How to Calculate Flammability of gas mixture 9

[Leclerc]

How can I calculate the flammability of a gas mixture from a knowledge of gas composition, and individual flammability limits?

 

[TD2K]

I came across one reference in “Explosions” by Springer-Verlag, Berlin Heidelberg, New York.

They claim (where Pn is the volume fraction of the individual components) that:

LEL =( P1 + P2 + ... Pn)/(P1/lel1 +P2/lel2 + ... Pn/leln)

P1 - vol fraction of first component
lel1 - lower explosive limit of component 1

For the UEL, the same equation is used except that lel is replaced with uel.

The book claims that this equation has been proven many times and gives a couple of references from German publications.

 

[Leclerc]

Thanks, TD2K.
the equation seems eminently plausible, so I will run with it whilst I investigate the reference.

The second part of my problem, which I forgot to add under Chem Eng.Process, but put under Combustion Engineering, deals with the addition of an inert (say N2) to the Gas/Air mixture. How do I calculate compositions of Gas/Air/Inert which are at the LEL and UEL?
Putting the question more visually, on a triangular diagram whose axes are Gas, Air, Inert, how do I draw the envelope delineating the flammable region?

 

[TD2K]

The best way I know is to get hold of the data from the Bureau of Mines on explosion limits of various gases in 02 enriched mixtures. I had to do this for an atmospheric ethane tank that we were purging out and I needed to confirm that the procedure the contractor proposed would not take us through the flammability region (basically, purging with N2 partway and then switching to air but they proposed switching to air at a higher concentration that I was used to but insisted it was safe).

I'm a little fuzzy on the details (this was a few years ago and I did not bring the reference material back from Arabia). I think I used the Bureau of Mines data to create a 3 phase diagram for AIR, N2 and ethane (I think the data was for O2, ethane and N2 but I converted it to air as I would be purging with air and it was easier to visually show this to management in this fashion). I drew the flammability region on the curve and then could see how the purging would progress wrt the flammability region. Basically, you looked at starting with 100% ethane moving along the straight line connecting it to the 100% N2 point. When you switched to air purging, you then moved along the line to the 100% air and this was the area that I did not want to pass through the flammability region.

Rather hard to put into words. Try playing around with it and if you still are stumped, I'll take another whack at it.

 

[Guidoo]

Please check out "Rules of Thumb for Chemical Engineers" by Carl Branan, 2nd edition, pages 276-278.

Formula TD2K mentions is called LeChatelier's Law, and can also be written as:

Lm = 100/(x1/L1 + x2/L2 + ... xn/Ln) %(vol)

where:
Lm = upper/lower flammability limit of gas mixture
Li = upper/lower flammability limit of component i
xi = concentration of component i in gas mixture

For combustible mixtures containing inert gases N2, CO2 as well as O2, following procedure can be followed:

- If O2 is present, the composition of the gas has to be corrected to render it "airless".
- Next, data from figures 1 and 2 on page 277 of Branan's book has to be used. These figures show flammable regions as a function of inert gas to flammable gas ratio for various combinations of flammable gases and inert gases.
- Finally Le Chatelier's law has to be applied.

Branan's book shows how this works with a detailed example.

Another possibility (for N2) I used myself is when you have literature data of flammable limits both in air and in pure oxygen. When plotting these upper and lower flammable limits in a composition triangle (with O2, N2 and the flammable gas in the three corners), and connecting the points by straight lines, you get a pretty good idea what the flammability envelope looks like.

Good luck!

 

[dbachovchin]

Limits of Flammability of Gases and Vapors by HF Coward and GW Jones, US Bureau of Mines bulletin 503.

And Flammabilty Characteristics of Combustible Gases and Vapors, by M Zabetics Bureau of Mines buletin 627.

They have all the gases, mixtures, etc.

 

[Leclerc]

thanks once again, people.
I did use the Bulletins way back in the early '70s for a similar job, but I thought there these days there would have been a more immediate form of the data. It appears that I was wrong. Never mind, I am perfectly happy with the bulletins, and I hope to have copies of the USB documents to hand within a few days.
regards, Leclerc.

 

[junnip]

I have also this same problem and I now know how to calculate gas mixture which contains nitrogen. But the problem is how to calculate flammability limits of gas mixture which contains water vapour?

 

[25362]

Nitrogen and carbon dioxide are geenrally used for inerting purposes and to reduce the MOC in a flammable vapour mix. Steam (water vapour) is sometimes used for the same purpose.

 

[junnip]

Yes, but how can I calculate flammability limits if gas mixture contains for example 30 vol-% CH4, 30 vol-% H2O, 30 vol-% N2 and 10 vol-% CO ?

