mbeychok]

ddkm:

I strongly urge you to read this article in the online Wikipedia:

http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure

<== click here

When you see how many different sets of reference temperature and pressure are in use, it will become clear why it is absolutely necessary to clearly define the reference conditions of temperature and pressure whenever expressing a volume or a volumetric flow rate. Just saying "normal m³" or "standard scf" is meaningless.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[SmartEngineer]

I agree with katmar, definitions of Normal & Standard conditions differ from various companies practises.
For instance, the project I'm working in is using Normal conditions as 0 C and 1.01325 bara. These figures might be different with other projects depending on the company standard and practices.
Back to the calculation, the method you have shown is correct as far as the gas is assumed ideal which is the case with low pressure cases.

Cheers

 

[sailoday28]

Using 0 or 25C can result in about a 9% error.
What accuracy is required AND what is the gas?
From my old IR Compressed Air and Gas Data, I note:
Z @14.696 psia 60F
N-jbutane C4H10 0.975
Air 0.9985
Ammonia 0.989
CO2 0.994
Ethane 0.992
Chlorine 0.988
Ethylene 0.994
Proplene 0.9836
etc., etc.
Regards

 

[mbeychok]

Sailoday:

I assume you meant to say that using 25 °C rather 0 °C (or vice versa) results in a "difference" of 9% rather than an "error" of 9% ... since neither temperature is really the "universally correct" temperature.

Read the Wikipedia article I referenced earlier at:

http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure

<== click here

and you will see that there are at least 12 different sets of standard reference conditions being used today.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[sailoday28]

Milton: I was refering to the accuracy required.
What I am trying to say is that using the perfect gas formula by itself can result in errors of up to 2% (Z variation) depending on the gas used. Further, depending upon the accuracy desired, the "standard" temperature used may or may not be that significant in the calculation.
Certainly, if one is rerering a mass or mass flow to volume,Then static temp and static pressure, must be known or specified, whether at "standard" or other conditions.


Regards

 

[ddkm]

So, it's 0ºC instead of 25ºC. Thanks guys, I'll amend my figures. In fact, with 0ºC, my figures now match the figures given by the supplier.

Milton and Harvey, thanks for the replies - you guys are always on the ball. Agree with you that it is important to recheck what is meant by "Normal" or "Standard" as used by the person using it. I myself hate to use such terminology, but it's just that in the industry, we have suppliers and manufacturers (and sometimes colleagues from other countries) using such terms freely.

As such, you start to wonder if maybe the definitions are not so diverse after all. Well, from the above discussions, it's still obvious that there is a varied interpretation of the terms. Something to note is Harvey's point that IUPAC had changed the Pressure condition from 101.325kPa (i.e. 1atm) to 100kPa (i.e. 1bar) and this again creates the disparity.

---engineering your life---

 

[Ashereng]

I agree with sailoday28's caution about assuming ideal gas law holds.

It depends on the gas, and I think also on how far from "normal" the data is. If the pressures and temperatures are very far apart, the ideal gas law may give higher errors than if the difference is smaller.

There are many other correlations for gas that may be better suited to your conditions than using ideal gas law.

The upside of the ideal gas law is its simplicity. The downside is that it is limited in its application.

 

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