Volume to Mass Conversion for Ideal Gas

[engpes]

I have methane gas (CH4) flowing 10 MMSCFD (115.74 SCFS) @ 515 psia. I am trying to convert this to a mass flow rate. The properties are as follows:

Pressure: 515 psia
Temperature: 120 F
Gas: Methane (CH4)
Molecular Weight: 16.043 gm/mole
Volumetric Flow: 10,000,000 SCFD (standard ft^3/day)
Mass Flow: ? lb/hr

I believe treating this as an ideal gas will get me close enough. It has been a while since I dealt with this conversion, and am a bit rusty. Can someone please point me in the right direction with the conversion equation to use? PV=nRT? Thank you in advance.

 

[zdas04]

For SCF the conversion to mass requires calculating density at your imaginary (standard) conditions. The equation I use for density is:
ρ_standard=(P_std*ϒ)/(R_air*T_std*Z_std)

The gamma term is specific gravity relative to air (0.5539 for pure methane). Compressibility (Z) of methane at 14.73 psia and 60F (a common set of values for standard, certainly not the only one) is 0.998 so calling it 1.0 is a pretty good estimation.

Mass flow rate = Volume flow rate * density

You just have to look up the specific gas constant for air and be careful with your units.

If you need velocity then you have to get out of SCF. Calculate density at actual conditions (careful here, at pressure that high your compressibility is far enough from 1.0 to give you crap answers if you ignore it and divide that into your mass flow rate (or take your SCF number times standard density over actual density).

David Simpson, PE
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

 

[MortenA]

David you did it right of course, but to the benefit of our young friend eho may or may not be doing his homework) a scf is a fixed no. of moles ( 1.19804 moles (0.0026412 pound moles)) this you can find e.g. on wikipedia the Danish energy agency also posts this.

So if you wish to covert a scf of C1 to mass you multiply by 0.0026412 and by the molweight of methane and voila, so for 10MM its

10000000*0.0026412*16.04=423648 lb

Best regards, Morten

 

[zdas04]

Morton,
It only becomes a fixed number of moles after you have defined "standard". If STP is 14.696 psia and 60F then your numbers are right. If STP is 14.5 psia and 59F is it slightly wrong. If STP is 15.025 psia and 32F then it is more wrong.

The problem that I keep having with this topic is that too many people believe that STP is only one thing (the one they are using). Every contract, every government agency, every processing plant PLC programmer has the ability to assign a different number to STP. The U.S. EPA has 5 different sets of values for STP in their various regulations. That is correct, if I calculate emissions under Subpart OOOO I have to use a different value for both standard pressure and standard temperature than if I'm calculating exactly the same parameter under Subpart W. It is very common for a gas sales contract to use a different value for STP than you have to use to report to the state. For a well in New Mexico on one of the reservations I have to be prepared to report standard volume to the state at one value, the tribe at another value, the BIA using a third value, and to the company's internal production system using a fourth. It is all very doable, but it requires considerable rigor in knowing the pressure/temperature basis for the value you store in your internal system.

We bought a cheep RTU one time that did not let you input a local atmospheric pressure or a pressure base. We had it installed and running of a couple of months before I tried to reconcile the system material balance. It took considerable effort to nail down that particular bit of programming stupidity. When I did, I replaced the RTU with a different brand and did a presentation at an international measurement conference where I didn't mention them by name but I included a photograph of their box. I hope I had a hand in their going out of business.

David Simpson, PE
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

 

[MortenA]

OK, I agree to that, i always also state the reference T&P - but if its 60 def F and 16.496 psia then its a fixed no of mol regardless og the gas.

I would (in Denmark where i live) get around this by referring to the Deanish Energy Agancy definition (and our company standard that i wrote myself).

A common mistake in the North Sea would be between Sm3 and Nm3. Offsgire Norway mostly refers to Sm3 - but continental gas transport usually refers to Nm3

 

[MortenA]

I remember that UIPAC some times back tried to re-define the pressure for Nm3 to 100,000 Pa (it remains at 101,325) - that was a mess.

 

[MortenA]

Here are "my" definition from "my" company standard

Pressure (abs)
Temperature
Eq. number of kmol

SCF
14.696 psia
60ºF
0.0011981

Sm³
1.01325 bara
15ºC
0.0422937

Nm³
1.01325 bara[1]
0ºC
0.0446158

 

[LittleInch]

Engpes - Just to be picky here, but your initial post quotes MW as 16.04 but states this as gm/mol. I think you'll find it is actually lb/mol, hence morten A is correct. Please be careful about your units.....