 

[25362]

It is well known that with additions of inerts such as nitrogen the flammable range gets smaller, up to a point where the mixture is no longer flammable at room
conditions.

For methane/air the internet tells us this happens for mixtures containing 21% CO₂, or 35% N₂, or 48% He, or 26% water vapour by volume.

To junnip, the example mixture you gave doesn't include oxygen or any other oxidant, thus at room conditions it wouldn't be flammable, right ?

I think every potentially flammable mixture should have its LFL and UFL checked in a specialized lab to be sure of its safe handling.

FWIW one may find the following site, on MOC with examples, of interest:

http://www.ntron.com/tb1.htm

 

[kenvlach]

This subject: LEL, UEL, Le Chatelier's Rule, the US BurMines documents, as well as the effects of increasing temperature or total pressure or adiabatic compression is covered in Perry's Chemical Engineers' Handbook, 7th Edn., pages 26-53 to 26-55.

 

[Flareman]

I posted a page which will estimate the flammable envelope for you, together with the effect of a diluent, based on published data for the flammable gas.

http://www.geocities.com/flareman_xs/Gases/Flammability_calc.html

I just posted it today so there has been no feed-back yet. If it needs 'tweaking' let me know.

David

 

[moquiton]

Hello everybody!
i have read you questions and answers and i still have the doubt of how calculate the flammability limits of a mixture with more than one inert gas (CH4 CO2 N2 air).

I have a diagram of Methane and these inert gases but i don´t know how i can take both into account?

thanks every body

 

[moquiton]

Is there a method or formula with which we can obtain the flammabily limits of whatever flammable gases + whatever non-flammable gases + air, in aproximatively way?

Could you recommend me any book for flammability limits of flammable gases + non-flammable gases (i am interesting when there are some non-flammable gases in the mixture) + air?

thanks again

 

[25362]

I haven't seen procedures to estimate the "corrected" flammability limits upon adding inert gases of many sorts to flammable mixtures.

Although LFL/UFL are based on fuel in air at 25^oC and 760 mm Hg, consider that in the process of inerting a combustible mixture one reduces the concentration of oxygen (the key oxidant) below a minimum "MOC".

For many (but not all) gases the MOC is ~10%, and for many dusts ~8%. I don't think there are procedures to estimate the flammability limits by adding inert gases of many kinds.

A control point of common use is 4% below the MOC, ie, 6% oxygen when the MOC=10%.

The MOC is expressed as % oxygen in the mixture of air + fuel. Below the MOC the reaction cannot generate sufficient energy to heat the entire mixture (including the inerts) as needed for flame propagation.

It is generally determined experimentally. If data are not available, the MOC is estimated using stoichiometry of the combustion reaction and the LFL. This procedure works for many hydrocarbons (acetylene is one exception) and organic molecules (ethylene oxide is another exception).

Following this procedure,

MOC = (LFL)(Moles O₂/Moles fuel)​

In a way, having the experimental MOC one could estimate backwards the LFL.

Take, for example, toluene whose MOC has been measured to be 9.5. Since the stoichiometric ratio is 9, the LFL would be 9.5/9 = 1.1, vs the published LFL=1.2 .

BTW, Steam is not always recommended for inerting, because its condensation, if the conditions allow it, would bring back the oxygen concentration into the flammable region.

Some published experimental MOC I've found:

methane, 12; propane, 11.5; ethylene oxide, 0; benzene, 11.4; toluene, 9.5; H₂S, 7.5; acetylene, 0; hydrogen, 5.

As it can be seen from these data, not all estimations are even in the "ballpark". It appears that the MOC also changes with pressure and temperature.

When googleing around just ask for minimum oxygen concentration for combustion, and you'll find a plethora of pertinent articles. For example;

http://www.banksengineering.com/about_flame_arrestors_and_detona.htm

 

[moquiton]

25362

So, there is a MOC, that represent the minimum concentration of oxygen for igniting a mixture.Is there also a maximum oxygen concetration? Could be represented by the next formula?

Max Oxygen Conc. = (UFL)(Moles O2/Moles fuel)

Thank you

 

[25362]

To moquiton, some sources also call the MOC a "maximum oxygen concentration", depending on what they mean by that.

Regarding you question: do you mean an oxygen-enriched atmosphere ?

 

[moquiton]

To 25362, that is what I want to say: a mixture in an oxygen-enriched atmosphere.

Thanks

 

[25362]

Visit:

http://www.eiga.org/pdf/Doc 04 00 E.pdf

for references

http://www.swagelok.com/downloads/webcatalogs/MS-06-13.pdf