6 replies to this topic

[Zaraf]

I've been given a concentration of H2S (dihydrogen sulphide) in a process as being 10 mol/kmol H2S. The medium that this H2S is in is natural gas. I'm not sure if I can determine the ppm of H2S in this without knowing what the medium is, based upon this 10 mol/kmol.

Is this possible? Or do I require the composition of the medium as well?

Thanks for any help!

[Qalander (Chem)]

Dear

I feel for any multi component system each of the component's %ages are needed to obtain the exact position.

[gvdlans]

First thing you need to know is whether you need to determine ppm (weight) or ppm (volume)...

[Zaraf]

I was thinking by mass, considering they tell me a molarity, and I can easily determine the mass from that. If I go for volume, then temperature and pressure become an issue.

[MrShorty]

ppm by mole seems to me the most obvious conversion: 10 mol H2S/1000 mol mixture=x mol H2S/1e6 mol mixture.

I've always been of the impression that ppm volume is the same as molar ppm. This comes from your gas law equation V=nRT/P. If R, T, P are constant (which shouldn't be too far fetched-- we are only talking about how to express concentration, not necessarily describe the actual state of the gas), then Vi/Vtot=ni/ntot as the RT/P cancels out. If this isn't correct, someone correct me.

If you are expected to convert to ppm by weight, you will need the composition of the gas, so you can calculate the mass of each "component".

[tarafdar]

Dear Mr. Zafar.
Only mentioning ppm is ambiguous.Please visit onlineconversion.com & then to Online Conversion Forums>Main Forums>Convert and Calculate and then to the post "Convert 1 ppm H2S in to Mole%.I think this will help you.

[kkala]

I agree with MrShorty, with a note:at high pressures there may be some difference between molar and volume percentage of natural gas components. But at high pressures composition is specified on molar base. At low pressures composition is specified on volume base (this is considered, in case of no clarification) and it is same as for molar base.
You can look at attached "realgas.xls", where a gas of molar composition of 90% CH4, 9% C2H6, 1% H2S is considered at 25 oC. Volume percent composition at 60 Barg shows lower C2H6 and H2S, due to their low compressibility factor under these conditions. The compressibility factors were taken from the spreadsheet by breizh in the thread http://www.cheresour...h__1#entry49288. Comments are welcomed.
In conclusion, "by volume" is same as "mol/mol" at law pressures or ideal gas conditions. What if there are high pressures and the percent composition is said to be "by volume"? I would ask clarification, because this "by volume" may actually mean "mol/mol". I have not seen "by volume" gas percent composition at high pressures (e.g. 50 barg).

Attached Files

•  realgas.xls   16KB   159 downloads