ZDAS04 - you have some job there with 4 STP definitions! The worst one I had was when we had liquid figures at about 75bar prior to entering a 3rd party processing facility. Not unsurprisingly by the time the processing plant had done its job the liquids volume changed (reduced). Attempting to explain that the plant had not "stolen my liquid" was only partially successful and I don't think they ever really got it....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

Morton,
Yep, SCF and SCM are quite good surrogates for mass flow rate and once you've defined "standard" you have defined how many molecules of a given gas is in a given imaginary volume.

The new ISO gas measurement standard uses 14.5 psia=100 kPaa=1 bara and 15C=59F. I think it is fantastic and wish everyone would go to it. The U.S. gas measurement standard stayed 14.73 psia and 60F.

The Sm3 vs Nm3 discussion is a really slippery slope. Everyone thinks that they have THE definition and that there is only one. There is not only one, there are dozens. I get around most of the nonsense by defining SCM as "the volume in cubic meters that a mass of gas would occupy at 100 kPa(a) and 15C" (or whatever temperature and pressure the parties agree to). There is simply zero benefit to defining two separate imaginary conditions, it is simply a self-deluding method to keep from doing the job properly (and always ends up with someone spending more effort reconciling where things went so horribly wrong). If I'm doing serious Engineering calculations I get to mass as quickly as possible and duck into actual volume flow rate as necessary to get velocities, etc. I reserve SCM and SCF for contractual/regulatory things. As bad as Nm3/Sm3 are, they are not as bad as the Canadian e3m3 to refer to 1000 standard or actual cubic meters with no agreed upon way to tell the difference.

As an aside, in my class I teach
[ul]
[li]SCF = the volume in cubic feet that a mass of gas would occupy at 14.73 psia and 60F[/li]
[li]MSCF = One thousand SCF[/li]
[li]MMSCF = One Million SCF[/li]
[li]SCM = The volume in cubic meters that a mass of gas would occupy at 100 kPa(a) and 15C[/li]
[li]kSCM = One thousand SCM[/li]
[li]MSCM = One million SCM[/li]
[li]Let the confusion begin[/li]
[/ul]
I hate the Oil & Gas use of Roman Numeral "M" (1000 in Rome) and then using "MM" (2000 in Rome) for one million. Nothing about that makes mathematical or historical sense. When I see MMSCM or MMnm3 I just want to cry. I reserve any m^3 or ft^3 for actual conditions. Not everyone follows that (obviously) so I've started being really careful to say MACF or kACM, but even that leads to confusion too often. Therefore I'm on a one-man crusade for this industry to stop being stupid. My chance of success is exactly equal to zero, but that didn't stop Don Quixote.

LittleInch,
I picked that example because it has an extra interested entity. I rarely see a single pressure/temperature base for all involved entities. When I am asked to review a gas-sales contract (which I seem to be doing 5-6 times a year) the first thing I do is go to the definitions section and look for a definition of "standard". I don't care what values are in there, but if that definition is missing I stop the process until I have an agreement from the parties. It is amazing how often the people negotiating a contract don't know what their company uses for internal reporting. I know it is pretty small beans (the difference between 14.606 and 15.025 is 2%, on a 100 BCF gas sales contract that is a mere 2.5 BCF (or $7.5 Million, chump change), but it can really be an ugly discussion in court.

I looked at one international company and found that their annual report program just accessed the production data from their subs and added it together even though it was stored at 9 different pressure/temperature bases. Fixing it did not make a material difference in their economic performance, but it got rid of a half dozen footnotes trying to justify numbers not adding up.

David Simpson, PE
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

 

[MortenA]

I think the 100,000 Pa is a nice round numbers - but all existing definitions uses 101.325 Pa and would hus be wrong. So i dont back that definition. I belive that the M is actually not roman but french (mille) and its mille mille (or a thousand thousands). Fortunately its very few european engineers that uses MMNm3 - they would use MNm3 I would like to help you with the MM vs. ISO prefix

 

[zdas04]

I can point to 6 different countries that use a number other that use 101.325 kPa as their standard. Also I can point to a dozen companies that store volume data on a different basis and convert on output. There is nothing in the world less standard than STP. It really should be NTP (i.e., "negotiated pressure and temperature") but that would be admitting our deep dark secret.

I looked up the French mille and can't find a commonly agreed upon symbol for it in a quick search. I'm going to look further.

David Simpson, PE
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

 

[milltir]

Contrary to one of the comments above, there is nothing wrong in engpes giving the molecular mass of methane as 16.043 g/mol, the mole is by definition the g mol. If using English (Imperial) units you state the molecular mass as 16.043 lb/lbmol - but the mole (g mol) and lbmol aren't the same. The lbmol is approx 454 moles.

 

[MortenA]

@David NTP i like that

@milltir: You are quite right!