Natural gas composition 2

[mielke]

Is there any standard composition for natural gas (a certain percent methane, ethane,...) Or is natural gas an ambigious term tat covers a range of percent composition?

 

[MortenA]

I would say the latter. However - The countries in Europe that I know of has a natural gas specification, stating certain requirements with regards to composition, heating value and so on.

Best regards

Morten

 

[Homayun]

of course natural gas has a composition...it should..

 

[JLSeagull]

MortenA is correct.

Natural gas from the well is processed to remove more desirable - and less desirable components then sold according to a pipeline specification for heating value and moisture content.

Liquid hydrocarbon compounds from ethane and higher may be removed leaving sufficient ethane etc. for the heating value. Water is largely removed. Other components removed include hydrogen sulfide and other sulfur compounds, carbon dioxide, etc. Many different levels of processing exist. The simplest may only include dew point suppression and perhaps compression.

 

[dcasto]

the specifications for natural gas are as follows:

Maximum amount of water in ppm or pounds/MMSCF (US)
Maximum amount of sulpher compounds in ppm
Maximum inerts N2 or CO2 in mole %
Maximum O2 in ppm or mole %
minimum or maximum heating value in BTU/cf or energy/volume
WOBBIE number = BTU/CF / (sqrt(sp gr))
Dew point temperature or the cricondenterm point
maximum amount C4 and larger in mole% or gallons/MSCF
Other contaminates such as; free liquids dirt, dust, gum
Pressure and temperature limits

 

[zdas04]

The problem is that the term "Natural Gas" is applied to two grossly different categories of stuff. The gas that the utilities deliver to their customers as "Natural Gas" has a very rigidly defined composition that results in an homogeneous energy capacity from location to location and day to day. This definition includes very tight limitations on allowable inerts, heating value, and allowable condensable vapor limits.

The other "Natural Gas" is what comes from the well. The gathering system constraints on this stuff are very much looser and the gas can be what it is. I've sold wellhead gas with 25% CO2, 5,000 lbm/MMCF water vapor, and a heating value below 850 BTU/SCF. This mess goes to a sweetening plant, the CO2 is removed, the gas is dehydrated, and sold as commodity "Natural Gas".

It is unfortunate that the two widely different things have the same name, but the world is what it is.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat" The Master on Dr. Who

 

[FOURe]

I would suggest that when doing basic engineering calculations, one can indicate that methane (CH4) is a good representative for natural gas ...

http://www.engineering-4e.com

 

[mbeychok]

mielke:

If you are interested in the processing of raw natural gas (including a shematic flow diagram), read this article in the Citizendium online encyclopedia:

[tab] Natural gas processing

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

 

[srfish]

For preliminary process calculations the following composition can be used:
CH4 - 85 Mol%
C2H6 - 10 Mol%
C3H8 - 5 Mol%
This gives estimated physical properties equivalent to 0.65 gravity (18.85 Mol. Wt.)gas.

 

[dcasto]

the 85% 10% 5% gas would have a BTU content that would not meet most US specifications as marketable gas.

HEATING VALUE (GROSS) BTU/SCF 1161.38
WOBBE NUMBER BTU/SCF 1437.04

 

[jaychem]

Yes there is specific composition for the natural gas depending on the pipeline specification.

Below are some common specs you will have to meet for natural gas:In US units

Hydrocarbon Dew Point at ceratain pressure (F)

HHV or LHV (Btu/ft3)

Water content (#/Mmscf)

CO2,H2S content.

So depending upon the specs provided by the customer you will have a certain Gas composion after processing of the raw gas from the wellhead or from other sources.

In common Fuel gas has a HHV value of 1000 to 1100 Btu/ft3 and has CH4 content more than 90%.

 

[MortenA]

1) None of the specs below is specifically concentration (maybe except co/h2s)

2) The question is not specifically oriented towards pipelines - it may e.g. be for LNG production or e.g. with reference to feedstock for chemical processes

I tink your statment is too "square". The last two posts from srfish and dcasto shows that its not easy to throw a definition on the table - because its not just Europe og US that uses NG - and we dont have the same definitions!

Best regards

Morten

 

[meanderson3d]

Natural gas is simply a mixture of hydrocarbons that happen to be in the gaseous state at the operating temperature and pressure you're looking at. Coming out of a wellhead, you usually have a natural gas phase in equilibrium with a crude oil or condensate phase, with a water phase (liquid) thrown in there as well. The composition of the gas phase will be determined by the composition of the entire hydrocarbon stream along with temperature and pressure.

It gets a little more complicated because as you separate the gas from the liquid and then send the oil to lower pressure separators, treaters or tanks, more hydrocarbons will flash out of the crude oil as it goes through the pressure drop. This gaseous hydrocarbon phase will be called "natural gas" too, but it will be a lot heavier (less methane, more ethane, propane, butanes, etc.) than the natural gas at the well head.

In many calculations, we don't need to know the composition. We just use typical values for specific gravity, specific heat ratio, etc. and go from there. That usually provides close enough results to design equipment that works. If we need more exact results, process simulation software works really well.

Andy
New Orleans, LA
Petronyx Consulting Engineers, LLC

http://www.petronyx.com

 

[MortenA]

BUT he asks for coposition... - and you dont know why!

And i dont agree with your definition of natural gas

Best regards

Morten